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THE ROLE OF SU13MERSED MAÇJWPHYTES IN PHOSPHORUS CYCLING /

by

@ Richard Carignan

\ A thesis submltted to the Facult'Y_ of Graduate

Studies and Research in partial fulfilment of

the requirement for t~e degree" of Doctor' of

Phil9sophy.

Department of Biology February 1980 McGill University _ t_

'" MontrEal t Canada

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Ph. D. Biology

Richard Cari'gnan

The role' of submersed rnacrophytes in phosphQrus cycling. .. ~

ABSTRACT

The specifie aetivity of the 32p labeled sediment-P taken up by'submer-•

sed macraphytes was 'shown to be identical to the spedfi C' activi.ty of the

sediment mobile P, as measured by isato,pic dilution. The mobile P therefore . \

represents the sediment-P available to aquatic maerophytes .

. The abi 1 ity ta aecurately measure the specifie actlv,ity of the available

sedimeRt-P was applied to problems pertaining to the rolè of macrophytes in P \

cyeling. The relative contribution of water and sediments in supplying P to "

l' macrophytes WpS measured by growing macrophytes if1 situ, roçteq in 32p labeled "' -

sediments, and with the shoot in free contact with ~the .unlabeled overlying

water. Macrap~ltes grown. ,in mesotrophic and eûtrophie sites cterived"more than

95% of their P from the sediments alone. When grown in a hypertrophie site, , . 1

the sediment.s still supplied 70% of the P.

Rates of P release by macrophytes and;ignificance ... to their periphyt~n

and surrounding ph,ytoplankton were e~t;mat/d by using<tul:1Y labeled pla~ts. The periphyton derived only 6.5% o! its P form ~he macrophytes. Myrioehy11um

, released 0.32 Ug,g-~.h-~ P., ~ast,of whith. being read,ily'available to phyto­

plankton.

The high vertical mObility of the available sediment-P was demonstrated

bath by exper.iment'al manipUlations ami 'direct observations.

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O. Ph. Biologie

Richard Carignan

Le rôle des macrophytes sUbmergés dans le cycle du phosphore.

RESUME

On- montre que l'activité spécifique du P sédimentaire assimilable par

le:; macrophytes submergés et marqué a'u 32p , .est égale a l' ,activité spécifi­

que- du P mobile, tel que mesuré par diluti,on isotopique. Le P mobile repré-

sente donc la fraction disponible du P sédimentaire.

, Cette possibili~é de mesurer de facon précise l'activité spécifique du

P sédimentaire disponible fut appliquée a des problèmes ayant trait au rO)e

("des macrophytes dans le cycle du P. On a mesuré la contribution relative de ,

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l'eau et 'des séd'iments da~s 1 a nutri ti on des macrophytes en fa; sant cro1 tre

des macrophytes in situ, enracinés dans des sédiments marqués, et avec la par­

tie verte en libre contac~ avec l'eau non marquée. Les plantes croissan,t dans

de~ milieux mésotrophes et 'eutrophes puisèr~nt plus de 95% de leur' P des sêdi­

ments seuls. En milieu hypertrophe, les sédiments fournirent 70% du Po

L'utilisation de plantes marqu~es -de'facon homogène a permis d'estimer ,

des taux' di excrétion de P chez Myriophyllum ainsi qùê d'estimer l'importance \ ,

de ce P pour le périphyton et le phytoplancton. On-a trouvé que le p'ériphyton

ne tire que 6.5% de son P des macrophytes. Le taux moyen d' excr'ét i on de P pour

Myriophyllum fut de 0.32 U9.g- 1.h-1, do~t la/majeure portion Hait rapidement

di sponibl e au phytoplancton.

On demontre la mobilité verticale élevée du P sédimentaire disponible

par ~es observâtions directes et manipulations expérimentales. ,

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---TABLE OF CONTENTS

PREFACE

ACKNOWLEDGEMENTS

GENERAL INT~ODUCTION ••••••••••••••••••••• ',' ••••• A • • • • • • • • • • • • • • • • • • 1

PART 1 Quantification of the sediment phosphorus avai1~ble to aquatic macrophytes ............................. .

Abstrac t ....... , .. ',' .... Il .'~ •

Introduc tion ••••..••••••..•• Materia1s and methods •.•.•.• Resul t s and discussion ••...•

· .................... . · .. ~ ...

• J ••••

. . / ... ~ · ... 1· ',' ..

· ............................ . Ref~rences •.••.•..• • • • • • • • • • • • • '. ,'li •• . . . . . . . . . . . . . .

"."'" e-

PART Il Phosphorus sources to aquatic macrophytes: W~ ter 0x: sediments? .................. , .......... If •••••••••••••

. ..... . · ...................... . Abstract ••••• Introduction. . . . . . . . . • .................. J • ••••••• ~ ......... ..

Methods •• • ~ .................... .. Results and discussion •..... Notes and referenees ••••••••

,.

. .. , , ...... " .. , .... . .. " . , ....... " " ........ " " ................................. " PART III Phosphorus release by aquatte macrophytes and signifi­

cance to periphyton and phytop1ankton •••••••••....•••

Abstract ... " .................... " ..... " .... " .. .. Inttoduct1on· .... ~." ..•• .. " 1 •• , ........ " ....... , . , . , .. ... , ........

/ Méthods .. " ... , .......... " ......... _,," ,. ..... .. Results and discussion-••...••• ' •.•••• .......... , ..... " ...

• " " ... Il ..... ~ ....... "\1 ....... ..

references ..... .- ............ " ........ .. .. • " •••• III" .1" ........... _' " ....... " ... , •

PART IV Post-deposiUonal mobility 0; phosphorus in lake sediments ••••••• . . . .. .. . .. .. . " .......... " " .......... " ............... ..

Abstract .•••. Introduction.

• .! ........ , ............... ..

.. .. .. .. .. .. .. .. " .. , ...... "Methnds ................ "" ... " ...... " .. .. Resul ts and discussion ••••••• Refelences ........................... , ... " .

.. . " ............ ~ ................... , ... ..

.. .......... " ............. , ............ " .. .. , .

.. .. .. .. " ...... .. ....... ., .... . .. . . . . .. . .. .. . . .. . . .. . . . .. .. . .. . . .. . . .. .. .. . .. . .. .. . . .. .. .. .. . ~ .............. . \

GENERAL CONCLUSIONS. " ........................... i ........................ .

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, LIST OF FIGURES

PART 1

Figure J. Partitioned container used in growth experiments wlth Heteranthera rooted in 32p labeled sediments ............ 8

Figure 2. Specifie aetivity of ion exchange eftraetable P _ (mobiJe p) with tlme .•...................•. \ ..•..•.• , ..•.•.. 12

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F j gure 3. SpeC'l fie act i v 1 ty of j on exchange ext ractab leP (lllObile p) in sediments Incubated at '.and 20°C ...... -; ...... 13

PART '1

Figure 1. Experimental apparatus used in phosphorûs uptake measU(ements ....... Il' •• Il , Il Il Il •• Il Il Il Il •• Il • Il Il'.11 •• Il • Il Il ••• Il Il Il Il'' •• Il 23

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PART 111

Fig,ure 1. Phosphoru~ release by the Hyriophyl lum-periphyton complex duri,ng a 24h cycle .. , ............................... ' .. 43

Figure 2., Phosphorus release by the Hyrloph'yl1um-periphyton . complex before and after addition of carrier P .... ,'" ......... 45

FI gure ). Remaining total, ,particulate and soluble 32p after addition of carrier-free 32P04 ln a chamber contalning

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an unlabeled Hyrlophy'1um-periphyton cortlplex ................ 52

PART IV

Figure l,. SRp· and Its speclflc' activlty versus tlme in the anaerob je conta i ners used for the measurement of mobile P by Isotoplc di lutlon ............................... 63

Figure 2. Lead-210 and total P profiles in Central Bas in, Lake , Hemph remagog ••••.•.•.•••....••.....••••....•••.....• '" .•.••• 65

Figure 3. Total, mobile and, Interstl,tlall P profIles after 5 weeks 1 n an 1 nit i a lly homogenous mud •..•••••••••...••••...•. 68 ,

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Figure 4. Total and interstitlal Fe profiles after 5 weeks_ ln an Inltlal1y homogenous mud .................. ~ .......... 69

1 < Figure 5. Total, mobile and Interstitial P profiles at station A ••••. 72

Figure 6. Total, mobile and interstitlal P profiles at station 8 •..•. 73

Total and mobile P profî les at 5 ta t ton C .• 1 Il •••••••• " • " •••• 7.4 1 !.

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LIST OF TABLES

PART 1 Page>

Table 1. Correspondence between plant aval1able phosphorus (âP) and mobile sediment phosphorus •••••....••••••.••• " ••.••• ~ ••.• ,15

PART Il

Table 1. Water and sediment P charae teristics for the three Southern Québec sites imiest igated ••••••••••••••••• ',' ••••••••• 27

Table 2. Mean percent P uptake_ from t~ _ sedimen·ts for six maerophyte species grown in Central Lake Memphremagog •••••••• ,28

Table 3. Mean percent P uptake from, the sediments for three .' macrophyte species grown under various conditions of water and sediment P availabilities •••.••••••. , ••••••••••••••• 29

PART III

Table 1. Total P content of macrophytes, associated epiphyte-P and % epiphyte-P derived from su~porting macrophyte for . nine macrophyte speeies cOllected\ in Central Memphremagog •• " •. :41

Table 2. P release rate by macrophyte+periphyton, macrophyte-derived P release rate a~d % macrophyte-de'rived P particulate after Sb for Myriophyllum spica tum •• , , , , ••••• , , , •. " • , , •• , , , .......... 48

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PREFACE

The thesis is presented as a series of four inter-connected pa pers in

publication format as permitted under the regulations of the Graduate Faculty

of McGill University. These regulations, also req,uire the following statement

as to the elements of the thesis that are considered to b\} "contribution to ..". -

ori gi na 1 'knowl edge" .

The technique presented in this thesis for quantifying in situ the re-

lative contribution of root versu's shoot i'n nutrient uptake by aquatic macro­

phytes is a contribution to original knowledge. It ihas never been possible

'before to measure quantitatively in situ the relative importance of: root- and

shoot in phosphorus uptate by aquat; c macrophytes. Thus" the da ta presented ~~ ,

>- here for nine species occuring in different environrflents are original contri-

bution to knowledge. It has' never been possible either to measure quantitati­

~ely in situ nutr;ent transfer from l11àcrophyt~s to their epiphytes and phyto-' 1 • (

plankton. !he data on nutrient transfer from'm'acrophytes to their epiphytes .

and the data on rates and availabil ity of the phosphorus released by macro-

phytes, as generated by the full labeling technique presented here, are also

1 contribution to original knowledge. Finally, the demonstration that sedimen-~ ,

tary Ph9sphorus i.ncludes a vert1cally highl~ mobile pool is a contributipn to "

original knowledge and radically changes our interpretations and concepts of

phosphorus distri'butipn and behavior in lake sedi~ents.

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ACKNOWLEOGEMENTS

1 wish to acknowledge the National Sciences and Engineering Research

Council of Canada and the Di rection Général e des Etudes Supérieures du Québec

for postgraduate scholarships'. 1 also wish to thank Jaap 'l<alff, my thesis , -,

. . superv,; sor, for encouragement and support during this- t\çearch.

. , Acknowl edgements are al 50 due to Rob Peters, Bruce Lazerte and Bob

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Flett, whose advices contributed si gnificantly to the- ful filment of t'hi 5

research. Special thanks are due to Bob Flett, who provided the lead-210

data. 1 wlsh to thank Tony Briza for skillful machining of the plexiglass

equip,ment and Robert Lamarche for photographie reptoduction of the figures. p , 1·

Most particularly, thank my wife, Susanne, who helped me more than

1 can tell.

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G'ENERAL 1 NTRODUCT ION

One of the 'most important achievéments'" of limnology in the last decade

is the observation, and subsequen.t 'gen~ralizatif)n, that phosphorus (P)' 1s a

l imiting nutrient in most aquatic systems. Thiso.finding si'gnificantly stimu-" , '

lated °research €'fforts on P 10ading pr,ocesses in lake!> and led to the deve-... ,IJ 7

lopment of predictive external nutrient loading models,that were used, in'

turn, to predict various biologicœl aspects of lake response'to P loading.

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Sin~e internal nutrient loading proces'Ses were also observed'to parti- ' 1

cipate in the nutrient, dynamics of l'akes; our efforts to eval,uate ,the impor- ' , ,

tance of internal P loading agents, such as sediments 011 macrophytes, were J

also intensified. Unfortunately",<?ur accumulated Q~servatiQn~ ~re not",yet

amemlbl e to general ized predictive theories or mode) s for re,àsons tha,t appear , (l ~ -

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~rn9re rel ated to our present inabi 1 ity to acclJrately quanti f,y, the importance " .. Q ,

of int~rnal loding 'agents, rather than"to sorne ;ntrins,;c ,irreductibility

property of biologicaT systems.

