Bedford and Moore 1984 Macrofaunal Involvment in Kelp Decay

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Estuarine, Coastal and Shelf Science (1984) 18,97-l 11

Macrofaunal Involvement in the Sublittoral

Decay of Kelp Debris: the DetritivoreCommunity and Species Interactions

A. P. Bedforda and P. G. Mooreb

University Marine Biological Station, Millport, isle of Cumbrae, Scotland

Received 18 March 1983 and in revised form 9 June 1983

Keywords: macrofauna; kelp; detritivores; sublittoral zones; Scotland coast

The fauna associated with sea-bed accumulations of decomposing Laminaria

saccharina has been studied by year-round SCUBA diving at two sites n theClyde Sea area. Seasonal changes in density of 64 species are reported. In the

autumn, large quantities of kelp are detached by storms. This weed carries withit to the sea bed a large part of its normal fauna. Additional species settle ontothe weed from the plankton whilst others migrate onto it from the surrounding

sea bed. Peakdensities f associatedpecies ere recordedn autumn.Litter bagexperiments in situ showed that, except during the summer, weed is lost from

sea-bed accumulations at a faster rate when macrofaunal animals are excluded.The macrofauna therefore inhibits decomposition. The relative importance ofinteractive cropping by three macrodetritivores, Psammechinus miliaris

(Echinodermata),Platynereis dumerilii (Polychaeta) and Gammarus locusta(Amphipoda) was studied by in situ containment f different species ombinations.The presence of Gammarus with Psammechinus resulted n lessweedbeing ostthan when Psammechinus was solated.This is becauseGammarus selectivelycrops rotting weed, retarding frond disintegration by microbes. Platynereis retardsmicrobialcolonizationof frond tissuesuptured during its feedingby repeatedcropping of the same region. Weed would decompose very rapidly were it not

for macrofaunal ropping.Macroalgaldecay husdiffers profoundly from that of

vascularplants.

Introduction

In temperate waters and on rocky coasts, arge brown algae (Phaeophyta) are the dominant

sublittoral macrophytes. The fronds of these algae resemblemoving belts of tissue (Mann,

1972), with erosion from the senescent,distal region of the frond being compensated or

by meristematic activity at the frond base. In a single year, these fronds may replace their

length up to five times (Mann, 1972), contributing large quantities of dissolved organic

matter (DOM) and particulate detritus to the sedimentsand water colnmn. In addition

to this process, winter storms can detach large numbers of intact kelp plants (Kitching,1937; Johnston, 1971; Field et ai., 1977). These plants can either be washed ashore,fuelling littoral detrital food chains (Backlund, 1945; Velimirov et al., 1977; Griffiths &

%esent address: 86 Slade Road, Ilfracombe, Devon, England .

bTo whom reprint requests should be addressed.

97

0272-7714/84/010097+15 $03.00/O 0 1984 Acad emic Press Inc. (London) Limited



98 A. P. Bedford 6 P. G. Moore

.

Stenton-Dozey, 1981; Koop er al., 1982) or else sink to the sea bed, below the photic

zone, there to decay by routes as yet little known.On the sea bed, large accumulations of detached kelp can be produced by tidal currents

acting over irregular topography. Locally, such macrophyte accumulations are dominated

by the alga Laminuria succharinu (L.) Lamour. Algae lack the structural complexity of

vascular plants and tend to be high in nitrogen (Tenore, 1977, 1983; Godshalk & Wetzel,

1978). As a result, decomposition rates are relatively fast (Hunter, 1974; Tenore &

Hanson, 1980; Griffiths & Stenton-Dozey, 1981) releasing large quantities of fine organic

material onto the sediment surface.

