------~-
ti..
.
I 11I
~.M.
Intcrnational Coimcil for the
Exploration of the Sea
'
• . J-
1995 / L:25 Ref.C
Bjolog!caI,Occanography
ommlttce
,
' ' ' ' , , '
DISTRIBUTION AND TRANSI)ORT OF
MESOZOOI)LANKTON DURING TIDAL CYCLES
AT A I?JXED STATION IN ,TUE NORTHERN GERMAN
\VADDEN SEA
.'
' "
by
Michael R. Gcorgc
•
•
Forschungs- und Technologiezentrum Westküste
Hafentörn, 25761 Büsum - Gcmlany
ABSTRACT
In thc northern Genrian \Vadden Sea sampies of mesozooplankton ,verc
takeri at an anehor station in a tidal ehannel at intervals of 1 h over 24 h;
Species eomposition arid quaritities ehanged during the tidal eyclc.
Summation of the mimber of individuals per taxonomie group shO\ved
that the speeies eoinposition differed with the tidal phase. Soine speeies
groups, e.g. the eirripede nauplius-Iarvae, eehinoderm larvae and molluse
larvae were foimd in higher ablJndanee during flood tide, \vhereas other
·organisms, e.g. cirripede eypds-Iarvae, were more abundarit during the
ebb tide. Tidal variations of the various mesozooplankton groups in the
\Vadden Sea are discussed in rdation to a direeted transport as an
integrated part of Iife strategy of these organisins.
INTRODUCTION
The sediments of the Wadden Sea are relatively ,vell investigated, in
eomparison with the pelagie system. BeealJse of its special hydrographie
environmental fadors, it ean bc expeetcd, that the pelagie system has
some eharaeteristies, which differ from eoastril \vater bodies. 1'0 qualify
and quantify these differcnees and the transport proeesses bet,veen these
regions, a German inter-disciplinary project, ealled TRANSWATT
(transport, transfer arid trarisformation of biomass elements in the
Wadden Sea) has started in 1994, \vhich includes this study. This projeet
is mainly eoneerned with eutrophieation and nutrient fllJxes behveen
North Sea and Wadden Sea and teITestrial eoastal zones. Quantifieation of
plankton, Le. baeterio- myeo-, phyto- arid zooplankton, arid its transport,
is an important part of this projeeL
The Wadden Sea zooplankton consists of organisms from the North Sea
and from braekish water (Fransz 1983), duc to the hydrographical
-----
inOuence of the estuaries. Mesozooplankton of the \Vadden Sea near the
island Sylt mainly consists of calanoid copepods in terms of abundance
and biomass (Hickel 1975, Fransz et al. 1991, Beusekom & DielChristiansen 1993). In the German coastal zone in general the
mesozooplankton species assemblage is temporary dominated by
planktonic larvae of zoobenthos, i.e. meroplankton, forming the largest
part of biomass and abundance. Especially polychaete larvae and
cirripede larvae are abundant in \Vadden Sea areas (Martens 1980, Heiber
1988). The mesozooplankton communities are governed by seasonal and
tidal variations. Tidal variation of species composition was also described
from southem German coast between the Eibe and \Veser estuaries (Giere
1968, Heiber 1988) and for the Dutch \Vadden Sea (Wolf 1973). The
main objective of this paper is to verify, if there is a transport of
mesozooplankton towards the \Vadden Sea or towards the North Sea in
the studied area.
MATERIAL and METHOnS
Zooplankton was collected in spring (May,12. 14:00 p.m. - May,13.
16:00 p.m.),and in summer (July,16. 11:00 a.m. - July,17. 12:00 noon) of
1994. Hourly sampling over aperiod of at least 24 hours was done at a
fixed station in the main stream of a tidal channel (Hever) in the northern
(Schieswig-Hoistein) German Wadden Sea, on board of RV "Victor
Hensen". Co-ordinates of the anchor position in spring and summer were
54°28,7 N / 08°40,7 E. A quantity of 30 litres of surface sea water per
.sample was taken, using a 301 water boUle. The sampie was screened
through a 160jlm mesh sieve, in order to coHect mesozooplankton of the
size fraction 200jlm - 2mm. Immediately after collection the plankton
was preserved in a 1% buffered formalin solution. In the laboratory
quantitative and qualitative analyses were undertaken, using a stereo
mlcroscope.
