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Limnologica 30 (2000) 175-182 LIMNOLOGICA http://www.urbanfischer.de/j ournals/limno © by Urban & Fischer Veflag

Institut ftir Hydrobiologie und Fischereiwissenschaft, Universit~it Hamburg, Germany

Long-term Investigation of the Distribution of Eurytemora affinis (; Copepoda) in the Elbe Estuary*

ANDREA PEITSCH, BRITTA KOPCKE • NIKOLAUSBERNAT

With 4 Figures and 3 Tables

Key words: Estuary, Eurytemora affinis, distribution, river discharge, transport

Abstract

The seasonal and longitudinal distribution of the Eurytemo- rytemora was described for the Baltic Sea (VuORINEN ra affinis in the Elbe Estuary was studied using long-term data from RANTA 1987; ERIKSSON et al. 1977), the Schlei Fjord 1986 to 1994. At an anchor station in the oligohaline zone of the es- (ScHNACK & BOTTGER 1981; CHRISTIANSEN 1988) and the tuary maximum abundance was always found between April and the Barther Bodden (ARNDT 1989). Different investigations de- beginning of June, but maximum numbers showed high interannual scribe Eurytemora to be an important zooplankton organism variations. During longitudinal sampling maximum abundance of of European estuaries: in Severn Estuary, England (MooRE Eurytemora affinis was found at different geographical positions be- tween the city of Hamburg (kin 630) and the North Sea (kin 720). et al. 1979); in Forth Estuary, Scotland (RODDm et al. 1984); Variation in temperature and salinity could not sufficiently explain in the Gironde (CASTEL & FEURTET 1986); in the Western the observed differences in distribution patterns of Eurytemora affi- Scheldt Estuary (DE PAUW 1973; BAKKER & DE PAUW 1975), his. River discharge turned out to be the most important factor con- in the Ems-Dollard-Estuary (BARETTA 1977) and in the trolling the geographical position of maximum abundance within the Weser (SOLTANPOUR-GARGARI~Z WELLERSHAUS 1984). estuary. The observed distributions along the axis of the river sug- From January 1986 to December 1995 the Elbe Estuary gest that the main habitat of the copepod is located in the freshwater was investigated under different aspects by the research pro- region of the estuary near Hamburg. The position of maximum ject: "Relationship between abiotic und biotic processes in abundance is located more downstream during periods of high river the tidal Elbe river". The calanoid copepod Eurytemora affi- discharge and more upstream during periods of lower discharge was the main subject of different studies on zooplankton rates. nis (BERNAT 1988; POSEWANG-KONSTANTIN1990; ORTEGA 1991; PE~TSCH 1992; KOPcKZ, unpubl, data). Special aspects of dis- Introduction tribution, population dynamics, production and grazing rates were investigated (PEITSCH 1992, 1993, 1995; BERNAT et al. Estuarine research has become more intensive in the last 15 1994). The present study tries to fill the lack of long-term in- vestigations by summarizing data on seasonal abundance years but there are very few long-term studies on zooplank- and longitudinal distribution of in the ton distribution. The zooplankton of the Elbe Estuary was in- Eurytemora affinis Elbe Estuary from 1986 to 1994. These data could give more vestigated by JENS (1953), KtXHL & MANN (1962, 1967) and insight into the factors controlling the distribution of the SOLTANeOUR-GARGAPd & WELLEr~SHAUS (1987). The cala- dominant copepod within this estuary. noid copepod Eurytemora affinis accounts for 90-99% of zooplankton abundance throughout the year (PHTSCrt 1992). Eurytemora affinis was first mentioned Material and Methods by DAHL (1893). BURCKHARDT (1935) already described swarms of this species in the region of Schweinesand / Hah- Study area n6fer Sand (Elbe km 630-650). Seasonal abundance of Eu- The Elbe Estuary (Fig. 1) is a coastal plain estuary, which openes * This paper is dedicated to Prof. Dr. HARTMUTKAUSCH on the occa- funnel-shaped to the German Bight (North Sea). The river has a total sion of his 60 th birthday. length of 1143 kin. Since 1960 a weir at km 586 (city of Geesthacht)

