Not to be quoted without prior reference to the author
C.M. 1993/ L: 25
International Council for the Exploration
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.
•
from 9 major aIgal groups.
185 taxa were recorded
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 08 0 51 to 09 0
3'W.
Location and station positions are shown in Figure 1. The study area
encompasses circa e 19 km 2 . 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.
exposed at low tide.
5m
SOUdl
South of Tyrone Pool the Yellow Rocks are
The region deepens into Mweenish Strait, to a depth of
of Mweenish Point. West of Mweenish Point and Island Eddy the
bottom gradually deepens, reaching a depth of 10m SE of Kilcolgan Point.
•
.'
.
. .:.:..... 0; 0::
GALWAY
BAY
N
l'
.......
Co. Clare
(a)
l-.-
--r-53'
L.-
Fig. .i..
(a) Location and (b) station Positior$.
N
l'
4
SE KILCOLGAN
POINT
~
.
~
";'" "
.......
,,'
. 0:
;
..........'" .... :0: ':, .. ;. ;0°:°•.::, ::.'" .
. ~:.'"
::::: ..
......
.t:!
Dunkelli~
EStua-ry.
!.
"-'1
Soul~
_W"VII:
i
aay
..-:;..:...- Wl.VS
/
I~
(
~.o
o
.
----~--.
.. " .
Fig.
2.
. .
1000
..
Bathymetric Contours & Place Naaes, Dunkellin Estuary.
.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.
following two years.
1984 was warmer and dryer than either of the
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
•
Ja
'"'
~
.
•
"Il
'~
t
13
•
E
•
~
'li
I.
I
0
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 J
rirru
TIf.t.IIII.t.'ONDITll.t.ll/J!SOIlDJTll.t.ll/ J
(~/u)
ri ..... (mDntA6)
Fig.3.1. :Mesn 1donthly .Hr Temps.. Gallfay, 1984-1988
Fig.~ lfindspeeds, DunkeJlin Enuary, 1984-1986
tSr---------
--,__-
ro
15
10
I
I
,
4
,
.
,
3<o.u/orl ScuJ..
•
Fig.3.2a
.
CLoud cO'"..wro.g. (oeHm')
Wind force, Dunlcellin Esluary, 1984-1986
Fig. 3.4.
Cloud Coverage, Dunl:eIEn &ltuary, 1984-1986
30,------
11.---------------------,
,
'"'
"
.,
13
~
G'
s:
~
'-
.i
~
...
~
~
~
"Il
~
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
, li A. XI
1 J. S O:i D i r liI A. W 1 J 4
~o X D 1 ., W
J..w
J 1
ri"", (rn.on Uu)
FI.g:a.5. Rlvertloll' Rates, Dunkdlin Estuary, 1984-1981
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
- 19.4 0 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 cn
eDDlng 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.
considerable fluctuations were recorded
For POC's,
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/m 3 .
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.i
-0111
-
-Olll
- -Ww
---------SL-t
---
lllJ,--------
ULi
~Ww
'lw
-OUl
-
-WUl
••
Zm
:i
~
o
t
~.~.
IIl1dntnlTlTJlTn1flT~11II1mlllllnt~~~II;~lTTilllllimrrßlI'IIIIlIlIrll1;;;1I1n
,,
1
J,"'J.W.J'.LSOHOJYWAWJJJ.HOr;DJrW ... WJ
f'J7uZ
Fig.
Oll
b"
1
o
10 -
~~~-
1111111 rrlllllll1nrmll 1111 1I111l1Tl1ll1lllllllllllllllllmilrullllmllnrll1llntllnrrrrITrm1l11T1mmllmm
JrW.l
.. JJ
.1S0UD
(l1~J\tlU)
Jj.
JP){.6.)(J
Fig.
,----------------...,---S·1.-~2-~
-011\
Gd
l\ /'
.y~ -- - V'
11/11 1II111111lllllnr rm11l1llnr I~ 11 111111 IIlllnrnllnmrrnlmlnrnrllfllTmmnrml1Tl1rrn IIIl1l1f11rrrrrmTTlIl
SO)iDJrw.J.)t
"'SOHDJJ~AW.J
JrliAW)J
]"L"'U (rn.onOu)
Nilrale~, SL 4, Dunkellin Esluary 108-1-19&1
,
~/~;;~,/~vGJ'----/
ri.~
PO·I-P, Sl. 4, DunJ.:ellin r:.,.luary 1004-1986
Fig.