During active growth, the young frond tissues exude large quantities of phenolic sub-

stances (‘Gelbstoff) which are toxic to saprophytic microbes and also prevent larval settle-

ment by sessile invertebrates (Sieburth & Conover, 1965; Sieburth, 1968). The numbersof bacteria increase progressively along the algal frond (Laycock, 1974; Mazure & Field,

1980) corresponding with reduced exudation of these substances. When the weed is carried

below the photic zone, however, these protective mechanisms no longer function (Tenore

& Rice, 1980) and bacteria and associated macroconsumers flourish. The leaching rate of

inhibitor plant-chemicals was seen by Valiela et aJ. (1979) as a rate-limiting factor in

vascular plant decomposition processes.

In seagrass ecosystems the passage of plant debris through the gut of detritivores resultsin mechanical breakdown of the plant substratum. This fragmentation increases the surface/

volume ratio of the substratum, encouraging bacterial lysis of the material and hence

enhancing decomposition (Fenchel, 1970, 1972; Hargrave, 1970, 1975; Mann, 1972).Our investigations (Bedford i? Moore, in preparation a, b, c) have examined experimen-

tally the roles of three important macrodetritivore species, the sea-urchin Psummechinus

miiiuris (Gmelin), the polychaete ISatynereis dumerilii (Audouin & Milne-Edwards) and

the amphipod Gczmmurus Zocu~ta (L.) in kelp decomposition, mainly in the laboratory and

with individual species kept largely in isolation. These species, however, are part of a

complex macrofaunal community and pressures exerted by other members of that com-

munity may influence their behaviour in the field. Benwell’s studies (1980) on meiofaunal

nematodes should be consulted .for complementary data on smaller metazoans.

A field sampling programme was therefore undertaken to establish the composition of

the total macrofaunal community associated with sublittoral beds of detached L.

succhurina. The impact of this fauna on weed decomposition was examined experimentally

using litter bags. Experiments to compare the relative effects of various combinations of

Plutynereis, Psummechinus nd Gammarus on the rate of weed decay in situ were also

carried out. These aspects form the subject matter of the present contribution.

Materials and methods

Sampling programme

Sublittoral beds of storm-detached L. saccharina are found at a number of sites in the

Clyde Sea area. Chumley (1918) recorded large quantities of detached Luminuriu in Loch

Goil and Loch Fyne. Sizeable accumulations were found at the following sites (Figure 1)and depths: Kames Bay, Gt Cumbrae (30 m), Irvine Bay (50 m), Tomont End, Gt Cumbrae(18 m), Loch Ranza (16 m), Fairlie Channel (50 m) and Loch Riddon (40 m). The weed

beds at Tomont End (grid ref: NS 181593) and Loch Ranza (NR 930510) were selected

for routine sampling since, being in relatively shallow water, they could be sampled readily

by SCUBA diving. Being at the northern tips of the Isle of Cumbrae and Arran respectively



Kelp dent&ore community 99

Figure 1. The Firth of Clyde, showing major collec ting and experimental sites (@) and other

sites of detached weed accumulation (0).

and thus sheltered from prevailing south-westerly winds, these sites were sheltered from

winter storms. At both sites, detached weed was collected by diving at irregular intervals

between November 1976 and March 1978. The weed was collected quickly in strong

polythene bags (45 x 30 cm) and the neck of the bag sealed with a rubber band. Six bags

of weed were collected on each occasion. The bags were brought to the surface immediately

and treated with 5% formalin. Rapid treatment prevented significant predation occurring

within the bag. The formalin also served to extract the majority of animals from both the

blades and the holdfasts. Holdfasts were dissected to remove any individuals remaining.The Laminatiu was then washed in freshwater, oven-dried at 60 “C for three days and

then re-weighed. Following identification of the macrofauna, the densities of the various

species were expressed as mean numbers per 100 g dry weight of weed. Although some

of the density data are likely to be underestimates, it is unlikely that any important species

actually avoided detection during the sampling programme.

Litter bag experiments

Various authors have studied plant litter decomposition in the field by means of litter bag

experiments (Odum & de la Cruz, 1967; Burkholder 8c Doheny, 1968; Heald, 1969; Odum

& Heald, 1972; Odum, Zieman & Heald, 1973; Hunter, 1974). Enclosing the plant materialin bags of differing mesh sizes restricts the access of various size groups of animals. By

comparing the loss of weed from these bags, the importance of various animal groups in

the degradation of plant litter can be assessed.