0075-9511/00/30/02-175 $12.00/0 175 NORTH SEA )

~": ~ ~" 700 " ' " ' ...... " " "68~

I CUXHAVEN

0 5 10 15 20 km ~ STADE 0 11 '1 i I I I I

Fig. 1. Map of the Elbe Estuary. (Numbers = stream kilometers; black dot = anchor station, stream-kin 695).

forms an upstream boundary to the tide. The estuary is channelized Sampling throughout the year was done from 1986 to 1993 from to a large extent since the beginning of this century. Tides follow a an anchor station at stream-km 695 (Fig. 1) in the oligohaline part of semi-diurnal rhythm, and the mean tidal range varies between 2 m the estuary. Sampling throughout the year was performed weekly to and 5 m (DuwE 1990). Estuarine conditions in the Elbe river are fortnightly from 1986 to 1991 and monthly 1992 and 1993 during characterized by a steep salinity gradient, a zone of high turbidity the ebb phase. For details of sampling frequency and tidal phase dur- and high current velocities. According to its salinity profiles, the ing sampling see Table 1. In 1988 no data were obtained. estuary is classified as partially mixed (DuWE 1989). Longitudinal samples were collected between 1986 and 1994 from the research vessel "Valdivia" at different times of the year. Sampling Sampling sites were located between the city of Hamburg and the North Sea (near the island of Helgoland). Sampling was carried out Zooplankton samples were collected using an electric driven pump 1987, 1992, 1993 and 1994 in spring time, 1986 and 1994 in summer with a capacity of 401 min I which was connected to a tube. Integrat- and 1987 in autumn. No data were obtained between 1988 and 1991. ed samples were taken by constantly rising the end of the tube from For detailed information on longitudinal sampling see Table 2, for the bottom of the river to the surface using an electric winch. During position of sampling stations in the estuary see Fig. 1. longitudinal sampling in 1993 and 1994 samples were taken from a depth of 2 m. In most cases 101, in winter sometimes 201, were sieved through Results a 55 ~tm mesh concentrator. Samples were preserved with buffered (Hexamethylentetramin) formaldehyde (4%). Individuals of Euryte- Seasonal investigations at the anchor station mora affinis were counted using a dissecting microscope and the abundance was recorded as numbers per liters. Water temperature at the anchor station (Fig. 2) exceeds Salinity and temperature were measured by a probe (ME Meeres- 10 °C in spring time at the mid of April and reaches maxi- elektronik; Trappenkamp, Germany) connected to the end of the mum values of about 20 °C in June/July every year. Little tube. Data of river discharge were obtained by ARGE ELBE variation of temperature was found during summer. At the (1987-1995), measured at the weir near the city of Geesthacht. end of October temperature drops to values of about 10 °C.

Table 1. Date, frequence and tidal phase of sampling from the anchor station at km 695.

Year Start of End of Days of Sampling frequency Tidal phase sampling sampling sampling

1986 25.04. 17.12. 22 weekly, fortnightly 2 h before high water 1987 19.03. 21.12. 37 weekly, fortnightly 2 h before high water 1989 25.04. 01.09. 33 every 3-4 days, weekly 2 h before high water 1990 17.04. 11.12. 24 weekly, fortnightly 1.5 h before high water 1991 05.04. 10.12. 22 weekly, fortnightly 1.5 h before high water 1992 26.02. 11.12. 10 monthly Low water 1993 28.01. 06.12. 11 monthly Low water

176 Limnologica 30 (2000) 2 Table 2. Date, sampling stations and tidal phase of longitudinal sampling. HT = high tide; LT = low tide; Sampling = shaded area. km 1986 1987 1987 1992 1993 1994 1994 4.7.-5.7. 26.4.-27.4. 27.9. 27.3.-3.4. 19.4. 11.4.-12.4. 19.7.-20.7.

