Not to be quoted without prior reference to the author

International Council for the Exploration C.M. 1993/ L: 25

of the Sea Biological Oceanography Committee

------------------------------------------------------------------------------------------------

Seasonal Cycle of Phytoplankton in the Dunkellin Estuary, Western

Ireland.

J.H.T. O'Mahony,*Department of Oceanography, University College, Galway, Ireland.

* Current Address: Fisheries Research Centre, Department ofthe Marine, Abbotstown, Dublin 15,

Ireland.

------------------------------------------------------------------------------------------------

Summary

Phytoplankton species composition assemblages, concentrations, distributions

and sequences were studied for two and a half years in a small estuary in

Western Ireland. Physico-chemical conditions (temperature, salinity, currents,

meteorology, water transparency, nutrients, chlorophyll a and particulate

organic carbon) were also monitored concurrently. 185 taxa were recorded

• from 9 major aIgal groups. The classical bimodaI peaks in both cell

concentrations and chlorophyll a concentrations were not observed in the

Dunkellin estuary, rather aseries of peaks were recorded throughout the

growing season (late Feb-Nov). Seasonal aspects of the dominant

phytoplankton groups and species are discusssed.

Introduction

Phytoplankton species composition, assemblages, distributions and sequences

have been littIe studied in Irish estuaries. As interest focusses on the potential

for urban, maricultural and industrial expansion botb in and around Irish

estuaries, it becomes important to document specles composition and the

factors that influence the phytoplankton assemblages that are currendy found

there. Occurrences of hannful phytoplankton in Irish waters since 1976 has

raised concern about therr potential impact on public health (vis a vis Diarrhetic

and Paralytic shellfish toxins), and on the economics of our expanding fish

and shellfisheries.

Trus present study is the first temporal, quantitative investigation of the

phytoplankton of the Dunkellin Estuary, from February 1984 to June 1986.

The aim of the resesarch was a detailed investigation of the phytoplankton

species composition, abundance, distribution and sequences in the estuary with

reference to both temporal (interannual & seasonal) and spatial (horizontal,

depth & tidal cycle) patterns. Results are presented here for the seasonal eycle

of phytoplankton recorded during that period.

Site description, Bathymetry, Topography & General Oceanography

The Dunkellin Estuary lies within the eastem portion of Galway Bay, Ireland,

bounded by latitudes 53 0 11'36" to 53 0 13'12" N and longitudes 080 51 to 090

3'W. Location and station positions are shown in Figure 1. The study area •

encompasses circa e19 km2. Figure 2 shows the bathymetric contours and

place names. The Chuinbridge Oyster Bed located in Keave Narrows, covers

an area of approxirnately 2 km2. Trus is a very shallow area, and can be less

than 1m in depth at low tide in some places. Dunbulcaun Bay, E of Keave,

dries out at mean low water spring tides along the north and south shores. From

Corraun Point westwards, the seabed gradually deepens unti] a maximum of 7m

is reached in Tyrone Pool. South of Tyrone Pool the Yellow Rocks are

exposed at low tide. The region deepens into Mweenish Strait, to a depth of

5m SOUdl of Mweenish Point. West of Mweenish Point and Island Eddy the

bottom gradually deepens, reaching a depth of 10m SE of Kilcolgan Point.

.' .

(a)Co. Clare

. .:.:..... 0; 0::

GALWAY BAY

N

l'

53'l-.- L.- --r-

.......

Fig. .i.. (a) Location and (b) station Positior$. Dunkelli~ EStua-ry.

; ~:.'" ::::: .... 0: ............'" .... :0: ':, .. ;. ;0°:°•.::, ::.'" ..

Nl'

4SE KILCOLGAN

POINT

~";'".. ~ ....... " ......

,,' .t:!

"-'1

Soul~ aay i

/

I~(~.o

Fig. 2.

!.

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

Bathymetric Contours & Place Naaes, Dunkellin Estuary.

o

_W"VII:

..-:;..:...- Wl.VS

1000

.1

The surrounding land is low-lying, with bedrock composed of limestone from

the Upper Carboniferous Period. The substrate types vary between the

sampling stations, ranging from mud and silty cIay at station 1, coarse sand and

shelIlgraveI (station 2) to a mixture of clay, sand .and maerl (remains of

coraIIine algae) at station 4. The principal sources of freshwater are two small

rivers the DunkeIIin and the Clarin, with a combined catchment area of 496

km2 approx.. These two rivers can floQd circa 4000 hectares of low-lying land

in winter. Due to the porous nature of the underlying Carboniferous limestone,

freshwater "sinks", called swallow holes, are present and water presumably

penetrates into subterranean drainage channels which may be a source of

additional freswater input.