60
, '-----------------,'---;0;-;:----,
bS
lASONDJYWAWJ
(m.. ontJu)
Silico leg, SL4, Dunkcllin ßsluary 1984 -19&1
---------------r--=-=--~
Sl. 2
Sl. 2
. - ÜLl
-0", .- 1m
-Ow .- ün
00
00
'0
l~~ /~---'\ j\"..P,.
'0
,
~!."
•
,
o IIllnlllllllllmrm1lTIll1ln1l1111mll1l1mTIrmlllllIlI1I111l1tmrliinl1ll1l11l1ll11lJ1l1lllllllTllllllllllnilllllfllll
J rllAA WJ J
4S0
J r:w i\l.lJ
ND
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
L=--..9
ttl
~~
20
..
10
-
,
\
\
\
'
......
~,,'
),I
A lof J J
I'
j,
~
0
}l
D J ,),(
.l
JAS 0 N IJ J , W
104 J
j,
~
St.
~
~
DUllkcllill t:utullry 1064-19UO
._._._.
__..
..
1\ t.-
~,~~-\:j\2\,
JrWA"'JJ
J J
TnHe (lt\OH/h.:f)
Pho.ph~lc.,
\
1\/
/~,
A
.r-l/'\
I
nrrnll(JmmmrrnrnnnlrfrJIllnnrnJrJlI1rrnnTlmrrri~m~mnmrmnnnrrmIflßlrnnnmmmrri'rrur
1lIIlIllI1I1IlIlIlIllIlIlIrrl1rnrllnmnrrrl1l1ll1llfll1l1111l11T111T1111111111I11I1I1IlI1Il1T1rnllnrnml1llllrmllnr
J ,
.----·---··-----·---------I-·St~I--·-
80
~
I
I
Yig. IC'(:.
10
I
\
I
I
I
_. ---'--
60
..
/'
.g. I::>h
i.80HDlrW.LUJ
_..
JA80NDJ'WJ.WJJ
T"lne (TT\(YuU13)
Stllc"Le., St. .!, DUllkellin K.luary. 1064-1068
-----------
Sl.1
_ . _ ~ _
-Url\
--2'1\_
Sl.1
-011\
.-
Z,,\
... Z"l
'10
iO
'0
~., ,~~Ak, , J:~-'v:~4V'/ v'Lj-~Vv~.,
11
11
'~A
ITrrlllllllmirfilnmmTTnflrrlll1llll1llmTIll1Irllll1lllllll1mllniiilfJlllllllml1lll1llllmllfllll1l1III 111 1111T
I" W" \l J J " S 0 N D J r l/ A l/ J J A ~o N D I Y l/ All I
J
l_
Fig.~:
ri~
Nitrate., St.
(nlon(/u;)
1. Dunkellin &'luary, 1084-lOtllS
A-, j}'l
;'
\~
Illi 1IIIIIIIIm 1111111111 111 11 1111 111111 1IIT111111 11 111 11 In 111111111 m1I11f11l1l11l1l1ll1lll1llllmlllfll11111 11111 milli
J
rliAWJ
fig.
(.-.C
"-1
JA~ONU JYll.lWJ
Tunt:
J.
(nlonlh..3)
-
:l
00
SON
DJr",
... UJ
I'llo.phulc •• Sl. I, iJ\lUkdllll E.Luary, 108-1-1006
~
A".
-».;!\,
. ~U.~-
~ _j/~V-':~:"
mllmlllllrrrmmlllmllllrn11l1111llllllllllflllllllllmnrrnll1 mmnnmrllllflllflllllnllmlllrrrnnlirrni
J
,
~ .L
r .'
fig. Ic.<.
WO J J &. S
0 NO
J t' W J.. ~ J
Tün~
(11'l011
J.l
th,,)
SO N
0 J
)':W }. U
Silicule., Sl. I, DUllkellin &.lu~ry, 1084-1060
J
~ i-_-Om--_-.~--4.-.z .,,--.--------------,
I
,
1600
I:, \J'~;"~
t
~
.
~c
600
,"
:
,
I'
I,'.
: _,
I,:
\
'
::~.,
\
...
I
."
1/
J~.JII1111,mll11Tl1A11Tl1I1I11111/~1mA;nm31ßT110~NmD:mn1!1n:,~' m.llTlIrn,rr:'Jrn'~/mn1ß!I3lTO~N!Jll!D:rrn:/rn,!!Tl1:II~mll~~
nlllllT•
..