Here, Laminuriu was enclosed in bags (25 x 35 cm) of two mesh sizes-l mm and

50 mm. The 1 mm mesh excluded all macrofaunal animals. The 50 mm mesh served mainly



100 A. P. Bedford 6 P. G. Moore

to prevent the weed being carried away by tidal currents and almost all the fauna of the

detached Latinaria beds could gain access to it. Although the 50 mm mesh prevented thelarger animals from actually entering the bag, the weed could still be attacked through

the meshes. Adult Echinus escuientus L. were observed feeding in such a manner. For each

experimental run, six bags of both meshes were used. The bags were loosely filled with

freshly-detached Laminaria saccharina and secured on the sea bed in 20m of water at

the Wishing Well, Gt Cumbrae (NS 183556) (see Figure 1). At regular intervals, two bags

(one of each mesh size) were retrieved and the amount of weed lost from each determined.

Altogether, four experimental runs were carried out at four different times of year.

In order to determine the loss of weed over the experimental period, it was necessary

to know the quantity of weed at the beginning. Detached Lam&aria from sublittoral drift

accumulations has never been exposed to the air. Latinaria which has been dried and

rehydrated, however, loses its rigidity rapidly and shows a tendency to laminate. If once-

dried material had been used in these experiments, it would have blocked the meshes of

the 1 mm mesh bags and prevented water flow. It would also have washed through the

meshes of the 50 mm bags. Further, both the palatability of the weed to various inverte-

brates and its rate of decomposition would have been affected. Thus, it was impossible

to determine the dry weight of the actual weed used in these experiments directly. Reliance

on changes in wet weight would also have been contentious. Consequently the following

procedure was used to estimate the dry weight of the weed used at the beginning of the

experiment.

Whole Laminaria fronds were collected and, following the removal of attached macro-

invertebrates, hung up by the holdfast to drain. The frond, holdfast, meristem region and

all blade tissue beyond the first 6&80 cm were then removed. The weed remaining was

free from epibionts. A specimen of this weed was cut from the middle of the remaining

kelp. This specimen included the whole width of the frond so that lateral variations in

weed density were included. This specimen section and the remaining experimental weed

were wet-weighed separately. The specimen was then washed in freshwater, oven dried

at 60 “C for three days and re-weighed. The percentage dry weight of the weed was then

calculated. Assuming that the section of the frond selected was representative, the percent-

age dry weight of the experimental weed should be the same as the specimen. Hence an

estimate of the dry weight of the experimental weed at the beginning of the experiment

could be achieved. This procedure was employed for each frond used in these experiments.Excluding the meristem and distal regions, there is a general increase in density distally

along the frond. Thus, the specimen section taken from the middle of selected weeds repre-

sented the average density of this material.

The experimental weed (lOOf 10 g), following wet weighing, was placed in one of the

mesh bags. Several fronds were generally used for each bag. The complete set of exper-

imental bags was then placed on the sea bed. When the individual bags were retrieved,

the remaining weed was washed in fresh water, oven-dried at 60 “C for three days andweighed. Using the estimated initial dry weight, the percentage loss of weed was then

determined for the experimental period.

The Wishing Well originally was not a site with accumulated detached weed. It wasselected as a good site for positioning field experiments primarily because of its accessibility

in all weathers, its sharply slop’ing bottom (allowing the required depth to be reached

quickly from the shore), its proximity to the laboratory and its lack of disturbance by fish-ing vessels. A rope line was laid on the sea bottom running perpendicular to the shore,

spanning a depth range of -2-35 m chart datum. This line, together with a number of



Kelp detiivore community 101

other items of debris, trapped drift Laminariu carried by bottom currents and quickly

formed an artificial weed bed at 20 m depth. The first litter bag experiments were posit-ioned more than one year (1977) after this artificial weed bed had been formed. By this

time, the fauna of the weed bed was similar to that found within natural detached kelp

accumulations at other sites.