As rooted

reservoi r;, they

aquatic macrophyt~s have access to a ~large se,dimenJary P -constitute a potentiallY important pa~hWay~ 'for' .int~;nal p'

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loading through upward translocation of sediment~P," and subsequent ~

or sènescent P release. The po~ential importance of macrophytes ,in-P-cYéli~g .',

can easily be illustrated i'f one 'con~iders that in ,many l~kes hav.ing an--, , '

ex tens; ve Ji ttora l zone, the amount 0 f P Ued up in rnacrophyfes i S often n , (> 1

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~q~ivalent to, or greater than the ~nnual exterrial P 10ading to .~hese lakes.·1

Theo estimation o'f the importance of macrophY.tes a's internaI nutr1ent , . ,.,

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loading agents includes three basic questions pertainin~ to:

a- the source (water OP sedi~ents) of macrophyte-P;

b- the in vivo relea~e and availability of macrophyte-P;

e- the senescent release and availability of macrophyfe-P.

The objective of this study was to develop and use techniques for the

accurate measurement of the relative contribution Pt. wat~r and sediments in

P uptal<e by aquati c macrophytes ànd to es tab 1; sh the s tgnif5 cance ot" macro­

phyte released P to periphyton and phytoplankton. .,

" The general approach was to first investigate the feasibillty of labe-

2

1in9 the sediment-P fraction available to aquatic macrophytes with'32p, and of . measuring the degree ,nf label,ing of this fraction. This section has been

\ .published in the Journal of the Fisher,ies Researefi Board of Canada and

eonstitutes Part 1 of this th,esis.

The se~iment labeling technique developed in Part 1 was then applied , "

to a variety of ~pecies by grow;ng plants in sltu in labeled sediments of

known availabl~~P specifie aetivity, with the shoots in free contact with the .. , ... le "1 . ,

surr6undi~9 wate~, in o~der to quantify the relative importance of water and

sediments in nutrient ,supply to macrophytes. This section constitutes Part II

of }his thesis,and is being published in Science •

'Al' ~pe presently existing information on the rates of in vivo nutrient

release by macrophytes ând significance to their periphyton and surrounding

phytoplankton is questionable because of the highly artificial conditions

employed and because of non homogenous tracer distribution with1n the experi­

mental plants. These two problems were "eliminated by producing. in situ.

ful1y labeled plants that were ,used to quantify P release by macrophytes and

'to evaluate its, significance to the periphyton and phytoplankton (Part III).

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Since Part II demonstrated the overwhelming~;mportance of-sediments

as a source of P to ,macrophytes, the d_istribution and dynamics of intersti­

tial and available (mobile) P in lake sediments wer~ investigated. and are

presented as Part IV of this thesis.

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PART 1

QUANTIFICATION OF THE SEDiMENT PHOSPHORUS AVAIlABlE

TO AQUATIC MACROP~YTES

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ABSTRACT

The specifie activity of phosphorus taken up by three species of 'sub­

merg~nt macrophytes. grown in partitioned containers and rooted in,32p labeled

sediments was shawn ta be identieal ta th~ specifie activity of the sediment

mobile phosphorus as measured\by two isotopie dilution techniques. Therefore,

the mobile phosphorus, as mè~sured by isotopie dilution, represents the total \

pool of sediment phosphorus avàjlable to macrophytes. Jhe ability to measure

and to specifically label this pool will allow the testing of hypotheses con­

cerning the role of macrophytes in phosphorus cycling.

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) INTRODUCTION

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Several studies have shdwn qualitatively that maerophytes are able to

take up phosphorus (P) via the roots (MeRoy and Barsdate, 1970; Brisfow and

Whitcombe, 1971; Denny, 1972; DeMarte and H~rtman, 1974; Twilley et al., 1977).

However, because of a laeK of appropriate techniques, it has been unc1ear

whether sediment-P controls macrophyte biomass in nature and whether macro­

phytes act as nutrient pumps or sinks.

I~ situ measurements of the relative cont~butio~ ef water and sediments o

in the P nutrition of macrophytes are obtainable by growing plants in sediments

with their available P specifically labelled with a radiotracer. In addition,

·the potentially limiting role of sediment-P on macrophyte development could be, "- <~ 1

assessed by comparing available sediment-P to plant yield. However. to do . . . either or both requires accurate measurements of the sediment-P available to

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macroph):'tes.

Studies on the short term 32p partitioning between the aqueous and par-,

ticulate ~hases of sediments (li et al., 1972) hafe shown that the interstitial

P of lake sediments ij in dynamic equilibrium with a variable amount of P

loosely held by the particulate phase. The sum of both fraction can be termed \ ~ ~""It--__ --=-/~} ...

IItotal exchangeable" or "mobile" P.

This paper reports 'a 'simple tedhnique that allows the quantification of

the amount and specifie activity of mobile P in 32p labelled sedillJents'. It [ ; -

also demonstrates that for one lakè at least, the sediment mobile, is the

'only sediment-P source available to aquatic macrophytes.

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MATERIALS AND METHODS

General approach

The experimental approach consisted in growing macrophyte species in

partitioned containers (Fig. 1) in whith the sediment compartment was filled'

with 32p equilibrated sediments (see belowf and the, upper compartment filled

with 32P_free lake water of known initial total P content (lO-15,ug/L).

Young macrophytes were collected from a shallow bay in mesotrophic lake Mem-.~

phremagog (Quebec-Vermont), which isthe subject of a long term study (e.g.

Ross and Kalff, 1975; Peters, 1978). ,

The plants were potted individually in the labelled sediments with the "

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stems leaving the sediment compartment through a tightly fitted soft silicone

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stopper which prevented al'l nutrient exchange between the two compartments.

The seal was sa efficient that à pressure difference, createa by ga~ production

in the sediments, was always evident at the end of each e~periment. The initial

plant P content was estimated by regressing total P on fresh weight for a .

, dozen similar plants collected in the same area. the resulting correlation

coefficients were always greater than 0.90.

The plants were grown,for periods sufficient (30-60 9).to reduce to

negligible (smaller than 1%) the'statistical error resulting from the estima­

tion of the initial plant P content. Thus, any phosphorus, beyond the initial,

appearing in the plant or water com~artment.had to have been previously assi­

milated by the rO,ot system and, therefore, belonged to the available sediment-P. , .

The specifie activity of this assimilated P (~P) was subsequently compared

with the specifie activity of the mobile sediment-P (see below).

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Heteranthera rooted in 32p label1e~ sediments~

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Mobile P methodology , ! . , The sampling and quantification of the sediment mobile P was done in two

ways:

1- The procedure of Li et al. (1~72). with the fOllow;ng modifications:

The sediments were suspended in qistilled water instead of a a.lN NaCl solution

and oxYgen-free nitrogen was obtained by passing the gas through a column of

oeated copper.

A known amount of carrier-free 32P-P04 was added to a 1% anaerobic sedi­

ment suspension. Concentration and activity of the soluble reactive phosphorus

<-(SRP) were thenperiodical1y measured after membrane filtration (0.45 um) under , '

a/nitrogen atmosphere. Tracer equilibrium was normal1y approached within a few '"

weeks and. assuming that the tracer is indeed in equilibrium with the P pool.

the amount of mobile P exchanging bëtween the water and the particulate com­

partme~t was ealeulated using standard isotopie dilution equations (Wang et al .•

1975) :

-:-::--- = =---31psol 32pso1 at equilibrium

31pso1 x 32Psed 31Psed = ---:::~---32pso1

3lpmobile = 31psol + ·31Psed

(1)

(2)

{3}

where 31pso1 = amount of SRP in the ~uspension 31Psed = amount of 3Ip loosely held by the' sediment

32psol = amount of tracer in solution

32Psed = amount of tracer retained by the sediment, calculated by sub­tracting the amount of 32p in solution from the total amount added.

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2- The interstitial P of undiluted 321' labelled sediments was sampled by -

means of anion ex change filters (Gelman "Acropor", type SB-6407). The filters

were pre-washed by soaking them for 1 hour in D.2N NaCl, followed by several

rinses in distilled water. They were then suspended for 48 ho~rs below the

oxidized microzone of labelled sediments contained in glass containers, after

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which they were cleaned by four successive and vigorous distilled water washings

to dislodge any adhering sediment parti cl es. The filters were subsequently

stripped' of the adsorbed P by supmersing them for 20 minutes in 50 mL aliquots

of 0.2N NaCl that were analyzed for SRP and 32p activity. The amount of mo­

bile P present in the sediment samples was calculated as above (method 1) by .

substituting in equation 2 the amount of 31p and \32p recovered from the fi ltlft

for 31 pso1 and,32pso1 , respectively.

Phosphorus analysis

All phosphorus measurements were made spectrophotometrically after Johnson

(1971), with a Bausch and Laub Spectronic 88 with 10 cm cells. No arsenic ;nter-

ference was encountered. Water total P was measured after persulfate digestion;

plant tissue P"was measured after digestion at 200~C in a HN03-HC104-H2S04-H20

mixture (Li et al., 1974); sediment total P was measured after digestion in a

mixture of HN03-HC104 at 200~C (sommers and Nelson 1972). Aliquots of the di­

gestates were counted for 32p by liquid scintillation in IIAquasol ll (New England

Nuclear)\without si~nificant quenching, using a Beckman Liquid Scintillation

System.

Sediment labelling

The sediments used for the plant uptake experiments were col1ected with a 1

Peterson dredge from a depth of 1. 5 meter from~._macrO~hYte area. Th~ mobi l e and

total P of these sediments varied between 32 - 60 and 831 - 925 ug/g dry weight.

respectively. For each experiment. 750 Bq of 32p as o~thophosPhoric acid (New

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1 England Nuclear) was added'and well mixed with 20 kg (fresh weight) of sediment.

Before initiating the growth experiments, the sediments were a1lowed a minimum

of three weeks for the tracer to equilibrate with the mobile P pool. However, "

Fig. 2 shows that a true equilibrium had not been achieved even after three

months as there was always a slight decrease of the specifie activity of the \

motiile P. However. after three weeks, this decrease only ranged from 2 to 4% " -~-

per week between experiments and cou1d be approximated as linear over time for '. J

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the periods i~volved in the plant growth experiments (Fig. 2).

After initiating the growth experiments, the sp~cific activity of the

mobile P was followed by either or both the suspension or the ion exchange

techniques. Since Rre~iminary work had shown that the evolution over time of

the mobile P specifie activity was a1so temperature dependent (Fig. 3), care

was taken ta keep the labelled sediments used'for plant uptake and the sub-. samples used for specifie activity measurements at the same temperature (22°C).

Pl a'n~ growth

The plants were grown at 22°C in a temperature-controlled incubator with \

the light produced by a combination of fluorescent lights and incandescent

bulbs providing 15 w/m2 (PAR) at the container's surface. Filtered air was

continuously bubbled to provide COZ and ta prevent thermal stratification in ,

the vessels. At the end of the growth period, the shoots were sectioned at the

septum level and vigorously shaken in their own medium ta dis'lodge as many

epiphytes as possible. Wall grpwth was then suspended by means of a clean brush

and the water sampled for total P and 32p activity.

1 f ~ J t , l 1 î 1 !

'i

\

1

~\

Fig. 2. Specifie aetivity of ion éxchange extractable ~ (mobile,P) .

w;th time after addition of tracer10n day zero where the

mean mobil~ P specifie activity during growth is taken as

the estimated activîty at midgrowth period, as measured bj

linear regression.-

\

u

, , ,..,

\

()

1 1

1

i

l ,

l '

1-" ,

\ ," . . ~"i

: ' ,

,1 ' " " 1

L

L' 1

'" 1 1 1

;'1 1

"

(

1 () 1

(

,r" \

a o

.......... ..1

o Q)

,0 <O}

o v

o t\I

------~----~~----~~----~------~o ./ 0 CD CD, V ,,(\1

tpOI X Cd 6nj ~d~) A1IAI1~'1 OI.:U 03dS

, .. ..

en >­« Cf

1 .... 1

-1 l

" ~~ .~~

" -1" - ... ~,"'.J, ~ ... ~ .. ,,,,~~~,,.~ ,

1 .,....

1

---.- --:--' ----:- - - .-.. -, .. '-- -;--"--~--._--~. , , ~

,,~i9. 3. Specifie aetivity of ion exehange extraetable P (mobilë P)

with time after addition of tracer, on ,day Zëro,~sedi-

ments kept at 4°C (1) and- 20 DC (o). ' "

r _

\ ,

.\

,0

o

o

)

o , \ \ .

<1-

, 1 1

. ,

l\j----

- .-,...~~"", .. "."" .. ~ ... ,... ... -........ ~~ ....... ~ .... ~ ............ ~ ........ -... .. ----- ..-.--

, -

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u o V

----~~,-~._----------~---- , 13

,~ 7'

N

,

u o o C\J

. -

-o •

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.f

C\J

Cl)

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Cf)

>-<(

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1 1 •

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~ ___ _ ,"" ___ ,-',;r ,~~,,' t

RESULTS AND DISCUSSION

Myriophyllum spieatum L., Potamogeton zosteriformis Fernald, and Hete­

ranthera dubia (Jacq.) MacM. were tested in six identical upta~e experiments.