The abundance of selected main zooplankton groups during tidal cyde
were represented by hourly frequency distributions (Fig.l, Fig.2). Only
mesozooplankton groups were taken into consideration, that were
continuously present. One daily tidal cyde consists of two Oood and two
ebb tides. '1'0 study a possible transport of organisms into the Wadden Sea
or towards the North Sea, or the presence of organisms in the same water
column, the counted individuals·m-3 per sampie of the two Oood phases
and ehb phascs were added separatcly. The resulting two sums, consisting
of an equal sampie number (12 sampies, total number of sampies: 24),
were opposed to each other (Table 1, Table 2). The extra sampies (>24)
of euch experiment (Muy,12. 14:00-16:00 und July,17. 8:00 + 9:00) were
disregardcd. Thrce assumptions were made to explain the resulting
differences between the sum of individuals per Oood tide and ebb tide: (i)
If the sum of Oood sampies minus sum of ebb sampies is positive, some
zooplankton organisms are retained in the Wadden Sea, (ii) if the
2
•
•
.
diffefence is negative, there must be a transport towards the North Sea,
and (iii) if ihere is no differencc or only a slight one, the organisms
cxhibit a uniform distribution in this arca. A high differcnce indicatcs a
relation behveen abundan,ce of the zooplankton group and tidal cycle. '1'0
compare zooplankton with constantly high densities arid zooplankton with
low densities, the proportional differencc of outflowing number in
relation to inflowing number, is also displayed (Table 1, Table 2).
RESULTS
•
Frcqucney distributions of, seleeted mesozooplankton groups (Fig.l,
Fig.2), during iidal eycles, demoristrated tidal ehanges in abundariee. In
May the number of individuals'in-3 per sampie showed variations of up to
a fivefold qlmntity in terms of cirripede nauplius-:larvae (l000-4833
ind. 'm-3 ) and to a tenfold quantity in the ease of cirfipede eypris-Iarvrie
(367-3833 ind.·m-3 ). The highest abuildance of eypris-larvae was
recorded at low tide, just after dic onset of f100d tide (Fig.l). The
cirripede nauplii reached the highest numbers in the middle of ebb tide in
this tidal ehanneI. Calanoid copepods, both adults and sub-adult
copepodites, revealed the highest abundance (2900-3100 ind.·m-3 ) during
the night hours (22-23:00, Fig.l). Hydrozoans, gastropod larvae arid
echinoderm larvae reached their maximum (567, 667 and 900 irid.·m-3 )
short before high tide.
"
,
.
"
In July the ealarioi~ eopepods (Fig.2), rriainly consisting of adults and
copepodites of AcaTtia spp.; CeJltropages halllatus and to a lower extent
.of Tel/IOTa IOllgicorJlis and Pseudocalallus elongatlts, sho\vcd generally an
extreme high abundance (mean: 10551 ind.·m-3 )in comparison with May
(rnean: 1364 ind.·m-3 ). Again the maximum (17867 ind.·m-3 ) in number
- in surfaee sampies - was reachcd dufing night hours (23-0 1:00, Fig.2).
The included rotifers c1early revcalcd the highest abundance (up to 8533
irid.·m-3 ) in the f100d phases. Cirripede larvae, especially the eypris-:
larvae, were not abundant like in May. The cirripede nauplii only showcd
a slightly higher number during f100d tides. The polyehaete larvae (max.:
14700 ind.·m-3 ), mostly bClonging to the spionid family, ncarly reaehed
the same doininarice as the calanoid copepods (Fig.2). The abundance was
slightly highcr during f100d tides. Hydrozoans, mainly consisting of
Obelia spp., Rathkea oc/opuliciaia rind Bougaillvillia ra11l0Scl, showed a
threefold increase (mean: 729 ind.·m-3 ) in comparison with the tidal
cyclc iri May (rnean: 233 ind.·m-3 ).
.