620 630 640 65O 660 670 68O 690 695 700 ::~,;::~rr 710 707 712 716 718 720 727 728 730 737 738 740 745 748 i 750 760 ;i;; J 770

Maximum salinity values recorded at the anchor station 1986, 1987 and 1994 were years of high yearly river dis- reached 8.5 psu. Oligohaline to mesohaline conditions were charge, 1990-1993 were dry years. In 1986, mean river dis- found during sampling at the anchor station. charge was 715 m 3 s-1. A maximum value of 1800 m 3 s-1 was Values of mean yearly river discharge und maximum dis- reached in June, peaks with discharge values higher than charge in the years of investigation at the anchor station are 1300 m 3 s-1 were observed in April and June before and after shown in Table 3. Abundance ofEurytemora affinis and river population maximum. 1987 was the year with the highest discharge at the anchor station in the years of investigation mean river discharge during the investigation period (1092 are shown in Fig. 3. m 3 s-l). Maximum values of more than 2600 m 3 s-t were ob- served in January and April. Values of more than 1000 m 3 s-1 were measured nearly every day from January until July. Population maximum of Eurytemora affinis was found in Table 3. Mean and maximum values ofriverdischargeinthe years May when river discharge had droped to lower values. In 1986-1994. 1986 and in 1987, population maximum reached 347 Ind. 1 1 and 490 Ind. 1~1 respectively (Fig. 3) at the end of May. Year Mean river discharge Maximum river discharge [m3 s-q [m3 s-l] In 1989, mean river discharge was 875 m 3 s-1. A peak of over 1600 m 3 s J was measured in January. Highest abun- 1986 715 1800 dance of Eurytemora affinis was observed later then in the 1987 1092 2630 years before, at the beginning of June (547 Ind. 1 ]). 1989 875 1652 The following years were characterized by low river dis- 1990 519 1278 charge values. In 1990 mean river discharge reached only 1991 414 1017 519 m 3 s-l. March was the only time of the year with high dis- 1992 515 1588 charge values (1287 m 3 s-l). Lowest mean river discharge 1993 510 1827 was found 1991 with only 414 m 3 s-1. Maximum discharge 1994 859 2181 value of 1017 m 3 s i was measured in January. During these

Limnologica 30 (2000) 2 177 3O Fig. 3 shows that highest 1986 I ~ TEMPERATURE I abundance of Eurytemora affi- ~H-- . e 4FJ,---"-'-J~4~--4~,L~~...~._~ nis was allways found in years I0 of highest river discharge (1986 and 1987). "7 ...... I ...... I I I .... I I 2011987.~0 Longitudinal investigations

Copepod abundance and values 30 t ...... t I ...... I ...... I...... I...... of mean river discharge (RD) of 1989 the 20 days before sampling are 20 ,p,,_,_.~% shown in Fig. 4. This interval was chosen because a particle r--'-i 10 1 ~" O needs about 20 days to cover the O 30 distance between Hamburg and W 1990 the mouth of the estuary near Ca:: 2O Cuxhaven (value valid for mean tidal way; LUCHT 1964). 1°1 Temperature values during / I I I I I I longitudinal sampling varied ac- U.I cording to the prevailing season, 13. showing a more or less linear de- W crease between Hamburg and

