SEASONAL CHANGES IN THE NERITIC ZONE MESOZOOPLANKTON OF POMERANIAN BAY IN 2000

Juliusz C. Chojnacki, Eliza Antończak
Department of Marine Ecology and Environmental Protection, Agricultural University in Szczecin, Poland

ABSTRACT

Plankton samples as well as temperature, oxygen and salinity data were collected at 9 stations in Pomeranian Bay during the spring, summer, autumn and winter of the year 2000. Thirty four taxa were noted, including 7 Rotatoria, 10 Cladocera, 11 Copepoda and 6 meroplanktonic taxa.  Copepoda dominated numerically, while in warm seasons of the year Cladocera were subdominants. Taxonomic diversity of mesozooplankton was the highest in warm seasons of the year. Stations located in the neritic zone showed changeable abundance over time, peaking at station MIII (133.235 ind.x m^-3). Geographically plankton numbers were high (put a number in here) in samples collected from the vicinity of the Œwina River mouth. Transects from Międzyzdroje to the mouth of the River Dziwna (respectively average abundance for the entire year for stations within 1 Nm distance from the shore were 21.858; 30.890; 35.091 ind.x m^-3). Seasonal succession followed this sequence among Copepoda: in spring P. elongatus, while in the remaining seasons A. bifilosa dominated. Subdominants introduced diversity into seasonal succession, especially in the summer season, when subdominants were made up exclusively of Cladocera – E. nordmanni, P. leuckartii, P. intermedius, in the autumn Copepoda and Cladocera – T. longicornis, E. nordmanni, P. polyphemoides,  P. elongatus,  while in the winter these were exclusively Copepoda P. elongatus, A. longiremis, T. longicornis.

Key words: mesozooplankton, Pomeranian Bay, seasonal changeability, qualitative and quantitative structure.

INTRODUCTION

In the Pomeranian Bay there is a constant clash of fresh water, carried to the sea by the River Odra, with the brackish waters of the Western Baltic Sea. In the mouths of the Świna, Dziwna and Rega Rivers which empty into Pomeranian Bay the Odra estuary ecotone zone can be observed clearly marked in chemical, physical and biological respects [16,27,33]. However, in each ecotone zone of the Odra, the Vistula or other rivers near the sea the range, qualitative and quantitative structure of mesozooplankton differs, undoubtedly depending on the waters dynamics and consequent physical and chemical features of the environment [3,8,9,10,11,14,16,22,23,35,40,46,47,48].

The Baltic zooplankton is composed of microzooplankton, mesozooplankton and macroplankton with characteristic ichtyoplankton forms. The dominant group in the Baltic zooplankton is made up of Copepoda, whose abundance may seasonally exceed even 90% of all zooplankters, but the next groups in quantitative respect are Rotatoria and Cladocera. Among Copepoda the dominants are Acartia bifilosa, Acartia tonsa, Temora longicornis and Pseudocalanus elongatus. Cladocera are usually dominated by marine forms – Podon polyphemoides, Bosmina coregoni maritima, Evadne nordmanni, and out of fresh water species, found in the neritic zone and in estuaries: Daphnia spp., Chydorus sphaericus, Bosmina coregoni, Bosmina longirostris. Rotatoria are represented by the following genera Keratella, Brachionus and Synchaeta [2,4,5,6,7,9,10,12,15,17,18,21,22,24,43].

Shallow coastal waters, particularly in sheltered bays and near river mouths, fertilized by the influence of rivers and organic pollutants entering the sea, are the areas richest in zooplankton in the Southern Baltic. The analyses conducted within the framework of state monitoring of the environment show, however, a decrease in average mesozooplankton abundance, starting from 1999 [3], which may be linked to subtancial changes in the environment, especially to oscillations of biogenic substance quantity, on which sea productivity depends. Every year in the entire Baltic, there is a strong plankton bloom [10,11,12,15,36,38,43,48,50].

However, doesn't high zooplankton abundance, resulting from eutrophication, cause the growth of autochthonic sea pollution? Józefczuk et al. [22]  believed that week by week decrease of some mesozooplankton groups was caused by strong predation pressure of predators, resulting in  substancial variation in qualitative and quantitative results . Additional research is needed to test this hypothesis. It is known, that the plankton communities of coastal waters is "controlled" by plankton-eating fish and that in the Baltic it is herring and sprat who are the major Copepoda and Cladocera predators, preferentially feeding on these groups in the Summer [28,34,41]. In the Northern Baltic Viitasalo et al. [42] found interesting plankton behaviour, namely, some groups of zooplankton move from areas where the water column is stable,  preferring to live in  changeable environmental conditions.