Tides in this region are semi-diumal, and range from 4.7m - I-9m for mean

spring and neap tides respectively (Bary cl al., 1986). Occasional deviations

do occur, events usually of a short duration of 2-3 tidal cycIes and Bary (ap.

eil.), suggests that these are related to weather-induced changes occurring

outside the estuary. Booth (1975) and Harte cl al. (1982) found that the small

bays at the eastem portion of Galway Bay appear to be Iittle influenced by the

general circulation within the bay itself. Berthois cl al. (1972), considered that

the surface water movements in the bay are govemed by wind, and that in the

southem part of the bay (which incIudes the DunkeIIin Estuary), that wind

stress is probably the major factor affecting water movement. Booth (ap. eil.)

considered the whole of Inner Galway Bay as a partial or weIl mixed estuary

and also suggests horizontal and vertical mixing processes as the predominant

form of freshwater transport in this portion of the Bay. Vip (1980), and

Cronin (1987), found that water originating from outside Galway Bay can be

distinguished during the summer months.

Materials & Methods

Sampling and standard analytical procedures for physico-chemical data are

given in detail in O'Mahony (1992). For biological sampIes, quantitative

duplicate 30ml sampIes were preserved in acidified Lugol's Iodine (Throndsen

1978), shaken gently to distribute the preservative and stored in the dark. For

examination, sampIes were gently shaken 100 times to redistribute the

phytoplankton as evenly as possible, and were poured into 10ml Hydrobios

chambers and left to settle overnight (Edler 1979). The sedimented plankton

were then identified and counted with a Nikon Phase Contrast Inverted

Microscope using a modified Utermohl's Method (HasIe 1978). All species

were identifed (or coded) and counted at the same time. The whole bottorn of

the base plate was counted to reduce effors even though this is a slower

method.

Results

•

A. Meteorology

The climate on the west coast of Ireland is characterized by a small annual

variation in air ternperature with average winter and summer values differing by

about lO0e. Maximum precipitation usually occurs between later autumn and •

early spring. In winter and occasionally in late summer, the Galway Bay area

is subject to severe storms. Figure 3 summarizes mean air temperatures, wind

force and direction, windspeeds, cloud coverage and riverflow rates (the latter

an indication of rainfall) during the period of the survey. The coldest months

were January 1985 and February 1986, with the warmest periods occurring

between lune to September. 1984 was warmer and dryer than either of the

following two years. The prevailing wind was southwesterly and on most

sampling dates winds were greater than Beaufort force 2. Windspeeds were

higher in 1985 and 1986 than in 1984. August 1985 was notable for

particularly severe storms. Cloud coverage values of 0/8 octres were never

J P 11 .t. 11 J J .t. S 0 K D J P 11 .t. 11 J J .t. S 0 N 0 J P 11 .t. 11 J Jrirru (~/u)

Fig.3.1. :Mesn 1donthly .Hr Temps.. Gallfay, 1984-1988

Ja

•'"'~'-.~

t 13

E~

'li• 0I I.

"Il

•

TIf.t.IIII.t.'ONDITll.t.ll/J!SOIlDJTll.t.ll/ J

ri ..... (mDntA6)

Fig.~ lfindspeeds, DunkeJlin Enuary, 1984-1986

, li A. XI 1 J. S O:i Dir liI A. W 1 J 4 ~o X D 1 ., W J..w J 1ri"", (rn.on Uu)

FI.g:a.5. Rlvertloll' Rates, Dunkdlin Estuary, 1984-1981

, .CLoud cO'"..wro.g. (oeHm')

Fig. 3.4. Cloud Coverage, Dunl:eIEn &ltuary, 1984-1986

15

10

ro

30,------ ,

tSr--------- --,__-

N Nta ta !ta ! ;;sg SI ffiIl 8 B!l'II SW 1':J"f W YlIW tnr Ill!l'~ind .!>iroction

Fig.~2b Wind Direclion, Dunkellin Uluary, 1964-1966

, 4 ,

3<o.u/orl ScuJ..