I-- 'lL..~,ll the North Sea. Differences be-

1 I i tween maximum and minimum values never exceeded 6 °C. During longitudinal sampling in July 1986, salinity increased ---.~ 10-~ clearly downstream km 695. Maximum abundance of 123 I I '1" I ' I ...... | 30 Ind. 1-1 of Eurytemora affinis 2ol1993~ ~-~---"--'-~. __.~. was observed near Brunsbfittel at stream-kin 695. Downstream km 720 no individuals of the copepod were found (Fig. 4). I-J ...... ,AN FEB MAR APR I MAY dUN ' '~iUL AUG SEP I OCT NOV I DEC During sampling in April MONTH 1987, a strong increase of salini- ty was measured from the mouth Fig. 2. Water temperature in the Elbe Estuary at the anchor station (stream-km 695). of the estuary (kin 720) towards the North Sea. Abundance was higher than in summer 1986, two years abundance of EuGtemora affinis was low, maxi- reaching a maximum value of 235 Ind. 1-1 at km 695. In mum abundance occurred allready in April, several weeks September 1987, salinity increased clearly from km 695 to- earlier than in the years before. Maximum values of 55 Ind. wards the North Sea. Compared to the previous investiga- 1 ~ and 155 Ind. 1-1 respectively were determined. tions maximum of the copepod was located more upstream In 1992 and 1993, low mean river discharge values at km 630 near Hamburg (298 Ind. 1-1). (515 m 3 s-1 and 510 m 3 s 1) were found. Maximum values of In spring 1992, salinity increased strongly downstream more than 1500 m 3 s-1 were observed only in March and km 710. Maximum abundance of Eurytemora affinis (326 April. Maximum abundance of Eurytemora affinis in 1992 Ind. 1-1) occurred at km 660. and 1993 was 125 Ind. 1-1 and 96 Ind. 1-~ at the end of April During sampling in April 1993, a strong increase of salin- and beginnmg of May respectively. ity was measured from km 695 towards the North Sea. In 1994, mean river discharge was 859 m3s i Three peaks Maximum abundance of 579 Ind. 1-j was found at km 650. with maximum values of more than 2000 m 3 s -1 were ob- In spring 1994, salinity increased clearly downstream km served in January, March and April. 710. Highest abundance of the copepod was observed at km

178 Limnologica 30 (2000) 2 700 (627 Ind. 1-1), a smaller peak of 369 Ind. 1-1 was located at km 660. In July 1994 salinity in- '2oi ,-° i:o°o creased strongly from km 680 1ooo1 k oo towards the North Sea. Maxi- 30110 ] I I I t J I I / Gt.)0 mum abundance of Eurywmora affinis was found at km 630 near the city of Hamburg (370 Ind. 1-1). Summarizing the results of the seven longitudinal sam- 31]ooj...... 1980" ~oo plings it seems that in case of 21]oo~,_ 141]o low river discharge before sam- 11]o1]~2~%- _ -- - J-2oo pling (September 1987, April 1993 and July 1994) the steep- ~'F:: 311004 1990 ~ soo ~ ness of the salinity gradient was only moderate and highest abun- e"' oooi. d dance of Eurytemora affinis was i- I~~:~ ,,, found upstream between km 650 I I I I I I and 630. When higher river dis- _o 3oooI zo co 1991 400 < charge preceded sampling (April .,°2°°°I p o 1992) the population maximum >w 'nz was found at km 660. In July ,ooo~/~/~;;~hT_x~f/;~,_ ~ ~,~,.-rr~2°° I I I I I I 1986, April 1987 and April 1994 3000 ~ ~ 600 when river discharge before sampling exceeded 1000 m 3 s -1 .ooot ,,°,F.oo and reached values over 2000 m 3 IOO0-~_,~ ,~'/X I- 2012 s 1 the population maximum of

Eurytemora affinis was shifted 3000 i ...... ~ 600 towards the mouth of the estu- 28001 199~~400 ary.

Discussion

There are only a few studies about long-term fluctuation of 3ooo| 1994 zooplankton in estuaries (CAS- 2000-I,,i>. /2>,2/~ TEL 1993; ORSI & MECUM 1986). This may be because zooplank- I I I 1 i I I I I 1 I ton can not be measured by dAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC probes or analyzers and has to be MONTH carefully counted, which re- Fig. 3. River discharge (hatched area) and abundance of Eurywmora affinis (black bars) in the stricts the work of zooplankton Elbe Estuary at the anchor station (stream-kin 695). scientists to a practicable amount of samples. Another rea- son for the lack of data might be alized in this study because different investigators with dif- the variability of zooplankton samples in estuaries, which is ferent aims of their own studies sampled during the years even higher than in stagnant environments like lakes. This from 1986 to 1994. Nevertheless, all sampling was per- often makes the interpretation of data difficult. formed using the same method, all samples for seasonal dis- One of the main problems in estuaries is the variability of tribution were taken at the same geographical position the environmental conditions and as a consequence the diffi- (stream-km 695) in the Elbe Estuary even though not exactly culty of getting comparable samples (PEITSCH 1993). It during the same tidal phase. would be important to follow the same sampling strategy In the years 1986-1993 Eurytemora affinis always over the whole period of investigation. This could not be re- showed highest abundance in the Elbe Estuary in spring time