Heerkloss [19] gave an interesting explanation of the reasons for fast changes in zooplankton abundance describing non-linear nature of growth processes in particular populations of planktonic organisms. Whereas Żmijewska et al. [50] believed, that in the coastal zone, inhabited by meroplankton in particularly high numbers, it is icthyoplankton, and especially fish larvae, post larvae and juveniles (e.g. Gobidae), that feed intensively on  larvae and holoplankton,  creating very irregular spatial mesozooplankton structure in the neritic zone of the sea.

Research shows that planktonic organisms abundance depends on water purity, whereas both the Gdańsk Bay and the Pomeranian Bay, as well as the neritic zone of the Southern Baltic are areas where excess allochthonic biogenic substances are found, which results in high biological productivity [12,44,46,47]. Low stability of the environment conditions, as well as meteorological phenomena cause seasonal differences, clearer than in open waters, in abiotic values of water quality and biogenic substances, significant differences in phytoplankton and zooplankton productivity, characteristic plankton 'poverty' in winter and rich, rapid plankton growth in the period between May – November [3,38,48].  Fluctuations in such changeable environment are linked to the pollution load introduced by the Odra, which is currently decreasing, and there is a need to identify the reasons for the drop in zooplankton productivity and abundance [23,33,25,26,40].

On the basis of the results of water quality analysis and mesozooplankton analysis in the Pomeranian Bay during the 4 seasons of the year 2000, we will try to demonstrate a certain causality of the seasonality of taxonomic, spatial and abundance structures for this ecological group and selected abiotic indicators of the environment.

RESEARCH AREA

In order to conduct qualitative-quantitative research of zooplankton in comparison to hydro-chemical conditions, nine permanent sample collection stations were selected, located in the neritic zone from Świnoujście through the region of Międzyzdroje to Dziwnów (Fig. 1).

The research stations were located along the coast in the southern part of the Pomeranian Bay in the zone influenced by the waters entering the Baltic from the Szczecin Lagoon via the Świna Strait and from the Kamieński Lagoon via the Dziwna Strait. Stations I, II, III were located in the distance of, respectively, 1, 2, 3 Mm in the northern direction from the coastline.

MATERIAL AND METHODS

The research in the Pomeranian Bay was started in April 2000 and was completed in February 2001; samples were collected on the following days: 18.04.2000, 12.08.2000, 15.10.2000, 11.01.2001 from the R.V. "Nawigator XXI" of the Maritime Academy in Szczecin. The analysis of abiotic water indicators was limited to temperature measurement with an electronic thermometer with the reading accuracy of up to 0.01°C, dissolved oxygen and chlorides were marked with the application of WTW probe type 325 (the O₂ probe was calibrated in laboratory).

Zooplankton was collected with a Bongo type plankton net of Ř = 20 cm [20] equipped with a flow meter made by General Oceanics with speed curves No 2030 and 2031, with a mesh of #80 µm, each haul lasting 10 min. at an average vessel speed of 3–4 knots. The planktonic material was preserved immediately after sampling with 4% buffered formalin. In a laboratory the material was diluted in Folsom's sample splitter [31] 2–10 times in order to obtain a representative sub-sample, containing at least 400 individuals and then the entire sub-sample was analyzed in Bogorov's chamber. The abundance of individuals in 1 m³ was calculated according to the formula presented below:

(1)

Where:
L – taxon abundance in a water capacity unit (ind. x m^-3),
l_n – number of individuals in a sub-sample,
n – number of dilutions of original sample in Folsom's sample splitter, V – capacity of water filtered through water plankton net (m³).

V  =  M x F x r x 2.66^-1 = 0.00118 x M     (2)

Where:
M – number of full log revolutions during the mesh operation under water,
F – surface of planktonometer tube inlet of Ř 20 cm [m²],
r – 0.01 – planktonometer tube radius [m],
2.66 – constant for a log with speed curves 2030 and 2031 (General Oceanics).

Microsoft Office Excel software was used for graphic representation.