Fig.3.2a Wind force, Dunlcellin Esluary, 1984-1986

I I ..11.---------------------,

'"'

"13 .,~

G' ~'-s:

i ~.~

~...~ "Il

~

•

recorded on the sampling dates and in fact were full coverage 8/8 on most days.

Combined weekly river flow data show that minimum precipitation occurred

during the summer of 1984, and the maximum of 238 mm in August 1985,

which was nearly three times the long term mean for that month. Overall there

were higher levels of runoff in 1985 than in 1984.

B. Temperature, Salinity & Circulation

Temperature and salinity records are shown in Figure 4. In the Dunkellin

Estuary, a salinity gradient exists along the axis of the estuary which varies

with both river discharge values and tidally-induced mixing. The water column

at station 4 was vertically mixed for most of the year and the highest salinites

were recorded there at bottom depths. The other two stations are more

stratified and have lowered salinities due to a combination of dilution from

river discharge and slack ClliTents at low tide. Values ranged from S3.6 (station

1) to S34.35 (station 4). Water temperatures ranged from 4.l oe - 19.40 C

during the survey with winter minima and summer maxima. In contrast to the

salinity values, there was very little difference in the temperatures recorded at

each station and themlal stratification did not occur. Greater differences were

evident, however, between years at the time of the spring increases.

In the Dunkellin Estuary cunent flow was generally in an east-west direction

for the inner part of the estuary, and nOlthwest-southeast for the outer part as is

shown in Figure 5. Downstream residual cun·ents were found during periods of

high runoff (Bary et al. 1986). Flushing rates were high in the inner part of the

estuary, with swface water in particular being replaced on each tide whilst

lowered current speeds at the bottom may result in some water being retained

or retumed there on the flooding tide. This may imply that neritic/coastal

species of phytoplankton are recruited from outside the estuary, from Galway

Bay rather than endemie brackish species predominating in the inner estuary.

•

•

•

i1colg"1'1.

\Mweenish Serait

Ooorus St,~it

(ol

Flg.· S. Generallzed circulatlon pattern (surface, direction onlyl on (al cneDDlng tide ana (D) a floOdlng tld(: In the Dunkellin estuary

(after Bary et al., 1986)

D. Nutrient Distributions

Data on nitrates, silicates and phosphates are shown in Figure 6. Overall the

nutrient concentrations investigated in the Dunkellin Estuary did not appear to

exhibit clearly defined seasonal relationships over the two and a half years of

the survey. Occurrences of maximal concentrations were more often associated

with the highly variable and anomalous weather conditions than with the

seasons. Nutrients did not appear to be limiting in the estuary. Gradients in

concentrations were found, with a decrease in concentrations of nitrate and

silicate from station 1 to 4, and an increase in phosphate.

E. Particulate Organic Carbon & Chlorophyll a

Data for POC and ChI a for each station are shown in Figure 7. For POC's,

considerable fluctuations were recorded at each station with trends in

increases and decreases generally following the same pattern at each station.

No clear seasonal trend was apparent over time. POC concentrations were

higher in 1986 than either 1984 or 1985. Chlorophyll a concentrations in the

Dunkellin Estuary varied between each station and between years during the

survey. Maxima varied also both in timing and in magnitude. Station 2

exhibited a more clearly defined seasonal distribution that either stations 1 or 4.

Values ranged from not detectable to 14.5 mg/m3.

F. Phytoplankton

185 taxa were identified from 9 classes of phytoplankton. Percentage class

representation is shown in Table 1 and the dominant species are listed in Table

2. Phytoplankton numerical abundance (Figure 8.) ranged from undetectable

to 2.9 x 106 cells per litre and were generally highest between April and

November. CeIl concentrations were generally higher at stations I & 2 than at

station 4, and were usually lower at subsurface depths. Mean cell numbers per

•

--- ----------SL-t

-OUl - -WUl •• Zm

10 - , l\ /'~/~;;~,/~vGJ'----/ .y~ -- - V'

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ri.~ (m..ontJu)

Fig. bS Silico leg, SL4, Dunkcllin ßsluary 1984 -19&1

lllJ,--------'lw

ULi-Olll - -Ww

,,

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Fig. Gd PO·I-P, Sl. 4, DunJ.:ellin r:.,.luary 1004-1986

Sl.i-0111 - ~Ww

t ~.~.o IIl1dntnlTlTJlTn1flT~11II1mlllllnt~~~II;~lTTilllllimrrßlI'IIIIlIlIrll1;;;1I1n 1J,"'J.W.J'.LSOHOJYWAWJJJ.HOr;DJrW ... WJ

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Fig. b" Nilrale~, SL 4, Dunkellin Esluary 108-1-19&1

Oll ,----------------...,---S·1.-~2-~

-011\ .- ÜLl, '-----------------,'---;0;-;:----,

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J rllAA WJ J 4S0 ND J r:w i\l.lJ JA .sOH DJ Y'W j"W J J

Tl»tl; (n\O/dfu)Fig. (./:> Nitr"tc., St. 2, DUhkeliin K..tu"l'y 106·1-IOtllS

~~I ,

, I• I I \\ I \ ......