Limnologica 30 (2000) 2 179 36.0q -_i ___. July 19116 FT-600 So in terms of abundance of 24.0J "-,,,,,, [-400 Eurytemora affinis the Elbe 12.01 ~ ""--~ ~-200 Estuary is comparable to other RD:~ "---_ northern European estuaries like / 600 the Weser and Forth Estuary. 36.6| t Longitudinal investigations 24°1 April 19117 ~400 showed that the copepod is most 12,0] ...... ""~ I f200 abundant in the freshwater re- IRD. 23'~7m' ~" gion of the estuary. BARETTA & .I " I I I I I I ~ I I II I I ~ I ~6001 MALSCHAERT(1988) described 360 1 ----"--- September1987 400 Eurytemora affinis to occur 24.0 ~"-~ throughout the salinity range of

p12"0~" IRD:632r,Lj ' ' ' ' ' "~-~'~.' ' ' ~___' ....' "I !M iiii ~ winter (salinity measured in psu :,&0 j ~ ..... is approximately corresponding 240~ ~'~--"-..,, ar /A nil 92 to %0). During summer the 1201,,,,,, ~'~'---r'l,, ! ! ! ~ copepod was restricted to salini- ~'"-~"~ I I ~ ___ Z ties below 15 psu. VIITASALOet al. (1994) found Eurytemora 3&0 I -----'------. Ap[il1993 affinis to avoid salinities above 6.5%0 in the northern Baltic Sea. According to SOLTANPOUR-GAR- l GARI & W~LLERSNAUS (1987) Eurytemora affinis in the Elbe is most abundant in salinities from less than 0.1 up to 0.5%0. Eury- ~"\- ~0EI affinis is an eurytherm organism. In the present study 38.0 , ---~-----. - ~600 spring time maxima of Euryte- 24'0 t - ._~,, mora affinis were always found several weeks before highest t, ,-T'-<,--,--,, !! water temperature was measur- 780 770 760 7'50 740 730 720 710 700 690 680 670 680 850 640 630 620 ed. RODDn~ et al. (1984) found Eutytemora affinis to survive I -- ~or,~ I STREAM-KM I " Euq,temoraatfinis I best at 5-19 °C under laboratory conditions. DE PAUW (1973) de- Fig. 4. Salinity and abundance ofEurytemora affinis in the Elbe Estuary during longitudinal sam- scribed a water temperature pling (RD = mean river discharge of the 20 days before sampling). range of 2-24 °C for this cope- pod. Neither the measured values of salinity nor those of with maximum abundance varying between 95 Ind. 1-1 temperature can give suitable explanations for the observed (1993) and 508 Ind. 1 1 (1987). Spring time maxima of this differences in abundance of Eurytemora affinis. copepod are well known from other estuaries like the West- At the anchor station in the oligohaline zone of the estu- ern Scheldt Estuary (BANKER et al. 1977), the Weser ary (stream-km 695) highest abundance of Eurytemora affi- (SOLTANPOUR-GARGARI & WELLERSHAUS 1984; 1985) and his was observed during the two years of highest river dis- the Forth Estuary in Scotland (RODDm et al. 1984). Spring charge (1986 and 1987, Table 3). The other years showed time maxima in these studies never exceeded 500 Ind. 1-1. much lower values of abundance with the exception of one There are more productive estuaries like the Schlei Fjord peak in 1989. Sampling frequency was much lower in 1993 with highest abundance of 2000 Ind. 1-1 (CHkIST~ANSBN1988) and 1994, so the peak of maximum abundance could easilier and the Patuxent River Estuary with maximum abundance of be missed as in the years before. 3000 respectively 3100 Ind. 1-1 (HE~NLE & FLEMER 1975; The seasonal cycle of the Eurytemora affinis population is SELLNER & BUNDY 1987). Examples for less productive estu- determined by food supply, egg production and mortality aries are the Delaware River with 14 Ind. k ~ (CRONIN et al. rates. The rate of egg production depends on temperature and 1962) and the Bothnian Bay (KANKAALA 1987) showing on food supply (EDMONDSONet al. 1962; HEINLE et al. 1977; maximum abundance of Eurytemora affinis of 50-80 Ind. 1-1. PETERSEN & KIMMERBR 1994). Changes in food supply or