Combination experiment

Mesh bags (1 mm mesh, measuring20 x 30 cm) were Ned with equivalent known weights

(loo+ 10g) of fresh L. succharina. The dry weight of this weed was estimated as n the

previous section. The following groups of animalswere put into six separate bags: (1) 6

Platynereis dumerilii; (2) 6 Gammarus ocusta; (3) 4 Psammechinusmiliuris; (4) 6 Pluty-

nereis+6 Gummarus; (5) 4 Psammechinus 6 Gammurus; (6) 6 Plutynereis + 6Gammurus+ 4 Psummechinus.

The six bags were secured at the Wishing Well at a depth of 20 m. At the end of each

experimental run, the remaining weed was washed (in freshwater), oven-dried at 60 “C

for three days and re-weighed. The lossof weed from eachbag could then be determined.

A comparison of these kept together’ and ‘kept separately’ weed lossesor particular com-

binations of the three species ndicated the relative importance of interactive cropping in

weed decomposition. A ‘control’ with no animals was deliberately not included, since to

derive the absolute impact of this combination of species, experiments would need to be

conducted at ecologically realistic population densities (rather than at the artificially high

levels presently used).

It was essential hat the individuals of each species n different trials were of the same

size. The size of individual Psummechinus nd Gammurus could be compared quickly

by measuring respectively their test diameter (Bedford & Moore, in preparation a) and

head length (Bedford & Moore, in preparation c). Comparing individual Platynereis by

measuring the ‘bite size’ (Bedford & Moore, in preparation b) would have been an

elaborate and time-consuming procedure. To circumvent this problem, for each complete

experiment run, the same ndividual animals were used. This work was carried out in

January-February, 1977. The sea temperature was 5-6 “C. Two complete experiments

(1 & 2) were completed.

Results

Samplingprogramme

Table 1 records the data from the sampling programmesat both Loch Ranza and Tomont

End. The densities of the various species ound in associationwith the detached weed at

these two sites are shown for different periods of the year. Altogether, 64 macrofaunal

specieswere recorded. Figure 2 illustrates changes n the densitiesof the important species

Galathea intermedia, Aora gracilis, Platynereis dumerilii, Psummechinusmiliuris, Gibbula

cineraria and Rissoa ilacina throughout the sampling period. All these speciesshowed a

marked peak in density in the late autumn-early winter period with a subsequentdeclineto a minimum the following summer. Most of the species ecorded were probably carried

down with the weed following detachment since they have been reported elsewhere aspart

of the attached kelp fauna (seeFauvel, 1923; Clark, 1960; Scarratt, 1961; Pettibone, 1963;

Moore, 1971, 1973; Rasmussen,1973; McKenzie & Moore, 1981). Only rare specimens

of the polychaete Platynereis dumerilii, however, are found in attached Laminatiu holdfasts



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Kelp detiivore community 103

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104 A. P. Bedford & P. G. Moore

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Figure 2. Seasonal changes in population density of important species associated with

detached accu mula tions of k&vztk succhorina on the sea bed at Loch Ranza, Arran (solid

line) and Tomo nt End, Cumbrae (dashed line). For actual dates see Tab le 1.

(Scarratt, 1961; Moore, 1971,1973), certainly insufficient to account for the large numbers

here recorded. This species colonizes accumulations of decaying weed following larvalsettlement (Bedford & Moore, in preparation b) and the membraneous tubes built by these

worms serve to bind the kelp fronds together. The amphipod Gammarus Zocustu, which was

found in such large numbers in Kames Bay (Bedford & Moore, in preparation c), was

found in much smaller numbers with the detached weed both at Loch Banza and at Tomont

End. Adult asteroid starfish and decapod Crustacea probably migrate onto the weedaccumulations from the surrounding sea bed, in search of cover as much as food. On

occasions the brittle star Ophiocomina nigra (Abildgaard) was found in very large numbers

on detached kelp, e.g. 640 individuals (of all ages) on a single frond (personal observation).