At the end of the growth periods, the total amount of P absorbed by the roots

14

of each plant C~P) was ealculated by subtracting the total 'amount of P initially

present in the water compartment and the young plant from the sum of the quan­

tities present at the end of the experiment in the water, wall growth, and the

plant. The sum of the 32p activ;ties found in these same three locations was

divide~ by toP to yield-a specifie aetivity value which was then compared with r- . ./

ti,he mean specifie activity of the sediment mobile P as measured by the suspen-

sion or the ion exchange methods.

Table 1 summarizes the specifie activity values for t.P and mobile P.

It is evident that the mobile fraction of the sediment-P is the only signifi­

eant source of sediment-P for all three species, beeause the specifie activity "

of the available P (L\P) is not significantly different from the specifie acti-

vit Y of the mobile P. However, in one of the four Myriophyllum experiments, the

specifie activity of L\P exceeded by 15.5% the specifie activity of the mobile

P as measured by the suspension method. This discrepancy is probably attributa­

ble to poor subsampling or contamination of the sediment used to produce the

suspension. The smal1 variations in the remaining experimènts are attributable

to differences in' initial P content and to smal1 errors incurred by assu~ing a

linear P uptake over time in the deterf1)ination of the mean mobile P specifie

activity (Fig. 2).

These results contrast with the fi ndi ngs of Li et a 1. (1974) who repor­

ted that in sorne cases, the amount ,of ( taken up from the sediments by Myrio-~,

Phyl him during a 15 week period markedly exceeded the change in mobile P

\

, 1

1 1 / 1

t:)

~ .... ~ ...

/'"

,. 'i,t

, ~~ ... ::. ot , .

~

TABLE 1. Correspondence between plant available phosphorus (~P) and mobile sediment phosphorus.

/

Spec;es

MyriophyllumQspicatum

Myriophyllum spicatum

Myriophyllum spicatum

Myriophyllum spicatum •

n

3

5

3

6

Potamogeton zosteriformis 3 / '

!

U Heteranthera dubia 3

a: Suspension method

b: Ion exchange method

A

Mean.l!.P speèific activity ± SE

581 ±. 13

7453 ± 79

2020 ± 10

" 7461 ± 83

2092 ±33

3270 ± 22

SE: Standard error of the mean

')

B

Mean mobile P % A/B

specifie aetivity ± SE fSE

560 ± 7.2a· 103.8 ± 2.4

7680 ± 83a 97.0 ± 2.1

2060 ± 6la 2010 ± 19b

6460 ± 101a

7540 ± 77b

2060 ± 61a

"2010 ± 19b

3200 ± 48b

fi

98.1 ± 1.5 100.5 ± 0.5

115.5 ± 1.6. ·99.0 ± 2.7

101.6 ± 2.8

104.1 ± 1. 7

102.2 ± 1.2

f-I \.11

/'

./

1 1 l j

t 1 !

j 1 ~ 1

..

j

1 1 1

1 ! 1 .

i 1

1 t

\

16

as measured before and after growth by a short term (3 days) rad; otraeer me-. ,

thod. From this, they concluded thah additional sediment-P pools were involved

in plant uptake. The explamation for this discrepancy probably lies in the fact

that they examined only the short-term mobil ity of sediment-P and did not show\

that this short-tenn P mobility is equatable with the long-term mobirity.

Only the latter is relevant to pla,QtS,.' growing over periods of 2-4 months in r

nature. Our results .show that thé specifie activity of the mobi,le P, fol1owed '" "'

/

over time, never reaches a constant level (Fig. 2). This observed eontinuous

dilution of the tracer indicates that there was a continuous P eyel ing between c'

the short-term mobile pool, and ,other less mobile and ,unlabelled P compartments

charatterized by slow turnover. rates.

An important conclusion from th'is is that attempts -1;0 determine the amount

or the instantaneous availability of a nutrient by chemieal means or by short-,

term isotope dilution techniques is meaningless when dealing with relatively

long-lived organisms such as macrophytes drawing nutrients from eomplex sys­

tems like soils or sediments.

In addition to establishinq a biological significance to the sediment

mobile P, the almost perfect correspondence observed in this study between

available sediment-P and mobile P demonstrates the feasibility of specifically "

labell ing the available sediment-P. Such labell ed sediments are suitable for

in situ P uptake studies in macrophytes where the shoot is in "free contact \

with the u,nlabel1ed wa~er-P. Applying this technique, we have ,shown (Carignan

and Kal ff, 1980) that alll species of submergent macrophytes te-sted in lake ,

memphremagog obtain all their P from the sediments and, none fram the overlying

water.

" • 7'11 m

1

o

Ct

REFERENCES

Bristow,' J.M., and M.Whticombe. 1911. The role of roots in the nutrition of

aquat; c vascul a r pl ants. Am. J. Bot. 58:.8-13.

17

Carignan, R., and J.Kalff. 1979. Quantification of the sediment ~hosphorus

available to aquatic macrophytes. J. Fish. Res. Board Cano 36: 1002-1005.

Carignan; R., and J.Kalff. 1980. Phosphorus sources to aquatic weeds: water or

sediments? Science, in press.

DeMarte. J.A •• and R. T.Hartman. 1974. Studies on absorbtion of 32p, 59Fe , and

45Ca by water-milfoil (Myriophyllum exalbescens Fernald). Ecology 55:

188-194.

Denny, P. 1972. Sites of nutrient absorbtion in aquatic macrophytes. J. Eco1.

60: 819-829. \

Johnson, D.L. 1971. Simultaneous determination of arsenate and phosphate in

natural waters. Environ. Sei. Tech. 5: 411-414.

Li, W.C., D.E.Armstrong, J.D.H.Williams, R.F.Harris, and J.K.Syers. 1972. Rate

and extent of inorganic phosphate ex:~?~i }n 1a,ke sediments. Soi1 Sci. Î~~

Am. Proe. 36:279-285.

Li, W:C., D.E.Armstrong, and R.F.Harris. 1974. Biolog'ical availability of sedi­

ment phosphorus to macrophytes. Water Chemi stry Program, Uni vers ity of

Wisconsin-Madison. Tech. Rep. Wis. WRC 74-09: 26 p.

McRoy, C.P., and R.J.Barsdate. 1970. Phosphate absorption in ee1grass. Limnol.

Oceanogr. 15: 6-13.

Peters, R.H. 1978. Concentration and kinetics of phosphorus fractions in water

from streams ~n)ering Lake Memphremagog. J. Fish. Res. Board Cano 35:

315-328.

1

1 j

J

\

L , .' k" .-r ~ \ ~ ,

i ~

f l

()

.)

" , " /

Ross, ?"and J.Kalff. 1975.~hytoplankton production in Lake Memphremagog,

Quebec (Canada)-Vermont(U.S.A.). Verh. rnt. Ver. limnol. '19: 760-799.

18

\

SOlllllers~ L.E., and D.W.Nelson. 1872. Determination of total pnosphorus in soils: f

a rapid perchloric acid digestion procedure. Soil Sei. Am. Proc. 36:

902-904.

Twilley, R.R., M.M.Brinson, and G.J.Davis. 1977. Phosphorus absorbtion, translo­

cation, and secretion in Nuphar luteum. limnol. Oceanogr., 22: 1022-1032. 1

Wang, C.H., O.L,.Wil1is, and W:O.Love,land. 1975. Radiotracer methodology in the

biological, environmental, and physical sciences. Prentice-Hal1,t Inc.,

Englewood Cliffs, N.J. 480 p.

J'

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'::-". '.c ••

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_. --- -- -- ----.----------·-~I--··'

.. 19

PART Il

PHOSPHORUS SOURCES TO AQUATIC MACROPHYTES: WATER OR SEDlMENT~ _. __ _____. ~/

\

" .

..

1 ) ,

\

ABSTRACT

Nine common species of aquatic maerophytes took aIl tbeir

phosphorus from the sediments when grown in situ in both a mesotrophic

and a mildIy eutrophie bay. E~n under hypertrophie conditions the

sediments eontributed"an average of 72 percent of aIl phosphorus

taken-up during growth. These expél'riments unambiguously demonstrate for •

\ ' the first timè that submergent macrophytes in nature overwheimingly

depend on the sediments for their °phosphorus supply and characterize

them as potentiai nutrient pumps ta thr open water •

\ , \

\

-( "

INTRODUCTION

\

Whether aquatic ,macrophytes (1) take the!r nutr!ents from the \

sediments or from the \ open wa ter has been a long discussed but

unresolved question. t quantification of the relative contribution of

water and sediments in 'mutrient uptake would identify macrophytes as

nutriénts pumps or sinks and would contribute to rationalize weed

control strategies in are as where excessive growth, is&a problem.

. Several studies have attempted ta establish the relative importance

of roots and shoots in the phosphorus (P) nutrition of macrophytes under

artificial conditions (2). These studies have qualitatively shown 'the

ability of macrophytes to take up and translocate some P via the root

system. However, they were not designed to resolve whether macrophytes

in nature obtain t~eir nutrients from the water, from the sediments, or

from both sources.

21

1

1

\

METHODS

Our approach to the question was to grow in situ various species

rooted in 32 p labeled sediments of known available P specifie activity

(3), with the shoot in free contact with the unlabeled open water P. If

during growth the plants obtain P exclusively from the water tHey should

-not show any 32p activity; c.onversely, if the sediment is the only

source of P, the plants should show a specifie ae tivity equal to the

specifie ae tivity of the available sediment-P. If P is assimilated both

·from the sediments and the water, the ratio of plant-P ta available

sediment-P specifie activities should provide a direct measurement of

the relative importance of each source. As the relative contribution of

water and sediments in P uptake is probably related to the relative P

\ . availability of sediments and water, the study was carried out in three

locations representing a bràad range of relative water-P and sediment-P

availabilities (Table 1).

At each site, the sediments were collected with a Petersen dredge

and pooled as large single batches (lOOkg~. They were then labeled with

32p as H332p04 (?), carefully homogenized and allowed to equilibrate for

four weeks, after which, growth experiments were initiated. In early

June of 1977 and 1978, small sprouting pl,ants were colleeted ~ weighed

and potted i~ elosed l.~ litre polyethylene jars filled with labeled

sediments (Fig. 1). The plants and control jars 'were then returned to

their original location and allowed to grow for 6 to 10 weeks •. Control

• jars were sampled at intervals for measurement of available sediment-P

specifie activity using an ion exchange method (5). The available

sediment-P specifie activity did IlOt remai'n constant with time but

decreilsed at a rate of 2-4% per week during the growth perlod.

22

1 1

1

\

? • f

'.,

-~- ~-,,~ -., ~~. , ....... '" _,,.._ ................ -,-~ .. ~~-"'~.. ~--~ -.~-_ .. ...--.- ... _ ... ---~~-----------~_ ...... _--._-

Fig. 1. Experimental,apparatus used in phosphorus uptake c

~?

measurements.

a

o

~ l 1

, i

l .

\

, \

\

OPEN WATER

SOFT NEOPRENE STOPPER ~ .--

I:---OPAQUE' COVER

JAR

--+_3~p LABELED SEDIMENT

SEDIMENT

u ,

\ \

'~,_-,. -•• ,-... - •. -, -, -. - __ ..... _-_ ... _,..,-".~ •• -,,~-;--~. '-::-;-.!-'''~:-Ài--·-,-~;=-~-r-. ---:, '.:-:-' ~-----~~" .... " t

1.

+' "

([)

\...

Upon harvesting (6), the plants were vigorously shaken with

filtered lake water to dislodge as much periphyton as possible.. The

shoots and roots were then dried and analysed separately for total P (7)

and total 32p activity (8). For each plant, the amount of P derived

from the sediments was calculated the following vay:

%S := A.lOO

p x SA

where:

%S .. Percent total plant-P coming from the sediments

A .. To tal amount of 32p ac ti vit Y found in the plant

P 1 := Total plant-P less estimated initial P present in the

young sprout (9).

SA = l'le an available sediment-P specifie ac tivity during the

grQwth period (10).

\

\

.f

\ .

"

\

RESULTS AND DISCUSSION " ,

Tables 2 and 3 show that maerophytes growing in the mesotrophie

part of Lake Hemphremagog (Q';lébec-Vermont) obtained aIl their P from the

sediments. The plants grown in the eutrophie part of ~he lake showed a

roean uptake of 91.6% from thë sediments. Thi~ last value undetestimates

the real sediment contribution sinee the plants growing at this particu-

l?r site harbored a thick coating of periphyton that could not be com- ~

pletely rerooved upon harvesting, thus producing an unlabeled P contamin-

ation. The results obtained frbm Rivière-du -Sud are of particular

interest sincè this site represents· a natura.,l upper extreme of water-P

availability. With a mean surmner soluble reaetive P concentration of

167 g/L in the water, the sediments still provided at least 71.5% of-

the P requirement of the macrophytes.

These results show that in both -oligotrophie arid mil:d-ll eutrophie"

lakes, characterized by relatively high interstitial> P concentrations in

their sediments, the sediments constitute the only significant source of

P ta rooted macrophytes. " .1

Only in rarely encoyntere~ hypereutrophic

waters is th,ere significant P uptake from the water. Therefore, the

relative contribution of water and sediments appears to be e a fune tian of

their relative'~.r availability.