..
Time (montM)
........
1-0:.
Fig.
POCs. SL 4. Dunkellin Estuary. 1984-1986
'-am
_olm
;~
11
I ,
15oJO ,..
,
,:.r-~
'I; \,
./11\1
•
:t.,.;;,,;,;,_...,~
J1I1AII1/"8 OND /111.111
Fig.
:+d
I .. SON Dl1 11.011
I
Ti m.c ("",niM)
Chi a. SL 4. Dunkellin Estuary 1984-1986
I
I
{\h
SI.. 2
2600 i
-. Zaa
..,~
,'"
,
'.I
\
St.-t
-"WCD
:1
'1\
t
1000
;-Om.
St.2
._.=rn
-Om
f\j i
I
f \ ,r,
., "I
.'fll\'
h
L[ :\""yv\/: r~~ß 0\1 ~
1\
'I
!>:()i:
r
t
:)
!Ir!"
~:l ur 11" 1111 1Il iul 1111 i I1lll1 i 111 i d lIIl ililllilld ,LlII.. II;l,I', ,;l 1.10 I.tlli II! 11 11 ,11 Li, i' [, .. 11l1 d. t1Il t111 1111 d 111111
1111.IIJJ.SOllDlrll<IIJ/.SO!<1lITlI.1I11
rig.
250U
TiTnel' (TT\OT\tn.3)
:.:J- b
r
su
~o I
l-om
:
~ -Dm ·-Zm
:~
"
:~
Fig.":}- oe..
POCs. SI.. 2, Dunl<ellin Estuary. 1984-\960
:\
~~! 1
\1\
':i ~\lrfv \/ ~\ ~
o
~i i ili i i li i l"'i "'i i i li i i li i li i li 'i " 'i i li lili i i li i i li i li li "i 1I i 1i li'i "i lilil i il l
J
r
Fig.
111 A.
W. J I
1<:.
'" S
0 ND
J
r w.
A W l
I
A
so
N
D I
1 W.
A
Time (moT\!h.»
POCs. Sl. I, Dun keil in Esluary. 1984-1900
~o
St.l
--Zm
I-
t
10
/\
/I
_; " .~ ";. (", lr.r ><'~ \~\- - -: ; ; r.; ;";:;~,., , ;~
o '-mrririmmmiilimiiilrimmmnm-rir,rrn-m-ITlr;tr!m;111.
,I
I
•
11
1111.101,11
O:fO
w: J
Fig.
=1'f
Jrw
Ti'tT'\e
AW.l
1 ...
(Tnonth.3)
SOX
Dl7li1
A."l
Chi a, Sl. I, DlJnkellin tstuary, 1984-1900
, ...
!ahle ..1-
Percentage Class Representation - Dunkellin Estuary
CLASS
% CONTRIBUTION
80
43.2
Chlorophyceae
4
2.1
Chrysophyceae
4
2.1
Craspedophyceae
1
0.5
Cryptophyceae'"
4
2.1
8(;
46.4
Euglenophyceae
1
0.5
Prasinophyceae
2
2.1
Bacillarophyceae
Dinophyceae
•
NO. OF SPEeIES
1.6
3
Pyrmnesiophyceae
--------------------------------------------------------------
185
100
'" includes ~esodinium rubrum, unid. microflagellates, and Sp. F.
.."
·
.
.;
Do.inant Phytoplankton, Dunkellin Estuary,
Table . '2.
1984-1986
DIlfOFLAGELLATES
DIATOIiS
BACILLAROPHYCEAE
DINOPHYCEAE
(26)
(11)
Ceratium lineatum
Dinophysis acuminata
Glenodinium foliaceum
Gonyau1ax polyedra
G. spinifera
Gymnodinium simplex
G. splendens
Heterocapsa triquetra
Prorocentrum micans
Scrippsiel1a trochoidea
Unidentified dinoflagellates
Ceratulina pelagica
Chaetoceros spp. (sma11)
Chaetoceros spp. (1 arge)
C. curvisetus
C. decipiens
C. simplex
C. socia1is
Leptocylindrus danicus
L. minimus
Nitzschia spp.
