Zooplankton structure in midfield ponds in north-west part of Poland

Midfield ponds are characterised by a rich biodiversity. A significant role in the function of these specific water ecosystems is played by resident zooplankton, which is a valuable component of numerous trophic networks. The aim of the present work was to study the taxonomic composition, concentration and biomass of zooplankton in two midfield ponds situated in the area of one commune, but geomorphologically different from each other. Experiments were conducted in the summer seasons (June-August) in the years 2011-2013 in two ponds. The presence of 55 zooplankton taxa, commonly present in such water reservoirs in Poland, was observed. Most species were determined. The largest group comprised rotifers (Rotifera) - 35 taxa. The crustacean zooplankton was less diverse: 11 taxa of copepods (Copepoda) and 9 taxa of water fleas (Cladocera) were found. The reservoir in Stare Czarnowo exhibited a greater number of taxa, where as many as 40 taxa were observed. However, 35 taxa belonging to various zooplankton groups were found in the pond in Żelisławiec. The relatively low Jaccard index confirmed small qualitative similarity between the studied ponds. Both reservoirs contained representatives of species defined as characteristic for oligosaprobic and mesosaprobic waters, which may indicate small contamination of the two ponds and the occurrence of a large amount of organic matter undergoing decomposition.