Ophiocomina is a polyphagous feeder (Fontaine, 1965) and has been recorded including

algal material in its diet (Vevers, 1956). Ophiocomina n&a kept in the laboratory neveringested any Luminati and the above-mentioned frond supporting 640 individuals had

not been attacked. In both these instances, Ophiocomina showed the special raised-arm

feeding posture employed when using the mucous-net suspension feeding mechanism

(Fontaine, 1965). Fontaine (1965) considered that this method was only used when other

food sources, such as sessile algae, were not available. Clearly this is not so. Moore



Kelp detritivore community 105

1 Sept.-Nov. /

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June -Aug.

t , I I I I I I I i t/:;*/:*, I I I I I

10 90 0 IO 20 3040 50 60 70 80 90 IIO 20 3040 50 60 70 E ‘0

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Figure 3. Rate of weed los s in litter bags of mesh size 1 mm (A, solid symb ols) and 50 mm

(B, open symbo ls) secured on the sea bed (20m depth) at the Wish ing We ll, Cumbrae,

at different seas ons of the year. Eq uations for lines of best ti Sept.-Nov. (A),

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P<O.O5); Dec.-Mar. (A), y=O.73x-6.2 (r=0.91 d.f.=4, P< 0.05) (B),

y=O,55x-11.0 (r=0.85, d.f.=4, P<O,05);Feb.-May (A),y=1.36~+4.87 (r=0.93,

d.f.=3, P < 0.05) (B), y=O.82x+6.34 (rz0.96, d.f.=4, P<O.Ol); June -Aug . (A ),

y=1.61x-37.0 (r~0.97, d.f.=4, P-cO.01) (B), y=1~15x+1.81 (r=0.90, d.f.=4,

P<O.O5).

(1983) hasphotographed 0. nigra suspension-feedingn situ while clinging to kelp fronds

and has criticized Norton & Millburn’s contention (1972) that 0. nigru is herbivorous on

these grounds.

Litter bag experiment

The percentage loss of weed dry weight with time in bagsof both meshsizes (1 mm and

50 mm) is illustrated in Figure 3 for the four experimental runs. In all cases,significant

positive relationships were found. For each experimental period, the regression ines fitted

to the data for both mesh sizes were compared (Table 2) using covariance analysis

(Snedecor & Co&ran, 1967). For the December-March, September-November and

February-May periods, the ‘A’ regression ines (1 mm meshbags)had significantly higher

elevations than the corresponding ‘B’ lines. In September-November, the slope of the ‘A

line was also significantly greater. Thus, in all but the June-August period, weed was ostat a significantly faster rate from the 1 m meshbags.

Combinuzion xperiment

The results obtained from the two trials of the combination experiment are shown in

Figures 4 (a)-(c) and 5 (a)-(c). Each pair of graphs illustrates the loss n dry weight of




TABLB 2. Covariancc analysis of regression lines fitted to data for weed loss with tiw forlitter bags of two mesh sizes, A VJ. B (see Figure 2)

Time of year 4

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Elevationd.f.

Sept.-Nov.Dec.-Mar.Feb.--MayJune -Aug.

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Figure 4. Combination experiment 1. Reduction in dry weight of weed (g) with time whendiEerent combinations of species were kept together (solid line), compared with the com-bined ef fect of the same species kept individually (dashed line), (a) pDotymrcis andGammarus, (b) Psammchinus and (2mnnarus and (c) Pl~eis, Psammechinus andGam?narus.

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Figure 5. Combination experiment 2. Reduction in dry weight of weed (g) with time whendifferent combinations o f species were kept together (solid line), compared with the com-bined effect of the same species kept individually (dashed line), (a) &~~Iw&s andGamnrarus, (b) Psammechinus and Gamnurus and (c) Pkuynereis, Pawnmechinus andGam?narur.