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J , ),I A lof J J j, ~ 0 }l D J ,),( .l 104 J JAS 0 N IJ J , W j, ~ J J

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10 - \ 1\ t.- /~, A~,~~-\:j\2\, 1\/ I .r-l/'\

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.. /' _.. T"lne (TT\(YuU13)~ .g. I::>h Stllc"Le., St. .!, DUllkellin K.luary. 1064-1068

10 .----·---··-----·---------I-·St~I--·-

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r .' Tün~ (11'l011 th,,)fig. Ic.<. Silicule., Sl. I, DUllkellin &.lu~ry, 1084-1060

1111

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(.-.C Tunt: (nlonlh..3)fig. "-1 I'llo.phulc•• Sl. I, iJ\lUkdllll E.Luary, 108-1-1006

'0

~., ,~~Ak,ITrrlllllllmirfilnmmTTnflrrlll1llll1llmTIll1Irllll1lllllll1mllniiilfJlllllllml1lll1llllmllfllll1l1III111 1111T

J I" W" \l J J " S 0 N D J r l/ A l/ J J A ~o N D I Y l/ All Il_ ri~ (nlon(/u;)

Fig.~: Nitrate., St. 1. Dunkellin &'luary, 1084-lOtllS

00

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Fig. 1-0:.

-. ZaaI St.-t;-Om. -"WCD

:1..,~

:t.,.;;,,;,;,_...,~J1I1AII1/"8 OND /111.111 I .. SON Dl1 11.011 I

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Fig. :+d Chi a. SL 4. Dunkellin Estuary 1984-1986

1600

tt

1000 ~ .

600 ~ \

c

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Time (montM)

POCs. SL 4. Dunkellin Estuary. 1984-1986

~ i-_-Om--_-.~--4.-.z.,,--.--------------,,

'1\I:, \J'~;"~," : ::~., ' \, I' ...

I,'. : _, ,'"I,: \ I ,

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rig. :.:J- b POCs. SI.. 2, Dunl<ellin Estuary. 1984-\960

11I ,

L[!>:()i:

2600 i SI.. 2'-am _olm

Fig. =1'f Chi a,

/\ /I

o '-mrririmmmiilimiiilrimmmnm-rir,-rrn-m-ITlr;tr!m;-_;;"".~~";..(",,lr.rr><'~~\~\-----::;;;;r.;;;";:;~,.,,,,;~111. ,I I • 11 1111.101,11

O:fO Jrw AW.l 1 ... SOX Dl7li1 A."l

Ti'tT'\e (Tnonth.3)Sl. I, DlJnkellin tstuary, 1984-1900

10

~o I St.ll-om --Zm

~o I-

t

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"

:~ :\ \1\ ~~! 1':i~\lrfv \/ ~\ ~

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Time (moT\!h.»Fig. 1<:. POCs. Sl. I, Dunkeil in Esluary. 1984-1900

, ...

!ahle ..1- Percentage Class Representation - Dunkellin Estuary

CLASS NO. OF SPEeIES %CONTRIBUTION

Bacillarophyceae 80 43.2

Chlorophyceae 4 2.1

Chrysophyceae 4 2.1

Craspedophyceae 1 0.5

Cryptophyceae'" 4 2.1

Dinophyceae 8(; 46.4

Euglenophyceae 1 0.5

Prasinophyceae 2 2.1

• Pyrmnesiophyceae 3 1.6

--------------------------------------------------------------

185 100

'" includes ~esodinium rubrum, unid. microflagellates, and Sp. F.

·.." .; .

Table . '2.

