Spring phytoplankton of Lake Onego as a trophic resource for deep-water benthos

Abstract

In the past few years the relict deep-water crustacean Monoporeia affinis (Lindström, 1855) has become less abundant in Lake Onego, Europe’s second largest lake. Further studies are needed, therefore, to better understand the formation of spring phytoplankton as food for deep-water benthos. The aim of the present project was to study the temperature-dependent vertical distribution of phytoplankton in Petrozavodsk Bay of Lake Onego in the spring season. In May 2019, the total abundance, biomass and species composition of phytoplankton were assessed. The transparency and temperature of Onego water were also measured. The median values of phytoplankton’s total abundance and biomass at various growth stages were 1.15–1.67 M cells/l and 1.96–3.12 mg/l, respectively. The above indices were consistent with the long-term maximum growth of phytoplankton in Petrozavodsk Bay in spring and were not markedly different (p<0.05) from those for spring phytoplankton in 1989-2015, indicating the permanent trophic status of Petrozavodsk Bay in the spring season over the past 30 years. A significant difference (p>0.05) in the vertical distribution of phytoplankton, dominated by Aulacoseira islandica (O. Müll.) Sim diatoms, was found between the stations in May 2019. The reproduction of the relict deep-water crustacean M. affinis, taking place simultaneously with the intensive vegetation and the settling of its major food resource, A. islandica, on the bottom, suggests the formation of favourable feeding conditions for deep-water benthos in Petrozavodsk Bay of Lake Onego.