Kelp detrisivore commuttizy 107

weed over the experimental period when the species in question were either (i) kept in

the same experimental bag (solid line) or (ii) kept in separate experimental bags and indi-vidual weed losses combined (broken line). Figures 4 (a) and 5 (a) show the combined

effects of Gammarus and pSa$ynertis. Little difference was found between the amount of

weed lost whether these animals were kept singly or together. With Psammechinus and

Gizmmarus (Figures 4 (b) and 5 (b)) however, more weed was lost when the animals were

kept separately than together. Initially, little difference was observable. After two to three

weeks, however, the rate of weed loss for animals kept separately increased rapidly. Weed

was lost at a fairly constant rate when these two animals were kept together. Similar results

were found with the combination of all three species (Figures 4 (c) and 5 (c)); less weed

being lost when the animals were kept together rather than singly. There is little evidence

that accompanied Psammechinus is feeding on rotting weed, despite its preference for this

diet (Bedford & Moore, in preparation a). Presumably, the highly motile Gammurus con-

sumes this material before it can be located by Psammechinus. Geometric rates of tissue

loss in summated trials (Figures 4 (b), (c); 5 (b), (c)) result from the rapid onset of

microbial decomposition of weed in subcultures with no microbe-stripping gammarid

and/or cut-edge recropping nereid.

Discussion

In the autumn, large quantities of weed are detached by storms. This weed carries down

with it to the seabed a large part of its normal fauna. Additional species settle onto the

weed from the plankton, whilst others migrate onto the weed from the surrounding sea

bed. Autumn corresponds with the time of peak density of all these species (Figure 2).

Macrofaunal densities then decline, reaching minimum levels in summer. Mesh bag exper-

iments set up in September, December and February, had a detached weed fauna available

to colonize the kelp in the 50 mm mesh bags and this fauna seems to have controlled

decomposition effectively. In the June experiment, however, few macrofaunal individuals

were available locally to colonize the weed and decomposition continued at approximately

the same rate in bags of both mesh sizes.

Comparing rates of decomposition in the 1 mm mesh bags, weed decomposed at a slower

rate in December-March compared with similar material in the September-November

experiment. This would be an expected consequence of lowered seawater temperature.There was little difference in the water temperature between the December-March and

February-May periods (Moore, 1980). In February-May, however weed decomposed at

a significantly faster rate than in December-March. In February, Laminaria undergoes

a period of rapid growth (Johnston et al., 1977). The resulting kelp tissue is more flimsy

in structure, illustrated by its lower density at this period (Johnston et al., 1977). Thus

weed used to set up the February experiment would be less resistant to decay than the

tougher material used in December.

It is clear from the litter bag experiments that, except during the summer, weed is lost

from sea-bed accumulations at a faster rate when macrofaunal animals are excluded. The

macrofauna thus inhibits decomposition. Using litter bags to study sea-grass decom-position, however, Burkholder and Doheny (1968), Heald (1969) and Odum et al. (1971)

all found that macro-invertebrates, particularly amphipods, speeded up the rate of de-

composition. Marine angiosperms have (i) a large proportion of structural polysaccharides,

(ii) waxy coverings and (iii) they exude protective chemicals all of which make them rela-

tively unpalatable (Odum et al., 1971; Valiela et al., 1979). Very few herbivores naturally




graze this material (Odum et al., 1971; Kikuchi & Peres, 1977; Fenchel, 1977, but note

Cammen, 1980). Comminution of vascular plant debris by macrodetritivores increases thesurface available for microbial colonization and enhances decomposition. Compared with

algal detritus, however, the degradation of sea-grass litter, even with macro-invertebrate

involvement, is slow (6-12 months, see Odum & de la Cruz, 1967; Odum et al, 1971;