RESULTS

During the 4 seasons of observation it was confirmed, that because of larger amounts of water entering the Pomeranian Bay via the Świna, and large loads of biogens and organic material entering the sea with this water – high productivity of the Lower Odra course and of the Szczecin Lagoon was proved. Similar observations were made previously by Wiktor [43] and Tadajewski and Kubiak [40], Chojnacki [13]. In sub-samples 34 taxa were found, including 7 Rotatoria, 10 Cladocera, 11 Copepoda, 6 meroplanktonic taxa: Keratella cochlearis, Keratella quadrata, Synchaeta balthica, Synchaeta monopus, Synchaeta litoralis, Trichocerca marina, Brachionus sp., Diaphanosoma brachuyrum, Daphnia longispina, Daphnia cucullata, Chydorus sphaericus, Bosmina coregoni maritima, Bosmina longirostris, Evande nordmanni, Podon intermedius, Podon leuckarti, Podon polyphemoides, Acartia bifilosa, Acartia longiremis, Acartia tonsa, Centropages hamatus, Pseudocalanus elongatus, Temora longicornis, Eurytemora sp., Anthocyclops viridis, Cyclopina gracilis, Eudiaptomus gracilis, Cyclops sp., nauplii + copepodit Copepoda, meroplankton – nauplius Cirripedia, larvae nectobenthic form Polychaeta, veliger Lamellibranchiata, veliger Gastropoda, Crangon vulgaris (zoëa), Mysis mixta.

Seasonal changes of selected abiotic indicators in the neritic zone of the Pomeranian Bay
Temperature (°C) of waters in the Pomeranian Bay followed a logical pattern – the lowest values were in winter, slightly higher in spring, and then the warmest in summer slowly decreasing in autumn (Fig. 2). In the Bay waters in spring and summer, temperature fluctuations between stations amounted to 2–2.5°C, while in autumn and winter between 1–2°C. Another characteristic was, that in the cooler seasons of the year the waters of the stations located 3 Nm from the shore, in the vicinity of Świnoujście and Międzyzdroje registered a temperature higher by 1–3°C than at the remaining stations (Fig. 2). In spring, with the temperature ranging from 7.5–10.5°C, the highest calculated correlation between water temperature and Cladocera and Copepoda abundance r – amounted to respectively - 0.491 and 0.53. In summer, with the Bay water temperature ranging from 17 to 19.5°C, the highest correlation with water temperature was determined for Rotatoria and meroplankton – r amounted to respectively 0.424 and 0.904 (Table 1).

Salinity (PSU) showed a characteristic spatial distribution in the 4 seasons of 2000. The lowest salinity level was noted at stations in spring, from 2.8 to 4 PSU, in summer, except for the stations located directly in the vicinity of the Świna (SI and SII), salinity oscillated between 7.5–8.2 PSU. In autumn, when the waters were fairly well mixed due to storms, salinity leveled to approximately 8 PSU, except station SI located 1 Nm from the mouth of the Świna. In the winter at almost all stations salinity was at the highest level for the year ranging from 7.5–9 PSU, except station SI (3.8 PSU) and MI (4.2 PSU). A very clear influence of fresh waters on the salinity of waters in the direct vicinity of the Świna mouth  was noted in the neritic zone (station SI), as well as the formation of a tongue of less saline and unmixed waters to the east from station SI to station MI (Fig. 3). In spring, with salinity levels at 2.8–4 PSU, the highest calculated correlation between water salinity and Cladocera and copepodites + naupliuses abundance – r amounted to respectively 0.412 and 0.625. In summer, with salinity level at 7.5–8.2 PSU, the highest correlation with water salinity was determined for Copepoda – r – 0.497  (Table 1).

Oxygen content (mg O₂ x l^-1) reached highest values in spring season (12.2–15.8 mg O₂ x l^-1), which was probably related to a period of long-lasting storms and southern winds (low salinity values). However, in summer and autumn seasons, oxygen content dropped and remained at the level of approximately 10 mg O₂ x l^-1, and with the coming of winter, at steady temperatures, average oxygen content increased to ~ 12 mg O₂ x l^-1. Observations regarding "a tongue of non saline – fresh water", usually less oxygenated, were confirmed. In the course of oxygen content analysis in winter the tongue was spreading from station SI to station MI up to Dziwnów (Fig. 4) In spring, with high oxygen content of  12–15.8 mg O₂ x l^-1, the highest correlation between water oxygen content and abundance was calculated for Cladocera – r and it amounted to 0.453. In winter, with oxygen content at the level of 11.0-13.1 mg O₂ x l^-1, the highest correlation with water oxygen content was determined for Rotatoria – r – 0.551.