N. c10sterium
N. delicatissima
N. seriata
Unidentified pennate diatoms
}lICROFLAGELLATES ( 8)
CHRYSOPHYCEAE (1)
Apedinel1a spinifera (W)
Rhizosolenia delicatula
R. hebatata f. semispina
R. ?pungens
CRYFTOPHYCEAE (2)
lfesodinium rubrum
Rhodomonas ? minuta (Z)
Ske1etonema costatum
Thalassionema nitzschoides
Thaiassiosira spp.
Thai sp. AB
Thai. sp. C
T. gravidajrotula
T. rotulajgravida
T. nordenskioldii
T. polychorda
EUGLENOPHYCEAE (1)
EUtreptia marina
PRAS I NOPHYCEAE (1)
Pyramimonas sp. (Y)
PYIDlNESIOPHYCEAE (HAPTOPHYCEAE) (1)
Chrysochromulina sp. (V)
Unidentified microflagellates
Species F.
Tahle
3 .' lfean Phytoplankton Cell Concentrations, Dunkellin
Estuary (cells/1itre)
. station No.
Depth
Surface
1
2
199,507
164,610
73,354
42,057
l1id
Bottom
4
190,480
69,581
28,551
b't.1
--W:1ll
'Lw
SL -t. Surrnce
-Dt - - Vn
VI
T
\
\
o
iSONDJ,ll{J.WI J.l.SOKDI1W1WJ
1'ü"-I.: (Hl-DuthJl)
J
"}!l
IfllIl1ll1 rml IllIllmnrrml1TTflnmm nmmnnnmllmmmTTrmrmmmmmmmlmmmmmmm
lYllAIIJJ
ASONDIYlIAIII1ASONDIYlIAWJ
1'irn. (mont1ls)
Phyloplankton A!.>undance, Sl. 'I, HllH-I\Jßß
Fig.
C.l Ct
Phyloplnnklon Groul' Auuuduoce Sl.4, 'B4-'ß6
Sl. 2.. Surface
.....,
f
,I
,
,
2
-
1
-
o
\
1\
1
-
"\j':VAA, ~
-~'
- , ,
'v
I
\
\
\/
- -,
I
'
/
,
\
I
Dl
- - Du
Q
~
0
n'
:il
111
,"
~
"
~
I
'"
1,,1
o
\, },'
i
,,
~
"
I
0.,
2'
mnlllllnllllllllmlnUlllllllmnUlIIlIllrrlllnll1l1lll1l1l1l1llllTllIlIllllIlIlIIlllßlIIUllIlIlIllIllI1I111IIUnifi 11
J''WAUJJ
Yi/:,
B
ASOHDJrW1..UJ
"'"-1
JASONDJP)l.lUJJ
::
F'ig.
__ lJl
N
,'\~ ~
,
•
,-_J:,
,
,..
:"
::
,
11
11
\
\
11
j\
I
I
I
11
\
11
I
j
11
'I
\
I
I
I
11
11
\
11
\
tl
t
\I
I
::
I
I
\/
'':;
YiC'
U J J .1. S
,-,.e
0 ND
Tlmc
Phytopl"nklon
AU J
1
I
A
SO
r".lordh.,)
I~
0 J
., WAU
AbUl\lI~I1C<, Sl. I, lUIH-)UUU
J
!'
"I i
:
•
""
1
j
111111111111 m 1111111111111 11 11111111 rn milli I1 m 1111111 11 m11 fIllIIIIllTITIIIIIJ rr IIIIIIIIIITT rr I11 Ilmlll m111 1111111
F 11 A 11 I lAS
Fig.
U
r ~
I
1 I
\: \;
1
11
J
,
:/
o
IlnmflßlrrnI1lIlIIllTTI1lIl11llnmlOllllllllrrnl1f1I1lIIIflIlTlUfIlUIlIllIIIllIllUT1UIUTllnlTIIII1U111111111111
, W .l
"';1
: 1".
.;;.
11
I
';:,
~:,
I
I1
I I
I I J
t 11
I V
: :,
0
: "-,'
: : :
J
VI
~,tl
I
11
11
\
. - - Dn
_
r!v4 ~
--ZIt\
\
.
------- -----_.