Tenore, 1977). In temperate climates, this slow decomposition assures a relatively constant

source of energy for heterotrophic organisms throughout the year, in contrast to autotrophs

which are subject to seasonality due to variations in photosynthesis (Fenchel, 1977). By

contrast, algal detritus has a much lower level of tough, structural polysaccharides than

sea-grass debris (Tenore, 1977) and the distal region of the frond is already decaying when

the weed is detached (Johnston, 1971; Mann, 1972; Johnston et al., 1977). As shown here

by litter bag experiments, detached weed will decompose very quickly in the absence ofmacro-invertebrates. Decomposition of kelp, as of sea-grass litter, is greatly assisted by

the action of ciliates (Briggs et al., 1979) and nematodes (Tenore et al., 1977; Benwell,

1980; Findlay, 1982; Findlay & Tenore, 1982). Detached Laminarziz, however, does not

represent a ‘bottle-neck’ to energy flow (cf. Macfadyen, 1961); quite the reverse. With

the input of weed to sublittoral accumulations being pulsed in synchrony with storm

action, microbial decomposition of this material would be similarly pulsed. Left alone,

decomposing Laminaria fronds lose rigidity and would tend to blanket the underlying sub-

stratum. Synchronized decomposition would, at best, result in irregular, massive inputs

of line detritus to the sediment. At worst, it would result in severe anaerobic conditions

suffocating both the fauna associated with the fronds themselves and also the infauna ofthe underlying sediments. Such a situation often occurs on the sea bed in areas in receipt

of organic pollutants, e.g. wood pulp (reviewed by Pearson & Rosenberg, 1978).

The highly motile amphipods, however, actively seek out and crop the decaying algal

material, which may also be included as part of the diet of other invertebrates, e.g. certain

decapods. As with the sea-grass litter (Fenchel, 1970; Harrison, 1977), the amphipods

digest only the saprophagous micro-organisms (Bedford & Moore, in preparation c). Their

action of cropping the rotting weed from the margins of kelp fronds prevents the sapro-

phagous microcommunity completing decomposition. Since the weed would decompose

very quickly if undisturbed by macrofaunal croppers, the effect of the amphipods in this

instance is to seriously retard weed decomposition. This situation is in marked contrast

to that reported by Fenchel (1970), Harrison (1977) and Lopez et al. (1977) for sea-grass

ecosystems. The participation of amphipods in kelp decay processes ensures that the kelp

macrofauna (not saprobic micro-organisms) controls frond degradation, with the rate of

degradation being governed mainly by the rate of feeding of the largest macrodctritivores,

like Platynereis and Psammechinus.

Macrofaunal involvement in the sea bed decomposition of Laminariu detritus then dif-

fers in two important ways from sea-grass litter. Firstly, with detached Laminariu, a

considerable number of detritivores probably digest the detrital substratum (see also

Foulds & Mann, 1978; Cammen, 1980; Seiderer et al., 1982; Stuart et al., 1982). In the

light of these findings, the general conclusion of Fenchel (1977, see also Fenchel &

Jorgensen, 1977) that detrital matter derived from macrophytes and the energy containedin it has to pass through a bacterial or fungal link before it can be utilized will have tobe modified not only to distinguish macro-algal from vascular plant debris (see also Tenore,

1977, 1981, 1983) but also to take into account Christian and Wetzel’s point (1978) about

low microbial densities on detritus particles in many situations. Secondly, the detritivoresassociated with Laminaria detritus act together to inhibit rather than to enhance



Kelp detritivore communtiy 109

decomposition. The effect of the orchestrated action of the great variety of detritivores

associatedwith detached weed is then to stabilize both their own food supply (cf. Goreet al., 1981) and the supply of fine detritus to sediment-living heterotrophs.

Acknowledgements

The skippers and crews of M.F.V.‘s L.eander andEvadne are thanked for their assistance.

Mr I’. J. Lonsdale facilitated diving work. One of us (A.P.B.) is indebted to the Natural

Environment ResearchCouncil for ResearchStudentship.

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Bedford and Moore 1984 Macrofaunal Involvment in Kelp Decay

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