Background. Copepods are the major secondary producers in the World Ocean. They represent an important link between phytoplankton, microzooplankton and higher trophic levels such as fish. They are an important source of food for many fish species, but also a significant producer of detritus. In terms of their role in the marine food web, it is important to know how the environmental variability affects the population of Copepoda.
Methods. The study of the zooplankton community in the south-eastern Baltic Sea conducted during a 24-month survey (January 2010 to November 2011) resulted in 24 invertebrate species identified (10copepods, 7cladocerans, 4rotifers, 1ctenophore, Fritillaria borealis and Hyperia galba). Data were collected at two stations located on the open-sea deep-water station – the Gdańsk Deep (54o50’φN, 19o19’λE) and in the western, inner part of the Gulf of Gdańsk (54o32’ φN, 18o48.2 ’λE). Vertical hauls were carried out using two nets: a Copenhagen net with an inlet diameter of 50 cm and a mesh diameter of 100 µm (in 2010) and WP-2 net from KC Denmark with an inlet diameter of 57 cm and a mesh diameter of 100 µm (in 2011).
Results. The paper describes seasonal changes in the abundance and biomass of Copepoda, taking into account the main Baltic calanoid copepod taxa (Acartia spp., Temora longicornis and Pseudocalanus sp.). They usually represented the main component of zooplankton. The average number of Copepoda at station P1 during the study period of 2010 was 3913 ind.m-3 (SD 2572) and their number ranged from 1184 ind. m-3 (in winter) to 6293 ind.m-3 (in spring). One year later, the average count of copepods was higher, i.e. 11 723 ind. m-3 (SD 6980) and ranged from 2351 ind. m-3 (in winter) to 18 307 ind.m-3 (in summer). Their average count at station P2 in 2010 was 29 141 ind. m-3 ranging from 3330 ind.m-3 (in March) to 67 789 ind. m-3 (in May). The average count of copepods in 2011 was much lower – 17 883 ind./m3 and ranged from 1360 ind./m3 (in April) to 39 559 ind./m3 (in May).
Discussion. The environment of pelagic animals changes with the distance from the shore and with the sea depth. Although the qualitative structure of zooplankton is almost identical with that of the coastal waters, the quantitative structure changes quite significantly. The maximum values of zooplankton abundance and biomass were observed in the summer season, both in the Gdańsk Deep and the inner part of the Gulf of Gdańsk. Copepoda dominated in the composition of zooplankton for almost the entire duration of the research.. Quantitative taxonomic composition of Copepoda at station P1 (the Gdańsk Deep) was different compared to station P2 (the western, inner part of the Gulf of Gdańsk) due to a high percentage of a crustacean preferring waters with lower temperature and higher salinity – Pseudocalanus sp.