Seasonal changes of the spatial mesozooplankton structure in the neritic zone of the Pomeranian Bay in 2000
Average plankter abundance in 2000 in the Pomeranian Bay was relatively high (32.907 ind. x m^-3). Analysis of the data set from the 3 transects showed that  (Table 3, Fig. 4)  mesozooplankton concentration varied  characteristically being dependent on abiotic indicators of the environment. In the transect SI – III the highest abundance was noted in spring at station SIII – 9.532 ind.x m^-3, in autumn – 26.485 ind. x m^-3 at the same station, whereas in summer and winter at station SII (abundance was respectively 85.465 and 17.180 ind. x m^-3).

In the transect of Międzyzdroje (MI – III) the highest abundance occurred (Table 2, Fig, 4) at station MIII in all seasons (in spring – 9.373 ind. x m^-3 , in summer – 133.235 ind. x m^-3, in winter – 19.578 ind. x m^-3), except the autumn season, when the highest concentration occurred at station MIII (38.676 ind. x m^-3).

In the transects of Dziwnów, the Pomeranian Bay waters had the highest abundance (Table 3, Fig. 5) at station DIII in spring (12.241 ind. x m^-3), in autumn (27.871 ind. x m^-3) in winter (17.356 ind. x m^-3)  and only once at station DI – in summer (117.742 ind. x m^-3).

Out of the four seasons under study, the highest abundance at all stations was noted in the summer season (133.235 ind. x m^-3), while the lowest abundance in this season amounted to  55.327 ind. x m^-3. The season with the lowest abundance was winter with a maximum abundance of 19.578 ind. x m^-3 at station MIII, while a minimum abundance was 4.546 ind. x m^-3 at station DII.

Seasonal changes of mesozooplankton taxonomic structure in the neritic zone of the Pomeranian Bay in 2000
Changes in taxonomic structure followed the rhythm of seasonal changes of environment abiotic conditions. At all stations in the Pomeranian Bay neritic zone Copepoda were a quantitative dominant along with a supplementary group of development stages (nauplii + Copepoda). A particularly high Copepoda abundance was noted at station MIII but a not much higher abundance was determined at station DI and DIII in the summer season. A similar abundance level was observed in autumn and winter. Second groups playing the role of subdominants in the warm seasons of the year were Cladocera and then Rotatoria.

An analysis of species dominance among Copepoda (Fig. 6) demonstrated, that in spring P. elongatus showed the highest abundance (especially at station DIII – 5.418 ind. x m^-3), while A. bifilosa was a seasonal subdominant (highest numbers at station DIII – 26,38 ind. x m^-3)  as well as  A. longiremis (at stations MII, DIII respectively 1417 and  1491 ind. x m^-3). In the summer season A. bifilosa was an absolute quantitative dominant (especially at station MIII (93.812 ind. x m^-3 ) and DIII (84.733 ind. x m^-3 )). At the remaining stations the species occurred in low concentration. In the autumn of 2000 A. bifilosa was also a dominant but it occurred in particular abundance at stations MII and DIII (respectively 19.671 and 19.781 ind. x m^-3). In autumn T. longicornis was a subdominant among Copepoda (stations MII (4.467 ind. x m^-3) and MIII (3.320 ind. x m^-3)), yet another dominant species was ever more numerous P. elongatus.  In winter A. bifilosa dominated (at station MIII – 12.345 ind. x m^-3), and another subdominant were  P. elongatus (DIII – 4.447 ind. x m^-3),  A. longiremis (MI – 3.317 ind. x m^-3) and T. longicornis  (SIII, MIII, DIII – respectively 2.543; 2.364; 2.123 ind.  x m^-3).

Taxonomic structure analysis of an important zooplankton group, such as Cladocera (Fig. 6), showed in spring 2000, in the neritic zone of the Odra estuary, a filtering-predatory Cladocera – E. nordmanni  dominated the population, especially at stations SII, SIII, MII and MIII (the abundance was respectively – 369; 371; 401; 318 ind. x m^-3).