The ealculated differences betweeri fload and ebb tides, dtiring the spring
experiment, areshown in Table 1. The results revealed, ihat of ealanoid
copepods~ cirripcde nauplius-Iarvac, polychaetc larvae, eehinoderm
larvae, gastropod larvae, bivalvc larvae, cyphonautes-Iarvae of bryozoans
and in hydrozoans, more individuals entcred the \Vadden Sea with the
flood phascs, than rifterwards left during the cbb phases. In contrast to
these firstly mcntioned organisms, in other rriesozooplarikton grouiJs, as
3
..
the cirripede cypris-Iarvae, the trochophore-Iarvae of polychaetes and the
harpacticoid copepods; more individuals leave the \Vadden Sea, than enter
it. In calanoid copepods, in polychaete larvae and in hydrozoans the
number of inflowing and outflowing individuals is not distinct, meaning,
that they are probably distributed in a homogeneous way in this part of
the Wadden Sea. A small iriput of organisms into the \Vadden Sea can be
seen in echinoderm larvae, in larvae of molluscs and in the cyphonauteslarvae of bryozoans, which do belong to the meroplankton. The most
remarkable difference by number can be seen in cirripede larvae. Over
50% of the entered cypris-Iarvae were missing in the ebb tide and
therefore remained in the Wadden Sea, but 38% more nauplius-Iarvae of
the cirripedes, than entered the \Vadden Sea, were carried out. The
harpacticoid copepods, not representing high densitics in surface sampies,
seemed to showastrang offshore directed transport (Table 1, see also
discussion).
The species composition of the second tidal experiment in summer 1994
(16.-17. July) differed from that of spring. As a consequence, some more
species groups were included in Table 2 (ctenophore larvae,
appendicularians, rotifers, Noctiluca), others (echinoderin and bryozoan
larvae) were left out, because of neglectable low abundance. The data
revealed a more than doubled total number of mesozooplankton
organisms in summer. This was also displayed in the absolute difference
in number of individuals per tidal phase (Table 2). The second tidal
experiment showed a different pattern, in tidal distribution of
zooplankton groups, from that of May. Cladocerans (Podoll and Evadne
. species), which were always found in low densities, seemed to be
transported towards the open sea. With the exception of the cladocerans,
all main taxonomic groups exhibited positive values of numerical
difference, meaning the entering of organisms into the Wadden Sea was
stronger, than towards the North Sea. The highest difference is
demonstrated by the rotifers. 61,22% of the inflown individuals were
missing in the ebb tide. A strong iriput towards the Wadden Sea was also
recognized in cirripede cypris-Iarvae and trochophore-Iarvae. Referring
to the cirripede larvae the transport direction was in agreement with the
first experiment. Trochophore-Iarvae were also transported into the
Wadden Sea in summer, in contrast to spring. The abundance of
hydrozoans, the ctenophore larvae, the mollusc larvae, thc polychaete
larvae and cirripede nauplius-larvae was slightly higher during the flood
lides, meaning that about 20-35% of these organisms remain in the
Wadden area. The most important holoplanktonic groups, Iike the
appendicularians, the calanoid copepods and the dinoflagellate Noctillica
scintillans, revealed an approximately equal abundance in Oood and ebb
tides, probably duc to a completely mixed distribution in the studied area.
4
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•
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•
DISCUSSION
.
.
The attempt to describe a qualiiative transport of orgariisms from or irito
the \Vadden Sea may not yet be interprcted quantitatively. For this aim a
continuous .plankton recording over a coinplete tidal eyele would be
necessary, in addition to biomass estimates. Also thc water transport and
its vCiocity riced to be investigaied. A possiblc lateral transport of
zooplankton organisms in this study is disregarded. Nevertheless, the data
rcveal a distinguished reaction to tidal eycles in each taxonomie group. In
some meroplariktonie larvae the adaptations of their life eycles to
environmental conditions seem to rcsuIt in an active use of tidal eurrents
to bc transported into other regions. This is most obviously the ease in
cirripede larvac. Thc eypris-Iarvae, just before their metamorphosis to a
benthic organism and not feeding (Luther 1987), are transported into the
Wridden Sea, whefe the possibility to find an adequate substrate to seitle
dowri, is mueh higher near the, coast. In contrast to. this, eirripede
ririuplius-Iarvae, in the phase of distribution of their spceies arid feedirig,
are transported to offshore waters~ A high abundance facilitates the
distribution and the probability of survival in this developmental stage. In
siudies of other regions, e.g. the Duteh Wadderi Seri, similar, supporting
resuIts were established. Wolf (1973) assumed an aetive swimmirig of
cypris-Iarvae as, the only explanation for thc continuing presence of
groups, i.e. patehy oceurrence; of these types of larvae, even in tidal
currents with aceompanying high turbulence. In eirripedes the eggs are
kert \vithin the sheH, until grown-up nauplii are released (Luther 1987).