180 Limnologica 30 (2000) 2 quality could have influenced the abundance of Eurytemora the estuary when reproduction rate exceeds the rate of loss affinis at the anchor station. due to net river flow (KETCHUM 1954). Other parameters like Population maximum of Eurytemora affinis develops in temperature, salinity, food conditions and predation have to springtime because of high abundance of food. The main be considered during investigations on zooplankton, but food components for Eurytemora affinis in the Elbe Estuary river discharge should always be regarded as one of the are detritus and phytoplankton. Values of chlorophyll a dur- main factors controlling its abundance and distribution in ing the longitudinal sampling in the years 1992-1994 are re- estuaries. ported by WOLFSTEIN (1996), who always found maximum chlorophyll a values near the city of Hamburg. Acknowledgements: This study was supported by the Deutsche Longitudinal sampling confirms the relation between Forschungsgemeinschaft, Sonderforschungsbereich 327. We thank river discharge and the occurrence of Eurytemora affinis Prof. Dr. H. KAUSCHfor his help and friendly assistance. (Fig. 4). When high river discharge preceded sampling (1986, 1987, and 1994), the position of maximum abundance and of salinity gradient was shifted towards the mouth of the References estuary (stream-kin 690-700). In April 1993, after a period of medium fiver discharge the position of maximum abun- ARGE ELBE (i987-1995): Wassergtitedaten der Elbe, Zahlentafeln dance was found at stream-km 650. In July 1994, when very 1986-1994. Arbeitsgemeinschaft zur Reinhaltung der Elbe. low fiver discharge preceded sampling maximum abundance ARNDT, H. (1989): Zooplankton production and its consumption of Eurytemora affinis was recorded near the city of Hamburg by planktivores in a Baltic inlet. Proceed. of 21th EMBS: 205-214. (kin 630). These results suggest the main habitat of the cope- BAKKER, C. & DE PAtYW,N. (1975): Comparison of brackish water pod to be located in the freshwater region of the estuary near plankton assemblages of identical salinity ranges in an estuarine Hamburg. tidal (Westerschelde) and stagnant (Lake Veere) environment. II. When river discharge is high during spring downstream Zooplankton. Neth. J. Sea Res. 9 (2): 145-165. transport is fast and the maximum peak of Eurytemora affi- - PHAFF,W.J., EWIJK-ROSIER,M. W~N & DE PAUW,N. (1977): Cope- his reaches the anchor station. During years of low fiver pod biomass in an estuarine and a stagnant brackish environment discharge only low abundance of Eurytemora affinis is of the south western Netherlands. Hydrobiologia 52 (1): 3-13. found at the anchor station because the population declines BARETTA, J.W. (1977): Seasonal fluctuations in the zooplankton of before reaching the anchor station. River discharge is the the Ems-Dollart-Estuary. Hydrobiol. Bull. 11: 12-13. main parameter causing the displacement of the abundance - & MALSCHAERT,J.EP. (1988): Distribution and abundance of the zooplankton of the Eros estuary (North Sea). Neth. J. Sea Res. 22: maximum, and therefore it is also responsible for the ob- 69-82. served differences in abundance measured at the anchor BERNAT, N. (1988): Untersuchung zur Verteilung des Zooplanktons station. auf einem Querschnitt der Elbe bei Scheelenkuhlen unter beson- Some authors describe Eurytemora affinis to be incapable derer Beracksichtigung der dominierenden Art Eurytemora affi- of autonomous displacement, therefore behaving as passive nis (Copepoda, Calanoida). Diplomarbeit, Universitfit Hamburg, particles in estuaries (CASTEL & VE~GA 1990; CASTEL 1993). 