The sediment-P uptake values observed for the three specles -inves-

tigated in Rivière-du-Sud are very similar. This is surprising in view

of the large m?rphologicé}l differences (such as leaf area to biomass

ratio) between the se species. 'This interspeclfic uniformity suggestli! • J

lIhat a simple predictive model of relative water~P and sediment-P . '

contribution ln P uptake for macrophytes could he readily construeted •

. \

,'"

~~----------------------------------~-----

25

\

1

1

f t t

" ! , } '~.

~

~ f j:

~~ 't~ '. ," ;r: t . j' .l, , ~ ,

~ l" v' :, « , l,

", t, , t, , ~

:' "

1 "f' t

1 "

i-l, ~

_ l

\

(~'

,--------j---_.-~.--,_._---------

ln conclusion, the overwh~lming importance ,of root uptake in nature

shoWs the importance of, sediments ln determining the extent! of rooted

macrophyte development. Long,term maerophjte control programs will thus

need to 'loeus primarlly 01. ~he sediment compartment. Submergent

macrop1!ytes are very active sedj.ment-P recyc1ers and should he viewed 'ss

potential P pumps. However, the II11Portance of these plants alf pumps to,

their epiphytic cover and to the overlying waters will remain unc!ear

until information on P excretion rates and on the fate of macrophyte-P \

upon senescence in nature becomes available (11).

\

26

'~ ,

\ i

." ,

(

--_._._-_. __ .~------------

j

TABLE 1. Water and sediment P characteristics for the three Southern

Quebec sites investigated. The intersti~l SRP was sampled Dy

dialysis.

Water

Site inean TP mean SRP

c~

( g.1-1) ( g.l-l)

Memphremagog 9.7 0.5 Central

Memphremagog 29.8 1.9 South

Rivière du 290 l6Z Sud

\

\

Mean TP ( g/g)

799

785

1199

Sediment

mean available-P

mean 1nterstitial SRP

( g.l-l)" ( g/g)

66.7

• 195

228

1200

1490

" . Il '1

27

,

o \

TABLE 4. Mean percent uptake from the sediments for six rnacrophyte \ '

species grown in Central Lak~) Memphremagog. a: Stand~rd

error, b: number of replïcates.

Speci-es % uptake from sediments

Myriophyl1um alterniflorum

Potamogeton zosteriformis

Potamogeton foliosus

Ca11itriche hermaphroditiça

~lodea canadensis

Najas flexilis

j fj

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..

104.4 ~ 1.0a (4)b

107.4 ~(2.3 (3 )

98. 6 ~ 2.1 (6) ,

94.2 ~ 2.4 (3)

99.0 ~ 2.5 (2)

100.8 (1)

\ \

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, 1

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().

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TABLE 3. Mean percent P'uptake from the sediments for three macrophyte

species grown un~er ~arious conditions di, wat~r a~d sediment P

availabilities. a: standard error;~: replicate,s.

,,Si te

Memphremagog centI\~l

~!emphremagog

South

Rivière du Sud

Shoot Root Whole Plant

\

MyriophyÜum spicatum

93.2 (1)

66.7 ! 1.3 85.4 :t 2.1 70.0 :t 0.5

.\

(4) (4) (4)

/

% uptake from sediments

Heteranthera dubia

95.2 + 3.2(8)

95.7 ! 0.1(2)

.. + 68.7 _ 1.3(6) 84.,8 :t 1>.7(6) 70.3 :t 1.2(6)

Va1lisneria americana

103.1 ! 5.6(9)

86.0 ! 2.0(5)

69.4 :t 2.8(5) 81.1 :t 2.7(5) 74.2 :t 2.7(5)

\

( .~

NOTES AND REFERENCES

1. The terro "macrophyte" refers here ta the larger aquatic plants, aS

distinct from the microscopie planktonic and benthic plants [C.D.'

Sculthorpe, The Biology of Aquatic Vascular Plants (Edwatd Arnold,

London. 1967].

2. J.M. Bristow and N. Whitcombe, Amer. J. Bot. 58, 8 (1971); J.A. ~.

DeMarte and R.T.·-Hartman, &ology 55, 188 (1974); C.P. McRoy and

R.J. Barsdate, Limnol. Oceanogr. 15, 6 (1970); R.R. Twilley, M.M.

Br,inson and G.J. Davis, Limnol. Oceanogr. 22, 1022 (1977).

3. The "specifie activity" of plant or sediment phosphorus refers here

to the 32p/31 p ratio ,expressed as 32p activity (CPH) per unit

weight of 3l p ( g).

4. The amount of 32p added was adjusted to give an initial specifie \

aetivity of approximate1y 1 Ci per l,mg of available sediment-P. 1

's. R. Carignan and J. Kalff, J. Fish. Res. Board Cano 36, 1002 (1979).

6.

7.

The avai1able sediment-P specifie aetivities measured in jars with

and without plant, was not significantly different. The specifie \

activity of the available sediment-P as mè~~ured by the ion

exchange method was found to be identical ta the specifie activity

of the interstitia1 soluble reactive P sampl~d by dialysis.

" AlI species were harvested at or shortly after flowering time, with

the exception of E. canadensis and M. spieatum which did not flower

at the study sites.

L.E. Sommers and D.W. Nelson, Sail Sei. Am. Proc. 36, 902 (1972). /'

.'

30

1 t 1

"'\ 31

8. The samples were counted in aquasol (New England Nuclear) or by

Cerenkov counting in water using a Beckman Liquid Scintillation'

system, and corrected for efficiency of Cerenkov counting by an

internaI standard method.

9. The initial P present in the young plants was estimated by

regressing total P on fresh weight for a dozen similar plants

collected in the same area; the resulting correlat,ion coefficients

were greater than 0.90.

10. The mean available sediment-P specific activity was calculated by

integration, within the grawth periad, of the regression best

fitting the observed specifie activity vs time values. The best

fitting regressions were of the farm: y = ln(t) + c, where t =

time and C, a constant. r 2 was always greater than 0.96. This

method assumes a constant P uptake rate with time.

Il. R. Carignan and J. Kalff, in préparation. Our results for

Myriophyllum show that although macrophyte excreted P Is readily

available to epiphytes and aigae, excretion rates are .verY' low.

12. Supported by grants from the Québec Department of Education, 'ne ----.. ' .. -...'-.

Inland Water Directorate of Environment Canada to the ,}kmphremagog

Project and a predoctoral scholarship from OGES, and the NSERC to

Richard Carignan. We thank Dr. D. Planas for providing SRP data

for Rivière-du-Sud.

1 (

J

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32

1

PART III

PHOSPHORUS RELEASE BV MACROPHYTES AND SIGNIFICANCE

TO PERIPHYTON AND PHYTOPLANKTON

\

/ /

(

(

ABSTRACT

- The contribution of nine species of macrophytes in the P

nutrition of their epiphytes, and the rates of P release by

Myriophyllum were measured in situ on fully labelled (32p)

plants. The epiphytes derived only 3.4 to 9% of their P from the

supporting 'macrophyte, indicating a relatively minor importance

for previously suggested macrophyte-epiphyte nutrient

Ir interactions. Although high diurnal P release rates (x = 3.24

fg.g-lhr-l) were observed for the Myriophyllum-periphyton complex,

Myriophyllum accounted for only 9.9% (O.32pg.g-lhr- l , of the

total P release~. This Myriophyllum-derived P was refrased in a \, ,

highly available (minimum es timate: 60.3 %) soluble form.,

\

/

\ '

~--------__________ ._r __ --,-' \

33

1

,1 l

1 1

l r , j

!

l

\

t

{ Introduction

(

Several studies have shown that actively growing macrophytes

can re1ease both inorganic and organic compounds to their

surrounding waters (Allen, 1971; Wetze1 & Manny, 1972; DeMarte &

Hartman, 1974; MeRoy & Goering, 1974; McRoy & Barsdate, 1970;

Hough & Wetzel, 1975; Nichols & Keeney, 1976; Harlin, 1973).

However, the significance of these cornpounds to the periphytic or o

planktonic communities surrounding the macrophytes, whether

inhibitory or stimulatory, has still largely to be demonstrated.

Phosphorus has genera11y been found to be the most limiting

nutrient in temperate lakes. Since in oligotrophic 1akes

macrophytes have access to a large sedimentary P reservoir from

WhlCh they derive the totality of th~ir P (Carignan & Kalff,

1979, 1980), P transfer from macrophytes to their associated

periphyton and p1ankton might be of importance in determinlng the

produètl'oTr~~ these cOlrununi ties. Unfortunate1y, stud ies focused

on this particu1ar aspect of macrophyte-periphyton-plankton

interactions are almost non-existent. McRoy ~nd Barsdate (1970),

in a study on P translocation by Zostera in compartmented

containers, reported chat 33% (equivalent to O.22~gp.g-lhr-J) of

the P taken up by the root-rhizome was transported and released

by the 1eaves during a 50 h incubation. By using a similar

approach, Twilley ~ al. (1977) estimated that the subrnersed

leaves of Nuphar rel~ased 86% of the P translocated to them from

below ground structures. DeMarte & Hartmàn (1974) 'showed that

~ _.- . ----_. - - ._---_ ...... - .. ~

34

, ~

i 1

, ! ~\

\

(

Myriophyllum cao release available P to the water but their data

could not be interpreted quantitatively.

--Short term radiotracer experiments designed to estimate

rates of P release by macrophytes or rates of P transfer from

macrophytes to epiphytes have the disadvantage of submitting the

plal)ts to highly unnatural c_onditions. Moreover, this approach)

will always yield uncertain results since in short term 32p ~

label11ng experiments, 32p release rates cannot be directly

converted to P release rates because the specifie activitr (32p

activity/31p) of the' macrophyte-p is not knqwn and i5 no~ \

homogenous wlthin the plant.

One way to circumvent these two problems is to producé';'lfully

labelled macrophytes by growing them from seedlings to maturity

on a P source of constant specifie activity, resulting in a

homogenous distribution of the tracer within the different

internaI macrophyte P pools. When grown in situ under these

conditions, the contrlbution of macrophytes to the P nutrition of

their associated epiphytes can be directly found as the ratio of

epiphyte-P specifie activity over macrophyte-P specifie

activity. P release rates by macrophytes are aiso easier to

measure on fully labelled macrophytes since it is easier ta

measure accurately small changes in 32p activity than s~all

changes in 31p concentration. In addition, 32p release rates can

be directly converted to 31p release rates when the plant-P

specifie activity is known. Although the full labe'ling technique

has previously been used in measuring P rel~ase rates by

35

1

,

(

\

zoop1ankton (Peters & Rigler, 1972), it has never been app1ied to

macrophytes.

In order to measure the sediment contribution in the P 1

nutrition of severa1 macrophyte species, we have approximated the

full labelling condi,tions exp1ained above by growing macrophytes

in situ, in 32p labelled sediments (Carignan & Kalff, 1980). In'

this paper, we report the use of fully labelled macropohytes in

quantifying the importance of macrophytes to the P nutrition of

their epiphytes and report P release rates by Myriophyllum

spieatum and availability of the released P.

Methods

\ The study was eondueted in Lake Memphremagog (45°06'N,

72°15'W)" in a proteeted bay (Quinn Bay) that can he classified as

mesotrophie with a mean aestival total, soluble and so~uble

reaetive P of 10.5, 2.2 and O.5pg. Liter-l, respectively.

Sprouting plants ( Sem) were 'transplanted in sediments

labelled to equilibrium with carrier-free 32p-P04 (Carignan &

KaIff, 1979) contained in jars descrlbed in Carignan & Kalff

(1980) and grown in situ for 6 to 12 weeks between June and

August 1977-78.

a) Macrophyte-epiphyte P transfer

The epiphytes were collected by cutting ,the macrophytes at

the base and very gent1y' 1,ifting them out of the water. Some

very loosely attached periphyton was lost at ,this stage, but the

,,'

36

\ )

i J

! 1 i ,

J j

1 1

'- ,

i ;

, , " ! l

!

J

(

\

amount appeared insignificant as compared to the amount present

on the plant. The plant and periphyton were then vigoroûsly

shaken 100 times in one liter of filtered lake water and the

resul ting suspens ion immed iately passed through a 100 pm screen

to'separate the epiphytes from the macrophyte and from other

larger organisms. Although this shaking procedure did not remove

the tightly attached epiphytes (Cattaneo & Kalff, 1979), it was

preferred to scrap{ng methods because of their inherent risk of

macrophyte. contamination.

The total' P content of the epiphytes was measured after

filtration of 5-50 ml aliquot of the epiphyte suspension on \

Sartorius (O.45fm pore size) membranes and digestion of the

fil ter and epiphytes with potassium pérsulphate in 50 ml of

distilled water (Menzel & Corwin, 1965). After digestion, the

solution was refiltered through the same filter, analyzed for P

(Murphy & Riley, 1958) and the results corrected for the low

fUter blank.