--5\-.I---J
1
A~O ND I F l i A l l l l A SON P l r l l A I I 1
'1 b Phyloplo.nkÜrW 61!2>mt \i1undo.nce, Sl. 2. 84-86
7~-----------"'--------------~
Sl.1, Surface
Pbylopl.llklon A!.>und .. ilce, Sl. 2, )\)O·\-H.lUO
ö
_-Om
\/
o ilirmmimmmlmnlllTTlimrrllTllTTmlrrmlnrl rmmmnTllllTmmrlnrrrrtrmTllTTllmTrrrmnnlTllrr
Jl'llA 1111
Tone ('lnOnUt3)
b
.... UI
'---
\
'\
i:
\ :1 q
.. , :
rnmrrrrnnnnmnnnrrrrrmrrrnrrrmnnnnnrrrmrrrnnnrnnnmrrnlllTlllllnnlllJmmnJlmnflllnnI
JrW1WJ
:jU
I.!:
9 C.
0 N 0
I I' 11 A 11 I
Time
1 A
("'V:J,"ntll~)
~
0 N
D 1 I' l/ .A l/
Phyloplunlclon {;roolp Abunöo.nCG, Sl.J, tg8·1-Hlß6
1
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 «8 0 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
•
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,
Ceratium Iilleat1l11l at station 4.
and the dinoflagellate
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
In October and
Skeletonema costatum.
occurred frequently
R. hebetata, L. minimus, C. clln1isetus and
due
to bad weather.
ovember sarnpling disruption
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),
.--~~~-
----~-~--
microflagellates dominated at station 1, Thalassiosira rotlIla at station 2 and
Thalassiosira rot1l1a/gravida at subsurface depths at station 4..
Discussion
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.
•
.
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 offiddler
i
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.
L:25 (p. 9)
"I
••
3:),-------:::----
-,
:lO
Ii
2:l
"""
V)
'-
~
20
.~
10
~
Cl
V)
10
6
51--_ _-:::-:-_ _--,
st...4
st...4
-Om
-)(m
-Om
·····Zm
hm'~WiTinA"IT!'!Wm!iJ'iiJ1imA.!'1'!S~O~N~DITTI!J~,.1"!W'!lT1A.~IIl~IITTl!IITJ!lA~S~O~H~D~J~r~W~An;II~I~
0J
4-
Fig.
Temperalure, SL 4. Dunkellin E.!lt. 11184-1986
4pr------
-----:
·····Zm
O~TlTl1lTTT1TTll~1TTlml!ffi1!~!f1TT!1T1!ßT'1TTlm~~~nmrrfl11.!lTlTI!T!Tmnrr~ß!1
l r l l A l i l l A S OHD I111AWl JA SOH D I r II All
Ti"... (mDnUu)
Fig.
-)(m
Til'7U! (mDnt~)
4-
Sa1inily. SL 4. Dunlcellin Estuary, 1984-1986
36,--------:-----
_
15
20
10
ICi
10
Sl2
1\
-_olm
-Om
ohmmTIiiiinTITTmi~~ITI1J.I1l:!.ffi11IT~~f1!I1T1!~~PJ1!1Tl!JT1!.T1~n::rm!rrJ!rl
IrllAII.JJ ASONDlrllAWIIASONDIP)(AII.IJ
Fig.
Ti71U
4-
I FIIAIlJ 1 ASO ND I FII AIlI IA SOH DI P10I All I I
Time (mcm.th.3)
(m.cm.th.3)
•
__-,
-_olm
-Om
Temperature, Sl. Z, Dunkellin &.tuary, 11184-1986
~r-------------------
F----;:S:7"""""";;2--'
l
F'ig.
LI'
Salinily, St. Z. Dunkellin btuary, 11184-1986
35,---------------------~
\
30
\
\
\
VI
I
\
\
I
I
\ I
\I
20
20
10
\
\
I:>
\
I
\
10
su
5~--=_.,__-__,
SU
-Om --Zm
o
ImmrmmmTIf!lTfrITiinmT!fm!!~~~~rJT!'T'!'~fI!!.fTTT!~~~
J Filii lII.:rlAS OND IFIIIAllll 11. SON DIrIlI 1.111 J
F'ig.
Lt
Tim.a
(TnOnth..B)
Temperat.ure. St..l, Dunkelnn ElIluary. 11184-11l86
-0=
--Zm
ofnlTIm-mmiiTiiimm~~~J!11l!'T!~~rJ.l!l!lll:~~~
J FIIIA WJ 1 AS OND 1 rw Alo{l I I. SON D J POlo{ AW
Fig.
Y-
Ti7T\oll
(month..B)
Salinity, St. I, Dunkellin "E8tuary. 11l84-1IIB6