84 p. In this case distribution of the organisms would be related - KOPCKE, B., YASSERI, S., THIEL, R. & WOLFSTEIN, K. (1994): Tidal mainly to river flow. Other authors reported horizontal and variation in bacteria, phytoplankton, zooplankton, mysids, fish vertical migration of Eurytemora affinis as mechanisms to and suspended matter in the turbidity zone of the Elbe estuary; in- tenelationships and causes. Neth. J. Aquatic Ecology 28 (3-4): minimize population loss (HECKMAN 1986; VUORINEN 1987; 467~476. HAESLOOP 1990). Our studies confirm the results of CASTEL BURCKnARDT,A. (1935): Die Ernghrungsgrundlagen der Copepo- (1993) who reported transport of the population maximum denschw~'me der Niederelbe. Int. Rev. ges. Hydrobiol. 32: downstream or upstream with increasing or decreasing river 432-500. discharge for Eurytemora population of the Gironde Estuary. CASTEL, J. (1993): Long-term distribution of zooplankton in the CASTEL (1993) found the highest abundance of Eurytemora Gironde estuary and its relation with river flow and suspended affinis just upstream the zone of high turbidity and salinity matter. Cah. Biol. Mar. 34: 145-163. increase. The distribution seemed to be only shifted by river - & FEURTET,A. (1986): Influence des matieres en suspension sur la discharge. biologic d'un copepode estuarien: Eurytemora hirundoides The results of the presented long-term study show that (NORDQUIST, 1888). Coll. Nat. CNRS "Biologic des Populations", pp. 391-396. river discharge influences the geographic position of maxi- - & VEIGA,J. (1990): Distribution and retention of the copepod Eu- mum abundance in the estuary as well as the abundance of rytemora affinis hirundoides in a turbid estuary. Mar. Biol. 107: Eurytemora affinis in the oligohaline zone of the river. This 119-128. influence could only be shown by comparing the data of sev- CHRISTIANSEN,B. (1988): Vergleichende Untersuchungen zur Popu- eral years. High river discharge results in high net river flow. lationsdynamik yon Eurytemora affinis POPPEnnd Acartia tonsa This factor is of major importance for estuarine zooplankton DANA,Copepoda, in der Schlei. PhD thesis, Universit~t Hamburg, populations because a population can only maintain itself in 141 p.

Limnologica 30 (2000) 2 181 CRONIN, L.E., DAIBER, J.C. & HULBERT, E.M. (1962): Quantitative PEITSCH, A. (1992): Untersuchungen zur Populationsdynamik und seasonal aspects of zooplankton in the Delaware River estuary. Produktion von EuJytemora affinis (Calanoida; Copepoda) im Chesapeake Sci. 3: 63-93. Brackwasserbereich des Elbe-Astuars. PhD thesis, Universit/it DAHL, F. (1893): Untersuchungen tiber die Tierwelt der Unterelbe. Hamburg, 166 p. Preugerd Commission zur wissenschaftlichen Untersuchung der - (1993): Difficulties in estimating mortality rates of Eutytemora deutschen Meere. Kiel, 6. Bericht: 151-185. affinis in the brackish water region of the Elbe estuary. Cah. Biol. DE PAUW, N. (1973): On the distribution of Eurytemora affinis Mar. 34: 215-224. (POPPE) (Copepoda) in the Western Scheldt estuary. Verh. Inter- - (1995): Production rates ofEurytemora affinis in the Elbe estuary, nat. Verein Limnol. 18: 1462-1472. comparison of field and enclosure production estimates. 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