The 32p activity of the epiphytes was measured by filtering

5-25 ml aliquots of the suspension on 0.45 pm membranes and

counting to at least 2000 counts (net) on a Nuclear Chicaqo gas

flow geiger counter. The results were then corrected for

counting efficiency (41%). Filtration of increasing amounts of

epiph~tes showed that no self-absorption occured. Macrophyte \

total P and 32p activity were determined by ~ ~et oxidation in

HCl04-HN03 (Sommers et al, 1972) fo11owed by P analysis (Murphy &

Riley, 1958) and Cerenkov counting with a 37% efficiency. The

37

! l

1

1 1

r

)

~ \

l r f t

(

(

. ,

contribution of macrophytes in the P nutrition of their epiphytes

was then ealeûlated as the ratio of epiphyte-P over macrpphyte-P

specifie activities after having corrected for counting

efficiency and 32p decay.

b) P release and' ava ilabil i ty

P release rates were measured by' enclosing in situ entife

plants (fully labelled Myriophyllum) in cylindrical plexiglàss

chambers (length: 1.25 m; volume: 5.2 Liter). The chambers

consisted of two hàlf-cyli~ders joined by hinges, that could bé . ,

gently closed on the plant with minimal disturbance: a soft

neoprene lining glue? to one half-cylinde;. insured a perfect seal

upon clasure with no app~rent damage to the plant stem. The

chambers were maintained vertical and immobile by fastening them

ta a metal, rad driven into the sediments near the plants to be

stuùied. The plants 'were incubated in situ for 12 to 30 hrs. and

the water inside the'chamber sampled every 2-4 hrs.

The sampling was done ,by means of large syringes connected

te the chambers by 3-way stopcocks; three 140 ml samples were

successively pulled out from the upper, middle and lawer level of

the chambers te insure representative sampling. The external

water was allowed to flow inside the chamber during the sampling

through ten 18G needles inserted equidistantly along the neoprene \

1ining. This insured inunediate and homogenous mixing of the \

replacement water. The needles were. inserted prior ta, and ,>

removed after each sampling. The three samples thus obtained

were pooled and duplicate 50'ml aliquots used for measurements of

38

, , ,

i , /

\

c

f i . l > ,

t ! , .

1 o t

(

(

TP (Menzel & Corwin, 1965 ) and SP (measured ~as TP after'

filtration through pre-washed O.45pm membranes).

Total 32p (T32 p) and sQluble 32p (S32p ) were measured by

Cerenkov counting after concentration and wet oxidation of unfil­

tered and filtered samples. Th'is was done byevaporating (90°C)'

on a hot plate 100 ml samples in 125 ml erlenmeyer flasks in pre-\

sence of l' ml of "HCl04. After çomplete evaporation of the, water,

the HCI04 was ,refluxed for 10 m~n. and allowed to cool. Thirty

ml of dlstilled water was then added and the solution gently boiled

for 10 min •. The solution was reduced to approximately 10 ml and was

then quantitatively transferred to a LSC vial with a fina~ volume of \ "

18 ± .2 ml. Recovery experiments performed by adding a known amount

of 32p-P04 to lake water samples conslstently gave recoveiies better

than 98% with no significant change in counting efficiency.

Prior to each sampling, the outside water was also sampled for

TP, SP and, occasionally, for CerenKov counting blanks. Particulate

3lp and 32p were obtained by difference. Finally, aIl the concentra­

tions and release rates of 3l p and 32p were corrected for the simu~-/'

tane9us dilution of,chamber water by outside water during sampling

and for the progressive dilution effect of successive samplings.

ReSULTS AND DISCUSSION

a) Transfer of macrophyte-P to epiphytes

The epiphytes of nine species of fully labelled ma~rophytes

were collected between Ju1y and September 1977. Table 1 shows

39 ,r

,

1 1 1

l i 1

1 1 ,

l' 1

(

..

the mean,P content of the macrophytes investigated, the mean

amount of eplphyte-P present on them and the mean %

macrophyte-derived P found in the epiphyte-P. The tesu1ts show

that between 3.4 and 8.9% of the P present in the 1oose1y held

epiphytes was contributed by the macrophytes. Between species or

within species, no significant correlation could be detected

between the P content of macrophytes and the % ~piphyte-p derived

-from macrophytes.

With a mean TP as 10was 10.5 ,.g. Liter- l in the water co1umn,

the phytoplankton (Carignan, unpublished data)' and, presumably, the

epiphytic algae of the Lake Memphremagog alfe P 1 imited. Cattaneo &

Kalff (1980) have found a linear relationship between epiphytic

biomass or primary productiçn and TP in the water column a10ng the

troghic gradient of Lake Memp~remagog, suggesting that epiphytes

respond to increased P av~ilability by a proportional increase in r

biomass qnd primary production. If we assume that the

. macrophyte-deriv~d P found \n epiphytes (Table 1) constitutes an

extra source of P to which epiphytes respond by proportionately

increasing thèir biomass and/or primary productioQ, the epiphytes

should then, be, at best, only slightly advantaged as compared to

algae occurr ing on non-1 iving substrates. This suggestion is

supported by Cattaneo & Kalff (1979) who found that epiphytes

growing on living macrophytes cou1d not be distinguished from those

growing on plastic plants 'in terms of biomass and primary 0

produc tion.

The minor importance of P transfer from macrophytes to

epiphytes found' in this study does not support the hypothesized \

1 . \

1 1

, , i

(

TABLE: 1: Total P content of macrophytes, assoéiated epiphyte-P and % epiphyte-P derived from macrophyte for nine macrophyte species collected in Central Memphremagog. AlI values expressed as mean ± standard error.

Species

Myriophyllum spicatum

Date Collected

77-08-20

Myrioehyllum 77-09-13 spicatum

Myriophyllum 77-08-25 al terniflorum

Potamogeton* . 77-07-28 zosteriformis

Potamogeton 77-07-19 fol iosus

Heteranthera 77-08-16 dllbia

Vallisneria americana

Bidens Beckii

Elodea canadensis

Potamogeton P i'char!3son i i

77-08-22

77-08-28

77-08-28

77-06-28

'senescent on sampling

P conte'n~ . of

macroPh1te (pg~g- )

2500 :t 130

2210 ± 150

3790 ± 290

3160 ± 170

2570 :t 180

2650' 'j: 300

2730 :t 60

3230 ± 240

2870 :t 250

5020

Epiphjtic-P on

macroph1te (}'g.g­

macrophyte)

303 ± 32

252 :t 43

222 :t B

291 ± 28

14

± 61

124 :t 22

142 :t 21

164

% epiphytic-P derived frDIO macrophyte

6.82 ± 0.81

6.34 :t 0.36

4.28 ± 0.50

8.54 :t 1.31

B.BS :t 1.43

6.03:t1.77

4.73 ± 0.96

3.40 ± 0.95

8.99 ± 1. 30

7.3

n

9

2

4

4

6

4

4

4

6

1

~ 1 r

! 1

1 , 1 ~ -1 X· '/

1 1 1

1

1 l' 1

t _ 'symbiosis (Wetzel, 1975) between macrophytes and eplphytes for

nutrlent exchange.

1 ~

, \ \ f ,

f c: t

_l

1

\

b) P release rates in Myriophyllurn

Ten P release exper iments were performed on fully labelled

Myriophyllum between August 15 and Septernber 10, 197B. The

resul ts of two typical experiments are presenteQ in Figures 1 &

2 and will be used to illustrate severai aspects of P release

Myriophyllumn - periphyton complexe

Periodicity of P release

AlI three forms of P measured in the chambers (total,

particulate and soluble 3lp and 32p~ exhibited marked changes

with time. The apparent release of T31p or T32p, s3Ip or S32p

was maximum during daytime and minimum to negati ve duril1g

night. Although rates of TP release in the chambers were

usually high during day, net changes were small (,(1 p9. Li ter- l )

over a full 24 hr cycle (Fig. land 2). These observations

indicate that measurements of P re1ease rates by Myriophyllum

\ periphyton complexes are highly dependent on time of day.

Rates and sources of teleased P

The use of fully labelled macrophytes of known P specifie

activi:y allowed the distinction of the P released by,the

macrophyte + periphyton (Fig. la li 2a) from the 32p labelled

macrophyte-derived P (Fig. lb li 2b). For each plànt, the

diurnal macrophyte + periphy~on TP release rate was calcula,ted

42

, 1 • 1

1 1

i

1 1

l ! l l

(

\ .

( )

\

/

Fig. 1. Phosphorus release by a Hyriophyllum-periphyton community during

a 24h cycle. A: accumulation of 31p forms released by the macro-

phyte+pèr-i-phyton; symbols: A-total P,. e-particulate P, .-soluble P.

,Érror bars, when larger than symbols, indicate the range of dupli-

cate values. B: accumulation of macrophyte derived P forms (right

axis) as calculated from 32p evolution in chambers (left axis) and

specifie actlvity of ~~_macrôPhyte P; symbols: A-total 32p •• _

----partlc~li]ite 32P--:--;:solubl'e 32p. Error bars represent • one standard

deviat ion of the count.

\, ,;,

· l , . ~ ~ 1

1 i 1

~--\.

l '

J 1 , 1 , i 1

, . ,

\~.

\

C\J .,..

eX) . '

, r ~/ "- ' ::::sI

~~ ..........

/~ 0 0

'ë Q)

E @i=\

N .,..

.... -;--,-_. ""=""-----------------.,-------~.-""..", .. _.-.-.------

1

1

/ / , ,

,1 1

1 /

/ /

/"

1.

1 , )

, ,

( 1

, r

1 1 1

, ( 1 1 1

, . .; l j

{ Il . \ . i .. ( ) t

Ij

L __ _

~

CO "'-"

co d

co o

\

-------- --- _._-- ---- _ .. _~------""'_-.......-. __ .-, ,

~ '0

C\I d

o d

-"-~::J

0 ..c "-"

~ ('Ij

oCl '+-0 CD E 0--

C\l1-

CD ,....

C\J ,...

(

1 '!

1 '1

f .i 1 • 1 l

1

l j

"

) î

f 1 1 1

_r / '~

\

1

J

Fig. 2. Phosphorus release by a Myriophyllum-periphyton community.

Carrier P (l,OOOll.1Iter-l) was added at 13:45. legend as in

Fi g. 1.

\

-..

~)

1 1 i

r 1

1 1.

~ "

1

1

l, \

"-(])

"­"-CO

Ü --------------------

o C\I

CD ,...

C\I ..-. ,.... '-:J 0 .c.

'-'"

~ COQ

15 Cl> E .-..q-r-

o

0 C\I

0

-J

, " , , ,

l "

,{: ~

~

-) , .. , Î • ~ t d ~

i i

1

! . 1

Il . l

l 1 • , 1 l

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, 1

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: 1 , 1 . , , t \

i ()

"-,0> "­"-CO

Ü -------_ ...... _-----

,,. \

o d

o N

CD ,....

..q-

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'+-0 ID E .-t--

f '

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1 1

1 \

l i 1

~ j

1 ,1 . ~ l $

1

( from the observed rate of change in TP concen tration in

chamber. For the same time interval, the macrophyte-derj. ved P

release was ealculated from the observed mean rate of change in

T32 P acti v i ty in the water and from the macrophyte-P spec ific

activity. For example, if during a P release experiment, 100

CPM of T32p appeared in the chamber and the macrophyte-P

specifie acqvity was later found to be 200 CPt-f'pg-lp at the

time of the experiment, the macrophyte had released 0.5 jJgP.

The resu l ts (Table 2) show that on average, the macrophyte­

derived P (x = 0.32 fg.g-l.hr-l, excluding one high value)

accounted for on ly 9.9% of the total P released by the

macrophytes + periphyton Cx = 3.24 j'9.g-l.hr-l). Sinee

macrophytes grow ~ng in Lake Memphremagog have been shown to' take

their P exclusively from the sediments, the macrophyte-derived P

re!eased constitutes a net P loading ta the littoral zone. The'

remaining 2.92 JI gP g_lhr- l released i5 due to periphyton alone

and comes from the surrounding water since it is not labelled.

The P release due to periphyton can thus be characterized as a P

recycling effect of this corrununity instead of a net P loading.

The s imilar i ty observed in Figure 1 and 2 between the

accumulation of macrophyte + periphyton derived SP and,

macrophyte derived SP suqgest that both fractions have the same

periphytic origin. This i5 not surprising since most of t!1~

observed changes in P concentration are due to the periphyton

and that 6-7% of the epiphyte-P found on lo1yriophyllum cons~sted \

of labelled P of macrophytic orig in (Table 1). Therefore, very 1

little P would be transferred directly from the ,macrophytes Jo

l,

J 47 ,

, ,

\ 1

i

l' t

: ! 1

, , \

1 < ,

1 (} 1

48

, 1 1

1

TABLE II: P release rate by macrophyte + per.-iphyton, macrophyte-derived P release rate and % macrophyte-derived P particu1ate after 5 h for ~yriophyllum spicatum. 1

P 1a/lt Dry weight P content Diurnal Diurnal % macrophyte 4 enclosed in (pg. g-1 L P re1ease P release derived P

chamber rate by rate by particulate (g) macrophyte+ macrophyte after 5 h.

periPhiton alone (~g.g- .h-1 ) (pg.g-l.h-l)

~

l .832 2230 2.30 0.46 35

2 .623 2880 3.78 0.l9a 82

3 1.0I2 2030 7.90 0.25 77

4 .534 3740 2.57 0.22a 75

5 .766 2530 3.26 0.76a 54

6 1. 64 2650 1.94 0.06 54

7 8.76 1740 2.22 0.26 62 ,\

8 1.662 2340 2.25 0.32a' 43

9 .552 4940 7.85 1.18 46

10 1.822 2700 2.93 0.33 61

x:t:si 2538:U91 3.24:l:.61 O.32:t.07b 60. 3:l:5. 3

~ \

a: l1leasured under carrier flooding conditions

b: excluding plant .9

\

- 4_~--____ """"""'-_ ... ~-r--.·-",----_______ ~I""-o:a"lo..\\r", __ '_.""" __ " ____ --_ ... ~ _______ -l.-____ ---._~ ___ _

1

r l l'

1 1 J 1

J j 1

l 1

J

J ~ { } 1

" l i ;

l, 1

,1

( the surround ing waters. Rather, the release of

macrophyte-derived P would occur indirectly, as a 'result- of \

periphyton metabolism, itself slightly labelled because of

~------- previous assimilation of macrophyte-P. !