DIATOIiS

Do.inant Phytoplankton, Dunkellin Estuary, 1984-1986

DIlfOFLAGELLATES

BACILLAROPHYCEAE (26)

Ceratulina pelagicaChaetoceros spp. (sma11)Chaetoceros spp. (1 arge)C. curvisetusC. decipiensC. simplexC. socia1is

Leptocylindrus danicusL. minimusNitzschia spp.N. c10steriumN. delicatissimaN. seriataUnidentified pennate diatoms

Rhizosolenia delicatulaR. hebatata f. semispinaR. ?pungens

Ske1etonema costatumThalassionema nitzschoidesThaiassiosira spp.Thai sp. ABThai. sp. CT. gravidajrotulaT. rotulajgravidaT. nordenskioldiiT. polychorda

DINOPHYCEAE (11)

Ceratium lineatumDinophysis acuminataGlenodinium foliaceumGonyau1ax polyedraG. spiniferaGymnodinium simplexG. splendensHeterocapsa triquetraProrocentrum micansScrippsiel1a trochoideaUnidentified dinoflagellates

}lICROFLAGELLATES (8)

CHRYSOPHYCEAE (1)

Apedinel1a spinifera (W)

CRYFTOPHYCEAE (2)

lfesodinium rubrumRhodomonas ? minuta (Z)

EUGLENOPHYCEAE (1)

EUtreptia marina

PRAS I NOPHYCEAE (1)

Pyramimonas sp. (Y)

PYIDlNESIOPHYCEAE (HAPTOPHYCEAE) (1)

Chrysochromulina sp. (V)

Unidentified microflagellatesSpecies F.

Tahle 3 .' lfean Phytoplankton Cell Concentrations, DunkellinEstuary (cells/1itre)

. station No. 1 2 4

DepthSurface 199,507 164,610 73,354

l1id 42,057

Bottom 190,480 69,581 28,551

SL -t. Surrnce-Dt - - Vn VI

:jU i:\ I.!:

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1'irn. (mont1ls)

Fig. C.l Ct Phyloplnnklon Groul' Auuuduoce Sl.4, 'B4-'ß6

Tb't.1

--W:1ll 'Lw

Phyloplankton A!.>undance, Sl. 'I, HllH-I\Jßß

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1'ü"-I.: (Hl-DuthJl)

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Time ("'V:J,"ntll~)Fig. 9 C. Phyloplunlclon {;roolp Abunöo.nCG, Sl.J, tg8·1-Hlß6

n'111

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1,,1

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F'ig. '1 b Phyloplo.nkÜrW 61!2>mt \i1undo.nce, Sl. 2. 84-86

7~-----------"'--------------~Sl.1, Surface

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f-~',I 1\

, 1 \

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1 -

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Tone ('lnOnUt3)Yi/:, B b Pbylopl.llklon A!.>und .. ilce, Sl. 2, )\)O·\-H.lUO

2 -

station are shown in Table 3. The c1assical bimodal pattern in phytoplankton

standing crop (using numerical abundance), was not observed in the Dunkellin

Estuary ; levels fluctuated continually during the productive months, and the

highest spring concentration was usually exceeded by concentrations in June,

July and September. This does not imply that these were periods of greatest

biomass, as blooms of some very small species such as Nitzschia delicatissima

and Leptocylindnts minimus occurred. The group dominance (in terms of

nwnerical abundance), also varied between sampling dates as is illustrated in

Figure 9.

General Characteristics of the Annual cyde of Phytoplankton in the

Dunkellin Estuary 1984-1986.

Winter (December - mid February)

A very sparse flora was generally found between December and the end of

February, when low water temperatures «80 C) and light intensity were found.

Assemblages present at this time were characterized by low species diversity

and low cell abundance «10,000 cells/litre). This winter flora began to be

replaced in late February/early March at stations 1 & 2, by microflagellates

notably Eutreptia marina and Rhodomonas spp.

Spring (End February-end May)

The spring increase in cell concentrations began in the last week of February of

the years sampled, though not necessarily at all stations together, or at the same

station each year. These months were charactellzed by an overall increase in

species diversity and total ceIl concentrations as weIl as temperatures and

chlorophyll a. During this survey the spring peak usually occurred in April. In

March, species diversity increased when water temperatures rose ahove 100

C, with microflagellates and diatoms such as Thalassiosira spp. appearing at

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stations 1 & 2 and small Chaetoceros spp., Thalassiosira spp., microflagellates