At that time P. polyphemoides was subdominant (especially at stations SIII, MII – III and DI with abundance respectively 248; 258; 269; 245 ind. x m^-3). In summer, with abundance on average  12 times higher than in spring, Cladocera occurred in the entire research area, where P. polyphemoides dominated (abundance from 1.749–12.497 ind. x m^-3 ), while E. nordmanni was a subdominant (especially at stations MIII and DI)  (with abundance respectively – 4.812; 5.574 ind. x m^-3). In autumn the Cladocera quantity in samples in comparison to the summer season was on average 2.5 times lower, and E. nordmanni dominated here (stations SII, SIII, MII, MIII – with abundance respectively 2.436; 2.723; 4.526; 2.446 ind. x m^-3) and P. polyphemoides were a subdominant with average abundance in the season 1.623 ind. x m^-3. In winter D. longispina occurred in numbers not exceeding 23 ind. x m^-3 at stations SI, MI, MII and DI, that is at stations located in the direct vicinity of fresh water from lagoons, as well as accessory species but only at the stations enumerated: E. nordmanni, P. polyphemoides and P. intermedius. In that season the abovementioned species occurred in the entire region with average abundance of   2 ind. x m^-3.

DISCUSSION

Taking into account how difficult it is to specify direct and indirect influence of environment conditions on ecological structures of mesozooplankton [1,10,11,12,13,43], an attempt was made to determine this influence indirectly on the basis of changes in the structure of dominance in seasonal succession in the Baltic neritic zone, similarly to how Margalef  [30] did it for fully saline seas. On the basis of observation of mesozooplankton abundance and of selected environment indicators in the Pomeranian Bay in the course of 4 seasons at stations of Świnoujście, Międzyzdroje and Dziwnów transect, in Table 4 we present a simplified pattern of mesozooplankton seasonal succession in the year 2000.

A similar seasonal succession pattern to the one presented in Table 4 was described by Chojnacki [12] with regard to the entire Southern Baltic Sea, as well as to the Odra estuary [13], while more Cladocera, including fresh water species, occurred in the position of subdominants. Fresh water pseudo-populations  do not survive long in the neritic zone of the Bay, because physical and chemical indicators are changeable, and frequent changes of currents, salinity, temperature, strong wave motion cause their death, thus increasing the amount dead organic matter mass in this zone.

Against this environmental background the spatial structures of mesozooplankton showed characteristic distribution and seasonal changeability, dependent on salinity value and water temperature [18]. Ubiquitous species, showing high tolerance to salinity, recognized as the-so-called euryhalophilous species, are typically found in a group that is characteristic for brackish and ecotonal waters. In the Pomeranian Bay a characteristic, increasing plankton abundance was noted in samples from stations located near the mouth of the Świna, through transects of Międzyzdroje to the mouth of the Dziwna (respective average annual values for stations located in the distance of 1 Nm from the shore were 21.858; 30.890; 35.091 ind. x m^-3).

In the distance of  3 Nm from the shore, at stations MIII and DIII,  enclaves formed  "lenses" with specific, much higher mesozooplankton abundance (respective annual average values of abundance were 46.621 and 42.815 ind. x m^-3)  and with different salinity and temperature characteristics (Figs. 2, 3, 5).

In the coastal zone of the Baltic there are marine and fresh water species found, a situation typically observed in the Southern Baltic. Such species treat the Pomeranian Bay as a specific environment, where they can easily survive, and reaching seasonally dominant positions in planktonic zoocenoces [7,11,14,15,29,35,49,50].

During a detailed analysis of seasonal spatial structure of mesozooplankton, depending on abiotic indicators of the environment, no direct correlations were observed between particular abiotic indicators and total mesozooplankton abundance. One may claim, that after relatively harsh winters with low water temperature <2.9°C and the spring seasons with water temperature >9.1°C, in the summer season of 2000 plankton concentrations from 55–133 thousand individuals x m^-3 were observed (with average water temperature of 18°C). The lower temperature of the Pomeranian Bay waters in the summer season typically meant lower values of mesozooplankton abundance reaching 55 thousand individuals x m^-3 and correlations with water temperature (Table 3, Figs. 2, 3, 5).

Cladocera are a very characteristic element of coastal (neritic) zooplankton in the summer season, not only in Pomeranian Bay, but also in Puck Bay and Gdańsk Bay. They often form concentrations in well warmed up surface and near-surface waters [2,32,45,47,49]. Altogether, in the year 2000, in Pomeranian Bay, Cladocera occurred in significantly higher abundance than the numbers given for Gdańsk Bay by Siudziński [39] (approximately 4,300 ind. x m^-3), Bielecka [2] (5.000 ind. x m^-3) and (cite year) (8.526 ind. x m^-3) – since on average for all the stations it amounted to as many as 11.381 ind. x m^-3, and was nearly two times higher at station DI in the vicinity of Dziwnów. Similarly to Bielecka et al. [2] and Mudrak [32] in their studies on coastal zooplankton in the vicinity of Gdynia, the results of the abundance in the Pomeranian Bay in 2000 proved a significant diversity of Cladocera existing in the summer season. Only in the summer of 2000 at 9 stations of the Pomeranian Bay their abundance had average maximum values of 87.994 ind. x m^-3 and the dominant was P. polyphemoides, just like  many years ago in the Gdańsk Bay [39] and the coastal zone of Gdynia and Sopot [2].