Availability of released P . '

The observation of ra tes of release of macrolPhyte de'ri ved

TP, pp or SP gives no information on the availability or formes)

of P actually released by the plant or by i ts per iphyton. For

instance, the appearance of rnacrophyte derived pp in the

chambers could be due to the release of sorne read ily available

SP irnme_diately taken up by the surrounàing phy_toplankton.

Al ternatively, this pp could be directly releaseà in the water

as unavailable PP.

These two possibilities were testeà by uSing a carrier

Ilooding technique which consisted in sudàenly increasing the

pool of available P in the chambers to a disproportionate size

(1000 }Ag. Li ter-l ) • Prev ious tests haà shown that when the

P04-P concentration is increased to 1000 !Ag. Liter-1 in chambers

containing l>fyriophyllum of the sarne size as 'those used in the P

release experilnent, at least 90% of the aàded P still remained

in 'solution after 5 h. Although su ch a treatment probably

increased the rate of P uptake by the phytoplankton, periphyton

and macrophyte present in the chamber wh en expressed per unit

biomass, it practically stopped P uptake when expressed per unit

quantity of P04-P present. Under carrier flooding conditions,

j

49

l i

~ l 1 ,

1 . 1 1

\

( the hypothetically released available S32p molecules should then

accumulate in the chambers as S32p instead of being taken IIp by

the phy toplank ton.

Typical resul ts for a carr ier flooding experiment ar~

presented in Flg. 2. It can be seen that al thOllgh rates of T32p

accumulat~,on W'ere not significantly affected by the added P04-P,

p32p accumulation stopped completely, with 32p now being

released as s32p only instead of Snp + p32p. This shows that

in absence of carrier P, macrophyte derlved P is not released in

a particulate form but rather in a highly available soluble forro

rapldly taken up by the surround ing phytoplankton. A minimum

estimate of the short term availability of the released S32p can

then b'e obtained by looking at the fraction of T32p found as

p32p at the end of the incubation periode Because the P release

experiments varied ln duration and treatment, we arbitrarily

chose to calculate this p32p:T32p ratio at 5 h after the

beg inning of the re lease experiments. Table 2" shows tha t on the

average, 60.3% of the released T32p is present as p32 p after 5

h Consequently, at least 60.3% of the °macrophyte derived P

is readily ava i,rable to othe surrounding phytoplank ton wi thin a 5

h experiment.

When fully available carrier-free 32P04 was added to

\ control chambers containing unlabelled M,YriophYllum (Fig. 3), a \

relatively slow uptake was observed. Af ter nearly. 5 h , 33% of

the total -32p present in the water was still a soluble forme

Such a slow uptake of fully available 32P04 is presumpably due

50 1 1

1

\ ,

\ ~

'1 j

,!

1

(

" to the slight carrier flood ing effect of SP release by the

periphyton and strengthens the conclusion that our -estim_ate for

the aVai labili ty of macrophytic derived P (60.3%) i5 a minimum

estimate since even, the fully available 32p04 is only partially

taken up after 5 h under similar conditions.

Reassimilation of released P

The macrophyte-derived P release rates presented' in Table 2

do not take into account the possibili ty of reabsorption of

released' macrophyte-derived P by the macrophyte itself or by i ts

per iphyton. Confining macroph'}'tes in small 'volumes of water for

long periods of time can le ad to underestimation of true re1~ase

rates since the released P is at'tificially maintained in close

contact with the macrophyte and periphyton which could take up a

significant portion of it. However, our data suggest that

reabsorption of released P \lias not very impor}.ant. This can be

seen in Fig. 1-2 and Table II where carrier flooding did not

significantly increase the rate of T32p release. However, the

degree of resolut~on of the carrier flood ing experiments \lias not

very high because of 10'11 32p activities in to: water. On the o

other hand, Fig. 3 show,s that when 32P04 'lias add~d to a control

chamber, 35% of the total 32p addéd was lost ~rom the water ,

after 5 hrs ~nd presumably ta ken up by the rnacrophyte-periphyton

community. However, the type of experiment'where a single spike D

of 32p04 was added at som~ initial time differs from the

macrophyte-P release experiments where 32p was continually

ft

51 1

1 l :/

'r

'r

1 1

\

Fig. 3. Remaining total 32p (A), particulate 32p (e) and solu91e 32p (0)

with time, after addition of carrier-free 32P04 in a chamber

containing an unlabelled Hyriophyllum-periphyton cOl1lTlun.ity.

r 1

()

o .. \.J

, 1

, 1 f

1 ! l,

f-

(

• o

\

\

- '-.

) (

\

o

( :

, L{)

q-

......... r<) i....

:J 0

..c: -f' w

~

Nt-

• " . '

'-0 o

, 1 ~ 1

1-J

1 f

1

\

. ,

'0

Ci

l ' ' ~_ " ______ ....... ~ .. __ ~ _____ ~ _______ ..: __ .... --<,~~_ .. ___ ........... _._...,.....,_ .. «" ........... __ I~ ........ _-.,- __ -....,.... ___ ~~"--_" __ ' ___ ""'-'___""~_"''''_''_''' .. J ~~-<"L_......,...!........-.r~ ~ MA _~ ..... t -. - -

released in the chambers. After an incubation time of 5 h

on1y the macrophyte-derived P released at the very begi~ning of

the experiment 15 subject to the 35% reabsorption shown in

Fig. 4. The mean reabsorption thus lies between 0 and ~5%.

Triplicate 32P04 uptake experiments performed on control plants \

of simi1ar size (1.12 ± 0.15 g) than the labelled plants used in

the release experiments showed a mean 1055 of tracer in the \

water of 36.1 ± 3.0% after 4.6 h~s. Assuming that the P

movements ocourring in the control chambers were similar to -

those occurring in the experimental chambers and assüming that

macrophyte-P 15 released as P04, we can say that the fraction of ,

released P reabsorbed during a 5 h incubation lies somewhere

between 0% and 36.1% with an intermediate value of 18% that cao Q

be considered as a reasonable approximation for mean

reabsorption.

When corrected for reabsorption, the mean

macrophyte~erived P release rate of 0.32 pg.g-l.h-1 found for

Myriophyllum is increased to 0.39 pg.g-l.~-l. Whether such a

correction is ~elevant in measuring the rate of macrophyte-p

release depends on the question asked. The corrected value

represents the rate of transfer of macrophyte de~ived P to its

periphyton and surrounding water whereas the uncorrected value

represents the rate Of transfer of macrophyte,derived P to the

surrounding water alone, provided that the experimental plant

biomass:water volume ratio is similar ta the naturally ocè~ring /

ratio. '

\ ,

53

\

': " ) ,~

1 l

; <

1

1 1 t t

(

Importance of macrophytes in the P dynamics of the littoral zone

Although macrophytes are often an important compon~t of the

littoral zone of lake~, very litt1e is known on their role in p

cycling. In an estuarine system MeRoy et al. (1972) have

estimated that Zostera could transfer 62.4 mg p.m- 2.day-1 fr~m

the\sediments to the water. How,ever, their specifie re1ease rate

(2.86 ,. gP. g-l.h-l ) for Zostera appears doubtfulfor reasons

mentioned in the introduction and because the 2.86 JgP. 9~1.h-l

value was calculated from an observed release rate O. 22fg .g-l. h- l

which was measured in vitro under a P concentration of 30 g.

Liter-1 supplied to roots and 1eaves. This observed release rate

was then extrap01ated to natural sediment pore water P

concentrations by·assuming, witpout any experimental evidence,

that P release rates by Zostera leaves were directly proportional ~

to the P uptake rates by roots and rhizomes. ,

Our study differs from the one reported by HcRoy et al.

(1972) essentia11y by the fact~that we measured P release rates

on fu11y labelled plants and that P release rates were measured

in situ on plants grown in natural sediments and with minimal

disturbance.

If we accept the value of 0.32-0.39 fg.g-l.h-l found ln

this study for a specifie P release rate by Myriophyllum anj

that this release occurs mainly during the day, the f)I>3t dense

Myriophy1lum stands (88 :t 11 g.m- 2) occurrinlJ in Quinn Bay would

release about 0.37 mg P.m- 2.day-1. For a mean depth of occur­

rence of 1.6 m and a mean TP concentration of 10.5 p9 P. Liter-l,

\ i

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1 • 1

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( sueh a release would resuit in a 2.2% daiIy inerease in total P

concentraûon within the stands. Although small, a good ~art of

this P 15 readily available and might be of signifieance to the

epiphytes and phytoplanktqh oecurring in close proximity of the

maerophyte. In addition, ~h~. P released by maerophytes repre- ~

sents a net input to the Iittora~ zone sinee it is ultimately

derived from the sediments.

Perhaps of more impor tance to the li t torai zone is tlle P

reeyeling impact of the perlphyton present on macrophytes.

Figures l & 2 show that 90% of ,the observed diurnal release of P

is due to periphyton alone. In this context, macrophytes would

be more important as physical supports of an active microbial

community that plays an important role in p cycling.

We consider our esttmate of P.release rate by Myriophyllum

to be valid for Myriophyllum growing in Quinn Bay only. Release

rates might be much different for other speeies or might be

dependent on factors sueh as TP content or physiologieal status

of the plants. When this information becomes available, esti-

mating the importance of maerophytes as P load ing agents ta the \

water eolumn during their active growth will reduce to specifie

questions of macrophyte abundance, relative size,of littoral and

pelagie waters and degree of exchange between them.

----,.- ~

55

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1

Rt:FERENCES

1971. \ Prililary productivity, chemo-organotrophy anq

nutritional interactions of eplphytic algae on macrophytes

in the htteral of t't lake. Ecolo Honogr. 41: ~7-127.

CarIgnan, R., and J. KalEE. 197Y. QuantificatIons of the

sedIment phosphorus avallable ta aquatic macrophytes. J.

Fisl1. Res. Board Cano 36: 1002-1005.

Carignan, R., and J. Kalff. 1980. Phosphorus sources to

aqu<ltic weeds: watec or sedim~nts? Science 207=987-989.

Cattaneo, A., and J. Kalff. 1979. Primary production of alQae

growin',l on natural and i'irtificial aquatlc plants: 1\ Syldy

vf interactions between epiphytes, and their substr~t-e..

Limn01. Oceano~r. 24:103r~1037.

Ci:lttaneo, A., and J. Kalf(. 1980. The relative contribution of

aquatic Jll"crophytes and thelr elnphytes to the pro~uction

o( macrophytr; beds. Llmnol. Oceanogr., in press.

DeMarte, ,1.1\., anLl H.T. Hartman. 1974 • .5tudies on absorptIon *'

of 32p

, 59 Fe and 45C8 by water-milfOil (Myriophyllum

exalbescens Fcrnilld). Ccology 55: 183-194.

flarlin, H.N. 1973. Transfer of prodùcts betwecn epiphytic

marine algae and hast plants. J., Phycol. 9: 243-248.

Hough, R.A., an(l R.G. \~etzel. 1975. The release of dissolved

organic carbon fram sUbmersed aquatic macrophytes: Diel, 1 -

seasonal, and community relationshps. Verh. lnt. Ver. \

Limnol. 19: 939-948.

1 ________ " __

-~. --- -

56

i , ) l

1 1

1

l

r i l

\

1 1 ,

,1 •

-f 1 '

1

•. 1

c

MeRoy, C.P., and R.J. Barsùate. 1970. Phosphate absorption ln

eelgrass. Linnol. Oceanogr. 15: 6-13.

r'1cRoy, C.P. ~ R •• }. 8arsdate and N. Nebert. 1972. Phosphorus

cycling ln an \q,elgra;s (Zostera marina L.). Llmnol.

Oceano~c. 17: J5~67. \. ' r ---- __ ;, __ -2

McRoy, c.P., and J.J. G,~ednJ •. 1974. Nutrient transfer between

seagrdss Zostera marlna and ,its epiphytes. '- /

,\

173-174. \

Nature 249:

:v\enzele, D.W., anù N. corwi~. 1965. 'rhe measurement of total P \ in seaW'iiter based on the liberation, of 'organically bound

fractions by persulphate '~xidation.