As pelagie eggs usually undergo a high mortality, keeping the eggs in a
'safe cover and releasing lrirvrie; which are, able to swim, mriy result in a
higher survival rate, without increasirig the reproductive effoit. Larval
release of the nauplii in the cirripede genus Semiblilalllls, \vas found to bc
coupled with phytoplankton blooms, meaning the availability of food, and
,vas stimulated by a high concentratiori arid elose coritact of the diatom
Skelelollema ,(Starr el aL 1991). All these results demoristrate rionrandom distribution pattern of meroplanktoniC larvae, Iike in the case of
cirripedes, but all behaviouf, weil ridapted to the environmcntal
conditions.
The role of thc harpacticoid copepods iri the Wadden Sea; up to riow,
remairis unelear. Beeause of hydrographie conditions, as the strong
turbulerice, also meiobenthic harpacticoid copepods werecontiriuously
found in plankton sampies (Giere 1968, Heiber 1988, own data). As they
are relatively aburidant, it is ·intei-esting to kno\v, whether they also feed
in the water eolumri arid therefore also rriay play ari important role in
trophie relatioriships iri the pCIagie system of this area. After Bouwmari
(1983) meiobenthie hrirructicoid copepods are consumed by polyehaetes,
shrimps and f1atfishes~ The observed transport of harpacticoid copepods
to offshore regions in the spring experiment rriay not rcflcct normal
conditions. A transport of coastal meiobenthic copepods to\vards the
North Sea by tidal cycles would mean a loss of individuals into deeper
a
5
.,'
water. The summer experiment in contrast showed a reversed transport
of harpacticoid copepods into the Wadden Sea.
The high percentage of benthos larvae and the obvious seasonal transport
of meroplanktonic larvae in and out of the \Vadden Sea led to the
conclusion, timt this coastal zone mayaiso servc as a nursery area for
benthic organisms, similar as it is al ready described for fish larvae and
juveniles.
ACKNOWLEDGEMENTS
The TRANSWATT project, conducted by Dr. K.-J. Hesse, is financed by
the BMBF (German Ministry for Education and Sciencc) under the code
number 03F0130A. For reviewing the manuscript, I thank Prof. Dr. F.
Colijn.
REFERENCES
Beusekom, J.van and Dicl-Christiansen, S. 1994: A synthesis ofphyto- and zooplankton
dynamics in the North Sea environment. WWF International Report, pp. 1-146.
Bouwman, L.A. 1983: The meiofauna of the Ems estuary. In: Dankers, N.; Kühl, H. &
Wolff, WJ. (eds.) Invertebrates of the Wadden Sea. Rep. 4 Wadden Sea Working
Group, Balkema, Rotterdam, pp. 153-157.
Fransz, H.G. 1983: List ofthe zooplankton species ofthe Wadden Sea. In: Dankers, N.;
Kühl, H. & Wolff, W.J. (eds.) Invertebrates of the Wadden Sea. Rep. 4 Wadden
Sea Working Group, Balkema, Rotterdam, pp. 12-23.
Fransz, H.G.; Colebrook, J.M.; Gamble, J.C. and Krause, M. 1991: The zooplankton
ofthe North Sea. Neth. J. Sea Res., 28 pp. 1-52.
..
Giere, O. 1968: Die fluktuationen des marinen Zooplanktons im Elbe-Astuar.
Beziehungen zwischen Populationsschwankungen und hydrographischen Faktoren
im Brackwasser. Arch. Hydrobiol., SuppI. XXXI (3/4), pp. 379-546.
Heiber, W. 1988: Die Faunengemeinschaft einer großen Stromrinne des Wurster
Wattengebietes (Deutsche Bucht). Untersuchungen zur Struktur und Dynamik in
Abhängigkeit von Gezeiten und Jahreszyklus und Folgerungen zu
Austauschprozessen zwischen Wattenmeer und offener See. Dissertation: AlfredWegener-Institut Bremerhaven, Universität Bonn, pp. 1-398.