Positive correlations between Cladocera abundance and water temperature were observed in the northern and central Baltic [42] and in the spring of 2000 – in Pomeranian Bay (r – 0.491) (Table 2).

Copepoda were the only planktonic fauna element of Pomeranian Bay that occurred in all the seasons of 2000 (Table 2, Fig.6), only their abundance and taxonomic structure changed. The highest Copepoda concentrations in 2000 were noted in the summer and autumn seasons, or even in winter in the entire Pomeranian Bay. The abundance average values were respectively – 62.928; 16.207; 12.109 ind. x m^-3. The summer maximum was observed in the open sea and in coastal zone of the Baltic by other authors [9,10,32,39,45,49].  These highest Copepoda concentrations in the summer season of 2000 were clearly correlated with salinity level  (r–0.497) while there was negative correlation with water temperature and oxygen content at stations in the Pomeranian Bay neritic zone. Różańska et al. [37] made an interesting observation, finding a correlation between Eurytemora affinis abundance and NO₃ content in the Vistula Lagoon, although this correlation differed in the subsequent years. This suggests, that analyses such as this one will never fully answer the question of the relation between zooplankton organisms' number and the conditions of abiotic environment, because qualitative and quantitative changeability of marine zooplankton is too high.

CONCLUSIONS

The results of the research on qualitative and quantitative mesozooplankton structure in the Pomeranian Bay waters in the 4 seasons of 2000 in relation to temperature, salinity and oxygen content led to these conclusions.

1. In the Pomeranian Bay waters in the year 2000 34 mesozooplankton taxa were noted, gathered into animal groups: Rotatoria, Cladocera, Copepoda and meroplankton, with average seasonal abundance at 9 stations:  in spring – 8.309 ind. x m^-3, in summer – 87.994 ind. x m^-3, in autumn 22.233 ind. x m^-3 in winter 13.094 ind. x m^-3.

2. Characteristic, increasing plankton abundance was observed in the samples taken at stations located in Pomeranian Bay near the mouth of the Świna, through the transects of Międzyzdroje to the mouth of the Dziwna (respective annual average values for stations located within 1 Nm from the shore 21.858; 30.890; 35.091 ind. x m^-3).

3. In the Pomeranian Bay neritic zone, in the course of all the seasons of 2000, Copepoda dominated, the dominant species being A. bifilosa, except during the winter season, when cold-loving P. elongatus was a dominant among Copepoda (a subdominant in the winter season). Halophilous Baltic Cladocera E. nordmanni and P. polyphemoides were subdominants in the year 2000 in the Pomeranian Bay neritic zone.

4. In the year 2000 in the Pomeranian Bay, seasonal succession in the dominant group of mesozoolankton among Copepoda followed this sequence: in the spring P. elongatus, while in the remaining seasons A. bifilosa did not give over the succession in summer, autumn and winter. Subdominants introduced an important diversity into the seasonal succession, especially in summer, when these were exclusively Cladocera – E. nordmanni, P. leuckartii, P. intermedius, in autumn Copepoda and Cladocera– T. longicornis, E. nordmanni, P. polyphemoides, P. elongatus, while in winter these were exclusively Copeoda – P. elongatus, A. longiremis, T. longicornis.

5. In the year 2000 in the Pomeranian Bay, Cladocera (r – 0.49) and Copepoda (r – 0.53) showed the highest correlation regarding temperature in the spring, in relation to salinity Copepoda in the summer (r – 0.497) and  copepodites as well as nauplii – unmarked for species – in the spring season (r – 0.63), and in relation to oxygen content in the water – Rotatoria showed in the winter (r – 0.55) Cladocera in the spring (r – 0.45).

ACKNOWLEDGMENTS

I wish to extend my thanks to Mrs Eliza Antończak, Master- Engineer, for making the research results available for the purposes of this publication, which she was not able to do herself due to health problems. We specially thank you Dr. Antony Jensen from National Oceanography Centre, University of Southampton, United Kingdom, for verification of language correctness of text.

REFERENCES

Accepted for print: 17.12.2008

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