280-282. \\

Lil'lnol. OceanogCo 10:

Murphr, J., and J.P. Riley. 1962. A modified single solution

method for t~e determination of phosphate in natura1

waters. Analyt. Chim. Acta 27: 31-36.

Nicno1s, O.S., and D.R. Keeney. 1976. Nitrogen nutrition of

Myriophy11um sRicatum: Upta~e and translocation of l5~ by

shoots ·and roots. F'reshwater Biol. 145-154.

Peters, R.H., and F.H. Rigler. 1973. Phosphorus release by

Daphnia. Li.nnol. Oceanogr. 18: 821-839.

Somners, L.E., and D.W. Nelson. 1972~ Determination of total

phosphor\Js in soi 1 s: a rapiù perchlor ic acid digestion

procedure. Soi! Sei. Am. Prec i' 36: 902-904.

Twilley, R.R., M.~. Bcinson, and G.J. Davis. 1977. Phosphorus

absorption, translocation, and secretion in Nuphar 1uteum:

Limnol. Oceanogr. 22: 1022-1032.

57

/

1 , \ ,

\ ,\

1 l <Î

l -!

c): '1 , i

'1 1 1 1

i

1 1 J l '\ , '1 1 • 1 l i , 1

, \

/

i 58 ,

( V

Wet.lel, R.G. 1975. Limnology. H.B. Saunders Co.,

Philadelphia, 743 p.

ioJetzèl, .R.G., and B.A. i1anny. 1972. Secretion of dissolvecJ

organlc carbon and ni trogen by aguatic macrophytes. Verh.

Int. Ver. LlInnol. 18: 162-170.

1 1

(

PART IV

~J

POST DEPOSITIONAL MOBILITY OF PHOSPHORUS IN LAKE SEDIMENTS

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Abstract

Fiffeen sediment cores obtained from Lake Memphremagog showed

pronounced'concentration peaks of total phosphorus within the Lead-2l0

defined mixed layer. This suggested that P was migrating upward and

accumulating near th~ mud surface. The mobility of P in the se sediments

was confirmed by the rapid development (5 weeks) of a marked total P

maximum in the upper centimeter of a previously homogenized sediment.

Associated with this P migration was a steep concentration gradient of

soluble reactive P (SRP), presumably the result ~f a redox gradient in

the sediments. Similar total P and SRP gradients were observed in non­

disturbed sedimen~s, strongly suggesting that the same upward migration

mechanism also operates under natural conditions. This remobilized P

appears to bè isotopically exchangeable (mobile) under anaerobic condi­

tions. These observations indicate that profiles of sediment total P

may not always be useful in the determination:'of historical P loading

lor P sedimentation rates in lakes.

, ------~---------_._-- $

60

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Introduction

Lake sediments frequent1y exhibit marked increases i~ total \

phosphorus (TP) towards the sediment-water interface (Bengtsson 1975; \

B10esch 1976; Bort1eson and Lee 1972, 1974; Jorgensen et al. 1975;

> Kamp-Nielsen 1974; Kemp et al. 1974,1976; Shapiro et al. 1971;

Williams and Mayer 1972;-Wil1iams et al. 1976). Since most of the

1akes exhibiting this increase were more or 1ess subject to cultural

eutrophication, any authors attributed the, TP increase, often observed

in the top 15 cm of sediment cores, to recent increases in P loading and

sedimentation rates. • Williams and Mayer (1972), Kemp et al. (1974, 1976), Williams

et al. (1976), and Bloesch (1976) suggested the possibi1ity of an upward

migration of disso1ved P from the reduced zone, fo110wed by precipitation

in the oxidized zone, thereby modifying the original pattern of P sedi­

mentation. However, these authors did not provide any c~nc1usive

evidence that such a mechanism was occurring in the sediments they

studied. In this study, we demonstrate such a pronounced P enrichJœnt

in the surficial sediments of at 1east one lake .

•

Methods

Samp1ing. Short cores (30 cm) were obtained with a K-B

gravit y carer from Lake Memphremagog on the Québec-Vermont border

(lat. 45°06 I N, long. 72°17'Wi Peters 1979; Ross and Kalff 1975).

Within a few haurs of collection, the cores were extruded and sectioned

in 1 cm slices that were homogenized and subsampled for ana1yses. "

Interstitial water profiles were obtàined from the vicinity of the core

)

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sites by inserting a compartmented dialysis sampler (Hessleill 1976)

into the sediments. The samplers fitted, with a bio1ogica11y inert

,PVC membrane (Ge1man DM-450; 0.4.5\1 pore size), were left in the

sediments for 10 to 15 days before sampling.

Ana1ytical Methods. lead-210 activity was measured fo110wing

Koide et al. (1972). Sediment TP was measured by either an ignition

method (Andersen 1976) or by perch10rit digestion (Sommers and Nelson

1972) fol1owed by reactive P determination on the extracts us;ng an

ascorbic acid reduction procedure (Johnson 1971). Both extractions

yie1ded essentially identical resu1ts for Lake Memphremagog sediments.

Interstitia1 soluble reactive P (SRP) was measured after Johnson (1971);

interstitial and total Fe by atomic absorption of the interstitia1 water

or the perehloric digestate.

Sediment mobile P (exchangeable P) was determined by isotopie

dilution under anaerobic conditions according to the following

procedure: approximately 20 9 of fresh sediments were placed in 100 ml

glass containers; dialysis samplers were improvised by cutting two'large ~

windows (6 em2 ) out of 20 ml polypropylene scintillation vials and

covering thetJwindows with Gelman DM-450 membrane glued with "Pliobondll

, ,

(Goodyear Co.). The dia1ysis vials were filled with deaerated disti1led

water and p1aced inside the glass containers which were then a1so filled

with deaerated disti11ed water and a known amount of carrier-free 32P-P04

was injected at the bottom. The glass containers were then sealed with \

one layer of ,IIParafilm" 1 one layer of aluminum foi1 and a polyethylene

/ .-- ----~- -~ ~-::-_---""'----_~_'.rr_---:---:::-::-::'

62 { 1

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1. SRP <e) and its specifie activ~ty (0) versus tlme ln the

anaerobic containers used for the measurement 'of mobile P

by Isotopie di Jutlon.

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SPECIFie ACTIVITY (cpm·~g·103). .....

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snap-cap, thereby preyentiio,g any atmospheric 02 di ffus~on' into the

containers.' PreVious tes~s had shawn that under these conditions, the - . l' specifie .activity of the d1issolved P ,inside the dialysis vials reached

a nea~ly constant value wAhin 30 days (Fig. 1). The vials were thus

, incubated for this period lin darkriess at 2~oC and manlJally shaken every ,

two days. At the end of t e incubation period. the .dialysate was sampled,

and assayed for total.l an for 32p activity by Cerenkov counting. The

conten~s 0) the glass coJn ainers were then aried at 65°C and the sediments

weighed. lhe-amount of bile P was calculated from the amount of 32p "

- 1

initiallyadded and the sp dfic activity of the dissolved -P using standard

isotopie exchange equation (Wang et al. 1975).'

Results and discussion

Lead-2l0 and total~P of; es

During a study'of edimentation rates in Lake Memphremagog (Flett

and Marshall in prep.), 21 cores were collected at depths ranging fr?m

2 to 100 'm and analyzed fot 2l0Pb; stable Pb, 137Cs and .TP. Sixteen cores

,~hQwed 210Pb profiles withlsurfici.l zones (2-6 co) of'constant 2l'Pb

a~tivity fol1owed by expon ntial decreases with depth in the sediments

, ta a supported base level tfi9. "2). The remaini.ng cores showed either

~ very' low 210Pb activity 9r lack.ed a surficial zone of,constant activity

and came from either sandy~ compacted clay or from sediments overlain . \ ' . ,

by Mn nodul es.

The lack of eVi~enœ ;o~ Pb mobility in sediments ha. led mo.\

authors ta interpret tAe surficial zone of constant 21DPb act1v1ty as

,

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Fig. 2. lead-210 (e) and total phosphorus (0) profi les in Central

Bas in. Lake Memphremagog; error ,bars="I.,.. !

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CUMULATIVE WEIGHT (g ·cm2)

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the result of physical mixing or bioturbation (Robbins et al~ 1977).

According -to this interpretation, any other non-mobile compound or , ù

element should a1so show an homogenous distribution within this mixed ,

layer. However, as shawn for one core in figure 2, 15 out of the 16

cores with a 210Pb defined mi xed layer also had steep TP gradients within

this mixed layer. A possible exp1anation for the exi stence of a 'TP

gradient within the mixed layer could be a progressive minera1izati'on

and diffusion of recently sed5mented P as it is worked down in the mixed

layer. However, this process would lead ta a TP profile having its

maximum at the sediment-water interface and not at 2-5 cm as often ~

observed in Lake Memphremagog (Fig. 2,6,7). The existence of steep TP

gradients within the mixed layer, with well pronounced maxima below the

interface rather t1han at the surface, suggests that extensive post­

depositional migration and accumulation of P occurs in the sediments to

a depth at least equal to the extent of the miXed layer.

Ta verify the possibi 1 ity of rapid vertiêal P migration within

the first centimeteres of sediments, a laboratory experiment was done

on homogenized sediment. Twenty kg of sediments (si lt-clay texture,

8.5% 10ss on ignition) were collected from the South Basin of the lake

wi th a ;~tersen d;edge. hornogeni zed, pl aced i n, a 50 l iters -aqu~r; um and l' ft ~ ,

covered with 20 liters of'lake water lTP ='15 Jl9.1iter- 1) without re-

suspending any sediment. The aquarium was kept in darkness. for 5 weeks

and air was constantly biJbbled into the water to insure O2 saturati on ~

_ and homogenous mixing of this water. No measurable sediJœnt' compaction

occurred and, except for a few isolated turificid faecal mOunds, no

".,

1

66

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benthos activity was apparent during the exp~riment. Short cores ,

collected on day 2 showed that TP was initially even1y di stributed with

depth (1321 ± 27 \.I9.9- 1 ) sediments. Interstitial ~RP and Fe profiles

were obtained toward the end .. of the experiment py inserting a compart-,

mented dia1ysis sampler in the sediment on day 25. On day 35, the

sampler was recovered and si x repl i cate cores were ta ken from the sed;-

ments. Three cores were analyzed for total P and Fe at 0.5 cm intervals. ,

The three other cores were used for mobile P measurements.

, Figures 3 and 4 show that during the course of the experiment,

the total P and Fe content of the top 1.0 cm section increased sharp1y

and that this increase was lapproximately balanced by a decrease in the

1-2 cm· i nterva 1. The fact that the pos iti on of thi s decrease in total

P and Fe (Le. the onset of upward P and Fe movement) coincide a1most

exact1y with the position of steep interstit'ial SRP and Fe gradients

suggests very strong1y that upward P and Fe movements were induced by

the diffusion of interstital P and Fe a10ng concentration gradient gener­

ated by precipitation in the oxidized surface layer of the sediments.

Figure 3 also shows that the mobile P accounted for a significant portion

of the TP which had migrated to the sedimertt surface. These findings are

;l very similar to those of, Tessenow (1972) Whl~ found significant P ;and Fe ".

enrichments in 'the top 2 cm of homogen;zed lake sediments kept in a tank

for a period of 11 months.

( )

Our results and those of Tessenow (1972) both show that the rate '.

an? extent of upward P migration observed under lal.)oratory conditions

are ampl~ to ê'xplain the steep TP gradient observed in the mixed layer

of undisturbed lake sediments.

Q

67

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Fig. 3. Total (.), mobile (0) and interstitial (.) P profiles after

5 weeks in an initially homogemous mud. Error bars =.one standard

error of mean values obtalned from triplicat.e cores.

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(~g . Liter-1) 1

INTERSTITIAL P 1

l ( 0 500 1000 1500 2000

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MOBILE P AND TOTAL P ( f.lg.g-l) . l' , ' \

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( t

initially homogenous mud. Error bars =ilrone standard errOr of

mean values obtained From trip! icate cor~'s. 1-

~.;~ ;; ~':t:"".,1 .:'p,:,~" ''', ' . ~', '"" . , , . ,:, ' 4A~~~î:~~.~;..,«('t'~ t ~~~ .. -4;i ........... - ..... J--I.,..;;.>"""f"'~~ ... _..,."t._ ....

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by a dissol ution-migration-preeipitation cyele of Mn or Fe with, P

co-precipitating or cu-dissolving with Mn and/or Fe. As hypothesized

by Lynn and Bonatti (1965) for Mn, the interstitial SRP maximum

observed aro,und 10 cm (Fig. 5 and 6) could be explained by the existence

of a redox houndary moving upward as new sediment is being accumulated j rI

and below which P-Fe or P-Mn compounds are being reduce1.f, thus producing

~ anfi~terstitial P maximum. However, the transport of oxidized sediment

\

to the reduced zone by burrowing animals could be a second explanation

for the presence of this interstitial P maximum. This possibility is ~ 4

supported by the work of Robf:jins et al. (1979) who found that tubificids

redistributed an added thin surface layer of 137Cs labelled clay to a depth

of 8 cm after 69 days.,

/ There is a striking similarity between the upward inerease in 1 , ,

; TP' and the increase in mobile P for the two pelagie stàtions (~igs. 6 \ \ and 7). The detennination of mobile P by isotopie dilution (see methods) .

presumably measures the sum of the reactive P in solution and of the P

loosely held to the particulate phase, the dissolved and particulate

phases being in dynamic equilibrium (Li et al. 1972). The partieulate

phase would include labile o,rganic P, adsorbed P and amorphous or

cryptocrystalline P minerals capable of exchange with the solution P.