and the dinoflagellate Scrippsiella trochoidea at station 4. Tbe Scrippsiella

was interestingly concentrated at mid depths only,

By April, both species diversity and cell concentrations had greatly increased

at stations 1 & 2, and less so at station 4. Mixed populations of

microflagellates, dinoflagellates and diatoms were found at stations 1 & 2,

whereas station 4 was dominated by diatoms. At station 1 the dominant flora

was composed of Elltreptia marina, Rhodomonas sp., Heterocapsa triqllctra, a

small Gymnodinium sp., Thalassiosira spp., T. polychorda, small Chaetoceros

spp., Skeletonema costatum and Nitzschia spp. At station 2, the flora was

similar, except that E. marina and Gymnodinillm sp. were replaced by large

Chaetoceros spp., and more species of Thalassiosira. At Station 4, the water

contained mixed diatoms such as small Chaetoceros spp., Thalassiosira spp.,

Thalassiosira ?Ievanderi, T. gravida, T. polychorda S. costatllm and

Rhodomol1as sp.

In May, specles diversity decreased in companson to April. The diatom

Rhizosolel1ia delicatlila appeared in the plankton at all stations, together with

the dinoflagellate Glenodinilllll foliacellm at station 1 and S. trochoidea at

.station 2. At station 4, S. trochoidea dominated in the surface waters, with

mixed diatoms and microflagellates at subsurface depths.

Summer (June-August)

In June, species diversity again increases, particularly at station 1. The diatom

Ceratalilina pelagica appeared at stations 1 & 2, and the dinoflagellate

Ceratium Iilleat1l11l at station 4. The diatom Leptocylindrus minimlls also

appeared in very high numbers at stations 1 & 2. The number of Chaetoceros

spp. present also increased, with species such as C. curvisetus and C. sociale at

station 1, with Rhizosolenia hebatata. At station 4 R. delicatula was present

at all depths.. During this month red tide blooms of Glenodinium foliaceum

occurred. In July, the diatom Thalassionema nilzschoides forms an important

component of the phytoplankton assemblages at all stations, with small

Chaetoceros spp., Scrippsiella and microflagellates also important at station 2,

and C. lineatum at station 4. Species diversity decreases during this month. In

1985 there was a localized bloom of the dinoflagellate Gonyaulax spinifera

near station 1.

August was one of the few months of the year where sampling was disrupted

or cancelled due to bad weather. Phytoplankton composition generally shifted

from a mixed diatom/dinoflagellate assemblage to a predominantly

dinoflagellate one at stations 1 and 4, with a mixed assemblage at station 2.

Species diversity decreased at all stations and depths, except at station 1, where

increases occuned. The predominant dinoflagellates were Prorocenlrllm

micans and Glenodinillm foliaceum at station 1, P. micans at station 4 and

mixed small Chaetoceros spp., Elltreptia marina, G. foliacellm and mixed

microflagellates at station 2.

Autumn (September - ovember)

By September species diversity generally increased again with assemblages of

mixed diatoms, dinoflagellates and microflagellates. The diatom Leptocylindrlls

danicus and the dinoflagellate Gymnodinium splendens occurred during this

month at station 1, with R. hebetata, L. minimus, C. clln1isetus and

Skeletonema costatum. In October and ovember sarnpling disruption

occurred frequently due to bad weather. Species diversity generally

decreased. In October, the dinoflagellate Prorocentrum micans was dominant

at stations 1 & 2, with microflagellates at station 1. A very sparse flora was

present at station 4. During November, (sampies only from 1985),

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microflagellates dominated at station 1, Thalassiosira rotlIla at station 2 and

Thalassiosira rot1l1a/gravida at subsurface depths at station 4..

Discussion

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185 species of phytoplankton were recorded during this survey from 9 alga1

groups. This is a relatively rich flora when compared to other Irish estuaries,

due in part the re1ative1y 10ng duration of this study. Species were principally

neritic and temperate (north & south) in origin. Both dinoflagellates and

microflagellates were important components of the flora as weIl as diatoms.

Highest phytoplankton concentrations were found in the warmerlligllter periods

of the year from April to September. The c1assical bimodal peaks in both cell

abundance and chlorophyll a concentrations, so much a feature of many

temperate estuaries and coasta1 waters, were not found in the Dunkellin

Estuary; instead frequent higll concentrations were found throUgllOut the

growmg season. This has also been documented by other researchers,

inc1uding Riaux & Douville (1980), in the Penze Estuary, France; by Hu1sizer

(1976) in Narragansett Bay U.S.A., von Bodungen (1975) in the Kiel Bight,

Gennany.