Hickel, W. 1975: The mesozooplankton in the wadden sea of Sylt (North Sea).
Helgoländer Meeresunters., 27 pp. 254-262.
Luther, G. 1987: Seepocken der deutschen Küstengewässer. Helgoländer Meeresunters., 41 pp. 1-43.
Martens, P. 1980: Beiträge zum Mesozooplankton des Nordsylter Wattenmeers.
Helgoländer Meeresunters., 34 pp. 41-53.
Starr, M.; Himmelman, J.H. and Therriault, J.-C. 1991: Coupling of nauplii release in
barnacles with phytoplankton blooms: a parallel strategy to timt of spawning in
urchins and musseis. J. Plankton Res., 13 (3), pp. 561-571.
Wolf, P. de 1973: Ecological observations on the mechanisms of dispersal of barnacle
larvae during planktonic life and settling. Neth. J. Sea Res., 6 (1-2), pp. 1-129.
6
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'fable 1: Summation of flood and ebb phases by number of individuals I
m3 of mairi mesozooplarikton groups, during a 24 hour tidal cycle
experiment iri May 1994; (+) means transport into \Vadden Sen, (-)
means transport towards North Sen, (%) percentage ofindividuals
from flood, which lack in the ebb tide and remain in the \Vadden Sea
.-
taxonomie group
•
calanoid copcpods
harpacticoid
copcpods
cirripcdc eyprislarvae
cirripede naupliuslarvae
polychactelarvae
troehophore-Iarvae
cchinodcnn larvae
bivalve larvae
gastropod larvae
eyphonautes-larvae
of bryozoans
hydrozoans
L individuals of2
L individuals of 2
inflow phascs
16600
533
absolute
diffcrcnce
+400
-867
+2,41
-162,66
9533
20401
+10868
+53,27
36366
26300
-10066
-38,27
17734
19368
4332
8334
3000
1301
..
2532
18301
17266
5432
10432
3701
1633
+567
-2102
+1100
+2098
+701
+332
+3,10
-12,17
+20,25
+20,11
+18,94
+20,33
+303
+10,69
outflow phases .
16200
1400
"
-'-~"
"
,~."
,'"
.. '
2835
in%
"
.-
Table 2: Summation of flood and ebb phases by number of individuals I
m3 of main mesozooplarikton groups, duririg a 24 hour iidal cycle
experimerit in July 1994 (further explariatioris see above rind in text)
~
..•. ,.."
taxonomie group
•
hydrozoans
ctenophore larvae
gastropod larvae
bivalve larvae
I polychaete larvae
troehophores
cladoccmns
ealanoid copepods
harpacticoid
copepods
appendicularians
cirripcde cyprislarvae
eirripcde naupl.larVae
rotifcrs
Noctiluca/flagellatcs
.
L individuals of 2
L individuals of 2
inflo\v phascs
10733
2033
6667
18033
127933
2967
867
137233
8033
absolute
difference
+2500
+500
+2300
+6000
+30100
+1333
-200
+133,
+3067
14633
1367.
16833
2300
+2200
+933
+13,07
+40,58
15367
20333
+4967
+24,43
+12000
+205933"
+61.22
+5,37
outflo~.v
phases
8233
1533
4367
12033
97833
1633
1067
137100
4967
in %
+23,29
+24,59
+34,50
+33,27
+23.53
+44,94
-23,08
+0,10
+38,17
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7600
3626967
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HT
LT
HT
LT
-----'1
Tidal cJcle
HT
Fig.l: Hourly variation or the abundance (individuals/m3 ) in mesozooplankton groups during a tidal cycle experiment (27 h) in
the northcrn German \Vadden Sea (Hever, 12.-13.5.94), LT=
low tide, HT= high tide, shaded areas symbolize nood phascs.
.,
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I Tidal cJcle
LT
Fig.2: Hourlyvariation of the abundance (iiidividuals/m3 ) iri meso-
zooplankton groups during a iidal cycle experiment (26 h) in
the northern German \Vadden Sen (Hever, 16.-17.7.94).