, The fact that the increase in mobi leP i s almost equa1 to the inerease

in TP observed in the 0-10 cm zone shows that this P fraction could be

a good estimator of the surplus P whieh has migrated into this zone.

This ob?èrvation a1so shows that the reaction leading to the precipi~ation

and accumulation of P in the 0-10 cm zone is fully reversible under

70 '1

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In situ profiles of total, mobile ,and interstitial P

To explore whether the inverse relationship betwe~n the

interstiti al SRP concentration and surface increase in TP observed 1

with homogenized mud could a~so be observed in undistrubed sediments, ~ 1

r14nterstitial water samplers w,re placed in JU,lY at two stations in

the lake (A: littoral, 2.5 m, s'R,~rsely vegetated; B: pelagie, 10 m,

macrophyte free) and cores were obtained from the inmediate vic'inity

of the samplers. Mobile P (isotopically exchangeable) and TP were

determined from each 1 em section on companion cores for station A,

and from a single core for station B; the results a're shawn in figures

5 and 6. Total and mobile P were also measured on a single core obtained . t \

from a third station (C: 100 m) and are shown in figure 7. Station A

and B ~ielded interstitia1 SRP profiles with well defined maxima at 8 . . and 12 cm, respectively. As in the aquarium expedment, the sharp

interstitial SRP gradient observed over the upper 10 cm corresponds to

an increase in sediment TP particularly'evident in figure 6. ,This

strongly suggests that'dissolution of P in the reduced zone of the sedi­

ment, fol1owed by upward diffusio~ and finally by a precipitation in

the oxidized zone occurs in undisturbed sediments, as well as in h6mogenized ,1

sediments. Simi,lar profiles for manganese, also suggesting a post-

depositional mobil1ty, have been reported (Lynn and Bonatti 1965; Li et

al. 1969; Robbins and Callender 1975). Robbins and Callender (1975) , .

;n particular show a total and interstitial Mn profile from Lake Michigan

that, is remarkably simi1ar to the P profUes presented here. This suggests

that the upward movèment 'of P observed in Lake M~mphremagog 1s controlled

71

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Fig. 5. Total (e), mobile (O)'and interstitiaT\(.) P profiles from

station A (z=2.5m) J ,

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INTERFACE

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20 d MOBILE • • • • •

25 • • • INTERSTITIAL " • •

~----~----~~.--~----~~--~---o 200 400 600 800 1000 . TOTAL P'{J.lg.g-l) M,oà~-p (lJg~g-1·10-1)

1 1

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6. Total (e)"mobile (0) and interstitial

stat ion B (z""IOm).

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(.) P profi les from

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TOTAL AND MOBILE P (~g~g-,1) .'

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Fig. 7. Total (e) and mobile (0) P profiles from station C' (z"IOOm).

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reducing condltio~s sinee, under"such conditions, the prec1pi~ated

Pis isotopically eX,changeable. This finding may well be of practical . , ,

importance ;~ evaluating the size of the sediment P pool able to diffuse

out of sediments in contact with anoxie waters.

In conclusion, the laboratory experiments on homogenized mud ~

and our field observations show that a substantial part of the P deposited \ ,

in the· sediments can rapidly migrate vertically .and acculIlIlate in' the

upper layers of the sedimentary column. It then f'~'llOWS ~hat, for

Memphremagog and similar lakes, the total P content of the surficfal '

sediments may bear no relatiOn. to the P content of the Sedime.9ing

material. Whenever this i s the case, dated P profiles wHl yield erroneous

records of the history of P loading te these lakes.

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75

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References

,Andersen, J.H. 1976. An ignition method for' determination of total (

phosphorus in lake sediments. Water Res. 10: 329-331.

Bengtsson, L. 1975. Phosphorus re1ease from a highly eutrophie lake '

sediment. Int. Ver. Theor. Angru. Limnol. Verh. 19: 1107-1116.

Bloesch~ J. 1976. Sedimen~tion rates and sedimen~ cores in two Swiss

lakes of different trophic states, p; 65-71. _ln.: H.L. Go.ltennan ~

, (ed.) Interactions between sediments and fresh water. Junk &

Pudoc, The Hague. 473 p.

Bort1eson, G.C., and G.F. Lee. 1972. Recent sedimentary history of

Lake Mendota. Wis. Environ. Sei. Techno1. 6: 799-808.

Bort1eson, G.C., and G.F. Lee. 1974. Phosphorus, iron and manganese

distribution in sediment cores of six Wisconsin 'lakes. Limnol.

OceanogP. 19:' 794-801.

Hess1ein, R.H.' 1976. An in, situ sampler for close interval pore water

studies. Lfmno1. Oceanogr. /2]: 912-914.

Johnson, D.L. 1971. S1f1l11taneous determination of arsenate and

phosphate in natural waters. Environ~ Sei. Techno1. 5:411-414.

Jorgenson, S.E., L. Kamp-Nielson, and O.S. Jacobsen.

for\anae~ic Il)Ud-water exchaftge of p~osphate. 1975. A submo~e1

Eco 1. Mode 1. 1:

133-146. ) .

Kamp-Ni,e 1 sen, L. . 1974. Mud-water exchange of phospha~e and o~her ions l ' •

\ 1~ undisturbed sediment cores and factors affecting the exchange

rates. Arch. Mydrobiol. 73: 2]8-237~ \ 1 \

. ,

\ 1

l , 1 ,

'1

'. ,

! ' 1

(

f

l • \

----- "~ ~--

Kemp, A.L.W. t T.W. Anderson, R.l. Thomas, and A. Murdochova._ lS74.

Sedimentat'ion rates and recent sediment history of lakes Ontario, . '

Erie and Huron. J. Sediment. Petro1. 44: 207-218.

Kemp, A.L.W., R.L. Thomas, C.I. O~l1, and J.-M. -Jaquet. '1976. Cultural'

impact on the geochemistry of sediments in Lake Erie. ,- J. Fish.

Res. Bd. Cano 33: 440-462.

Koide, M., A. Scritar, and E.D. Goldberg. 1972. Marine geochrono1ogy

with 210Pb. Earth Planet. Sci. Lett. 14: 442-446.

, Li, Y.-H., J. Bischoff, and G. Mathieu. 1969. The migration of

manganese in the Aretic Basin sedi~nt. Earth Planet Sei. Lett.

7: 265-270. tif;

Li, W.C., D.E. Armstrong, J.D.H. Williams, R.F. Harris, and J.K •. Syers.

1972. Rate and extent of inorganic phosphate exehange in lake

sediments. Soi1. Sei. Soc. Am. Proe. 36: 279~285.

• Lynn, D.C., and E. Bonatti. 1965. Mobility o,f manganese in diagenesis

of deep-sea sediments. Mar. Geol. 3: 457-474-;

Peters, R.H. 1979. Goncentrations a~d kine~cs of phosphorus fractions

aJong the trophic gradient of Lake Memp~lremagog. J. Fish. Res. Bd.

Cano 36: 979-979. '

Robbins, J.A., and E. Ca11ender. 1975. Diagenesis of,manganese in Lake "

Michigan sediments. Am. J. Sci. 275: 512-533.

Robbin~, J.A., J.R. Kresoski, and s.e. Mozley. 1977. Radioactivity

in sediments of the Great"Lakes: post-depositional redistributio~

by deposit-feeding organisms. Earth Planet. Sei. Lett. 36: 325-333. , 1

..

,\ \ • 1 __ ---_-~':':::-7-:-:-F - ,_, '_- "_, ---r--.--........ ".--_______ ---------~

)

1

1

, ,

c • , , - '"'~ "''''' .... - .. " ~ 1 ....... ~ """ ............... ~.~ .... " ....

\ ' 78

\.

, 1 : l ;

--~-----;---' -~--

Ro~bjns, J.A~, P.L.' MéCall , J.B. Fisher, 'and'J'.R. Krezosl(i. -1979.' 1

Effect of deposit feeders on migration of l37ts in 1ake sediments. 1 . .:.

Earth Planet. Sei. Lett. i2: 217-287.

Ross" P.E .• and J. Kalff. 1975.' Phytoplankton produetton in Lake ,/

Memphremagog" Québec (Canada)-Vennont,(U.S.A~)" Vèrh. In't. Verein. , , \

Limnol.' .19: "760-769.. . , , '

, , .Sonmers, L.E., and D.W. ~elson. 1972. Detennination of total phosphbrus ..

in soils; a rapid perchloric aeid digestion procedure. Soil Séi. ,

Soc. Am. Proe. 36: 902-904.

Tes~enow, U. 1972. L~s~ngs: ~ Diffusions-' and S'orptionsprozesse in der

Obserschieht von See-séd'imenten 1. Ein Langzeitexperiment unter

~erOben und an~er~ben Bedingunlen im Fliess-g1eischgewieht.

Hydrobiol. Suppl. ,38: 353-39~.

Wang, C.H.,"D.L. Willis. and W.D. Love1and. 1975'. "Radiotracer '1

meth6dology in the biological, environmental, and phlsic?l

sciences. Prentiee-Ha:11.

Arch. ,

Williams, J.O. H •• and T. Mayer. 1972. Effeets of sediment diagenesis

and regeneration of phosphorus with special referènee to lakes

Erie an~ ~ntario, p. 281-315. 111: H.E. Allen an~ J.R. Kramer

(ed.). Nutrients in na~ural waters. Wile~terscienee.

Williams, J.O)H., T.P. Murphy. and:'T. Mayer ... 976. Rates of

aceumu1at;on of phosphorus forms in Lake Erie sediments. J. Fish.

Rés. Bd. Cano 33: 430-439.

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GENERAL CONCLUSIONS

The main ach;evement of this rèsearch has been to develop methods for

measuring t,he relat;v-e contribution of ~ater and sediments in the P nutrition

of ~q!Jatic macrophytes, and for measuri ng rates of mact'ophyte-P rel ease and

subsequent transfer ta periphyton and phytopliiTikton. -~

79

& u

This study has demonstrated the overwhelming importance of sediments \

as a source of P to macrophytes. Such a resul t was not unexpected si ncefor:

th'e three sites investigated, the sediment interstitial soluble reaetive p'

(SRP) was one to nearlY 'three orders of m~gnitude higher than the overl'ying

water SRP, Available data from the literatur~ inlicates that high water-SRP /

interstitial water-SRP ratios are conmonplace for a11 types of 1akes~ whether

oligotrophic' or eutrophie. Therefore, the high sediment dependence of macrb­

phytes for P uptake observed in this study can probably be extended,- with

little risk, to most situations.

From this wiewpoint;. the century old controversy on the ,relative impor­

,tance of root and shoot in nutr;ent uptake could perhaps be better explained

by the fact that very littleinformation on inte,rstiti'al water chemistry was

(or iS) available. rather than by the apparent difficuÙY of de'veloping

unambi guous methods for measuring root versus shoot nutrient uptake.

AH species inve~tig,ated derived approximately,the same amount of P

from the sediments. This strongly suggests that very simple models, ba'sed on

interstitial and overlying water nutrient status, could be constructed to

predict se~iment-P uptake ~y macrophytes and thus, quantify t~is particular

aspect of internal nutrient loading in lakes. -' \

Although this study has ,shown the relatively minor importance of' P trans-

Yer from macrop~ytes to periphyton, Myriophyl1um was found to release

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significant amounts of available P to its surrounding waters. However, this

study tîas only indicated one possible method of quantifying tüe in vivo P

release by mac-rophytes and availability to periphyton ànd phytoplankton.

Far more work will need to be done. On other species and under different . nutrient conditions, before useful predictive models can be constructed.

80

This study has only approximated the availability of macrqphyte released

P. More quantitative results C~ld easily be obtained by producing ful1y_ labe­

led plants of higher specifie activity.and eomparing uptake rates by phyto­

plakton of released mae;OPhyte-32p with ~lie uptake of fully ,available 3;P04.

The demonstrated importance of sediments in P uptake by macrophytes

indieates that more attention should be given to 'the sediments if we want 'to ~-....,

predict or to explain field observations on macrophyte species composition

a~d biomass. This study has shown that the usefulness of an i~stantaneous

nutrient availability concept is questionable for lake sediments since mea-, ...r

sures of isotopical1y diluted P (available p) are highly time dependent.

The same may apply for lake water.

(' The available fraction of sediment-P appears very mobile vertically apd

shows a pronounced ~tratification within the sediment column. In addition to

beJng of ~potential importance in our understanding of maC?rophyte-nutrient

relationships,' such as depth of rooting or uptake ki~etics strategies, this ,

verti~al mobility. of sediment-P alters'our current interp>etation of sediment \ \ d

total P profiles, as related to P sedimentation rates in lakes.

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