Short-term and seasonal variability of mesozooplankton at two coastal stations (Gdynia, Sopot) in the shallow water zone of the Gulf of Gdańsk

The idea of the study was to describe short-term and seasonal variations in the composition and abundance of zooplankton in the coastal area of the Gulf of Gdańsk at two shore stations and the influence of selected environmental factors (temperature, salinity, wind velocity) on it. The studies were based on material collected in the shallow waters of the Gulf of Gdańsk from stations located by the Marine Promenade (Bulwar Nadmorski) in Gdynia and the pier in Sopot. Samples were collected in 2001 mainly at weekly intervals using an open-type plankton net. Environmental parameters were measured at the same time. The zooplankton at the Gdynia and Sopot stations consisted of 30 taxa; Rotatoria, Copepoda and meroplankton were dominants. There was a much higher percentage of Rotatoria and a lower percentage of meroplankton at Sopot than at Gdynia. Considerable differences were apparent in the numbers of particular groups of zooplankton at these stations in the same periods. Simpson’s biodiversity index, here based on the zooplankton of the shallow waters off Gdynia and Sopot, was relatively high most of the time. Diversity of mesozooplankton was greatest in July and October. There was a significant correlation between the abundance of mesozooplankton and environmental factors in the case of cladoceran and polychaete larvae. Frequent studies at stations located close to each other illustrate the highly dynamic range of variations occurring in the coastal area: the concentrations of particular organisms fluctuated very considerably from one week to another, even by a factor of several dozen. Nevertheless, the statistically mean numbers of zooplankton at both stations are comparable.