Seasonal successions/sequences in phytoplankton often occur in areas that are

characterized by having seasonal temperature and nutrients, and the timing of

their community dominance is usually an indication of their environmental

preferences (Smayda 1980). In the Dunkellin Estuary, where microflagellates

were as important a component of the phytoplankton assemblages as diatoms

and dinoflagellates, succession seemed to occur within these groups over the

annual cyc1e. However, this was succes·sion not obvious between groups, as

these underwent continual rapid changes in dominance, as weIl as reversals to

previous assemblages, during the growing period.

A feature of the two and a half year study of the Dunkellin Estuary, and;

documented throughout much of the meteorological, physical and chemical

data collected, is the variability that occurred petween years. This was also

reflected in the phytoplankton data. Additionally, the annual maximum of

chlorophyll a concentrations did not necessarily,occur during the spring peak,

but in the months of September (1984) and June (1985). his has also been

found to be the case in Narragansett Bay, U.S.A. (Smayda 1983).

..,Due to the relatively short duration of this research (in comparison to over 30

years data from Narragansett Bay in eastern U.S.A.), it is difficult to speculate

whether the variations encountered in the Dunkellin Estuary during the study

are the norm, part of a longer term cyde, or unusual. They indicate that

caution should be observed in interpreting data' from one annual cycle alone,

where interannual variations are masked and timt longer term studies are more

desirable. Results from this study indicate timt the Dunkellin Estuary is a

complex ecosystem.

References

Bary B.McK, McMahon T. & Seale M. (1986) The Dunkellin Estuary,

Oceanographic and Biological Survey. Report No. 3 to the Office of

Public Works, Dublin.

Berthois et al., (1972) In: Booth D. (1975), The ~ater Structure and

Circulation of Killary Harbour and of Galway Bay. MSc. Thesis,

National University of Ireland.

Booth D. (1975) The Water Structure and Circul~tion ofKillary Harbour and

ofGalway Bay. MSc. Thesis, National University oflreland.

Cronin A. (1987) A Study of the Phytoplankton Ecology of the Inner Galway

Bay; ·Ph.D Thesis, National University ofIreland.

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Edler L. (1979) Recommendations on methods for marine biological studies in

the Baltic Sea : Phytoplankton and Chlorophyll. Baltie Marine

Biologists Publ. 5 : 38pp

Haines E. R (1976) Relation between the stable carbon isotope offiddleri

crabs, plants and soils in a salt marsh. Limnol. Oeeanogr. 21: 880-883

Harte A.M., Gilroy 1.P. & McNamara S.F. (1982) A computer simulation of

water ciircuhition in Galway Bay. Institute of Engineers of Ireland

Seminar: Galway Bay, an amenity and natural resource. University

College Galway, March 1982.

Hasle G. R. (1978) Using the Inverted Microscope. In: Soumia A. (Ed.),

Phytoplankton A,-fanual UNESCO, 191-196.

Hulsizer E.E. (1976) Zooplankton of Lower Narragensett Bay. Chesapeake

Sei. 17: 260-270

O'Mahony J.H.T. (1992) Phytoplankton Species Composition, Distribution &

Abundance in the Dunkellin Estuary, Galway Bay, Ireland. Ph.D Thesis,

National University of Ire1and. 279pp.

Riaux C. & Douville 1.-L (1980) Short-term Variations in Phytoplankton

Biomass in a Tidal Estuary in Northem Brittany. Estllarine & Coastal

Marine .)'cienee. 10: 85-92

• Smayda TJ. (1983) The Phytoplankton ofEstuaries. In : Eeosystems ofthe

fVorld (Ed. Ketchum RH.) Ellsevier, Amsterdam 65-102.

Throndsen 1. (1978) Preservation and storage. In: Phytoplankton Manual.

(Ed. Soumia-A) UNESCO. 69-75

Von Bodungen (1975) Der Jahresgang der Nahrsalse und der

"Primarproduktion des Planktons in der Kieler Bucht unter

.: :::Berucksichtigung der Hydrographie. Dissertation, Christian-Albrechts­

·.;tlnWersitat, ''Institut fur Meereskunde, Kiel. 116pp

Vip S.Y. (1980) The Ecology and Biology ofCtenophore species along the

coastal waters west of Ire1and, with special reference to Pleurobrachia

pileus Mu1ler 1776. Ph.D. Thesis, National University oflreland.

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