MALACOFAUNA OF SELECTED WATER BODIES OF THE CITY OF SZCZECIN

Studies on molluscs of selected water bodies of the city of Szczecin (NW Poland), carried out in 1997, revealed the occurrence of 11 snail and 5 bivalve species. Two groups of water bodies were distinguished, of different species composition: G3êbokie lake on one hand and the ponds with the Osówka stream on the other. Overall dominants were Potamopyrgus antipodarum (Gray), an expanding species, and Sphaerium corneum (L.).


INTRODUCTION
The malacofauna of north-western Poland, and especially the Szczecin voyvodeship, is still insufficiently studied, with only few literature data pertaining mainly to larger reservoirs, such as the Gulf of Szczecin or D¹bie lake. The malacofauna of the city of Szczecin is virtually unknown. Some information is contained in the paper of W£OSIK-BIEÑCZAK (1992). On the other hand, molluscs of suburban areas are constantly exposed to harmful effect of human activities. Because of this I have started a long-term study on molluscs of Szczecin water bodies. This note contains its preliminary results.

STUDY AREA
Field studies were carried out from June till September 1997. The material was collected from 4 reservoirs and the connecting stream: 1. G³êbokie lake, postglacial, maximum depth 5.7 m (own data), strongly polluted, mostly devoid of shore macrophyte belt; 2. Uroczysko pond, located next to the G³êbokie lake, depth ca.

MATERIAL AND METHODS
Molluscs were collected with various methods: catches with a round and a triangular net, mesh 1 mm, triangular drag net, mesh 0.5 mm, diving, collecting empty shells etc. Collecting localities (UTM grid coordinates of all the localities: VV 61) are shown in Figure 1.

RESULTS
A total of 737 mollusc specimens have been collected. Of these 486 were snails, 251 -bivalves. Eleven snail species and 6 bivalve species have been identified. The total of 17 aquatic mollusc species found in the studied area constitute ca. 12% freshwater malacofauna of Poland.
In spite of the fact that all the studied reservoirs are connected by the stream Osówka, two groups of localities of different species composition could be distinguished (Table 1): 1. the lake G³êbokie, 2. the ponds with the stream.

G£ÊBOKIE LAKE
In that lake the most numerous specimens were collected. Snails dominated clearly (99.7%), pelophilous species prevailing. The snails were represented by species that are common to the whole study area: Lymnaea stagnalis (L.), Stagnicola corvus (Gmel.), and by species specific to the lake: Bithynia tentaculata (L.), Valvata piscinalis (O. F. Müll.), V. pulchella Studer and Potamopyrgus antipodarum (Gray). The differences in species composition are associated with the history of this water body. The lake is post-glacial, practically devoid of affluents and surrounded by a forest. There is a canal connecting it with the pond Uroczysko, but the flow is small and often inhibited. In 1996 the pond was dredged which increased its depth but de-100 Janicki D.  stroyed the macrobenthos biocenosis and broke the natural way of migration of molluscs into the lake. The record of expanding Potamopyrgus antipodarum (Gray) is noteworthy. Besides the typical form, f. aculeata was also found. The way by which it got to the lake is mysterious, especially that it is absent from the remaining water bodies. Probably it was transported to the lake by aquatic birds, resting on the lake, from the Gulf of Szczecin; it was found there by WOLNOMIEJSKI & GRYKIEL (1994).
The absence of bivalves in the lake is surprising. One specimen of Anodonta anatina (L.) was found, but it is unknown if it was a remnant of an extinct population or an introduced individual. No members of the genera Sphaerium or Pisidium were found which is in agreement with the data of W£OSIK-BIEÑCZAK (1992).

OTHER WATER BODIES
The malacofauna of the remaining reservoirs is similar. The most frequent species are: Sphaerium corneum (L.), Lymnaea stagnalis (L.), Planorbarius corneus (L.) and Gyraulus albus (O. F. Müll.), found in all the ponds, S. corneum (L.) being recorded also from the stream Osówka. Rare species are: Musculium lacustre (O. F. Müll.), Stagnicola corvus (Gmel.) and Armiger crista (L.), found at one locality each (Table  1). S. corneum (L.) was dominant, being the most abundant in each pond. The most numerous species were found in the pond Ustronie. The total of 190 specimens collected there included 77 snails and 113 bivalves, representing six and three species, respectively. Only six species were found in the pond Uroczysko, 96.8% specimens being bivalves. The low number of species results probably from dredging. The stream Osówka which connects the ponds harbours only one species -S. corneum (L.).

CONCLUDING REMARKS
Eleven snail and five bivalve species were found in the studied area. With respect to their species composition, the studied water bodies can be divided in two groups: G³êbokie lake on one hand and the ponds with the stream Osówka on the other. The absence of bivalves in the lake G³êbokie is noteworthy. An expanding snail species Potamopyrgus antipodarum (Gray) was found in the lake G³êbokie. Some of the recorded species are on the Red List of Polish Animals (G£OWACIÑSKI 1992).