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Quantitative aspects of the spionid polychaete Pygospio elegans population in the Polish coastal zone of the Baltic Sea, in estuarine areas of Pomeranian river mouths were investigated. The fre-quency of occurrence (F) of P. elegans in the Polish coastal zone averaged 54% (permanent spe-cies) and ranged from 20 to 80%. The polychaete abundance along the Central Pomeranian coast peaked at 1 837 ind. m-2, the mean abundance being 175.7 ind. m-2. The density of the spionid worm in the Middle Pomerania was low and rarely exceeded 1 000 ind. m-2. The mean wet weight biomass of P. elegans in different areas was low (max. 0.29 gww m-2). The abundance of P. elegans in the river mouth areas (estuaries) was basically higher west of the mouth, in areas less exposed to polluted and freshened riverine water; water in those areas, however, carried lower bioseston loads, which affected trophic conditions.
The populations of common Baltic bivalves Macoma balthica, Mytilus edulis and Cerastoderma glaucum were studied in the Polish coast (up to 4 Nm), both in estuary and open coastal zones of the Middle Pomerania as well as in the Puck Bay and the Gulf of Gdańsk (up to 2 Nm). The following parameters were investigated: density of the bivalves and environmental conditions (distance from the shore, substrate type, coast type, depth). Relationships between environmental conditions and abundance of the consecutive bivalve species were analysed with the help of ordination method (redundancy analysis RDA) and multivariate regression trees (MRT). RDA analysis indicated that C. glaucum density depended mainly on the localisation along the coast while M. edulis occurrence was governed by the distance from the shore and depth. Only density of M. balthica depended evenly on all the environmental parameters. MRT analysis revealed that the main factors influencing the distribution of bivalves were sediment granularity and depth.
A population of Cerastoderma glaucum Poiret 1789 was studied in the Polish coastal zone (up to 3 Nautical miles) of the Baltic Sea within the Puck Bay and part of the Gulf of Gdańsk. The frequency (F) in the sheltered coastal zone of bays was high (65%), whereas at the exposed open coast of the Middle Pomerania it was lower (43%). Also, abundance in the Puck Bay and the Gulf of Gdańsk was over 3-fold higher than in the coastal zone of the Middle Pomerania. By means of classic statistical methods (essential tests) we proved the significance of differences between mean abundance at west and east transects in estuaries and between open coast and estuaries. The multivariate regression and classification trees (MR&CT) indicated that biomass and abundance of C. glaucum in the Baltic Sea were affected by the kind of bottom, whereas depth and distance to the bank influenced cockles' abundance and biomass in bays. The population's structure in the Puck Bay and the Gulf of Gdańsk was determined mainly by the depth. In the Wieprza River estuary only specimens from age group 1+ were found. On all the other studied profiles cockles lived up to 3 or 4 years and the dominant group was 2+. The highest mean width of shell was in the Słupia River estuary, and in the bays shell width increased from west to east. In comparison to the other bivalves inhabiting the discussed area, C. glaucum does not play an important role as food source for fish and other consumers but may serve as an indicator of environmental conditions.
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