Seasonal succession, vertical distribution and long term variation of phytoplankton communities in two shallow forest lakes in eastern Finland

Abstract

Two pristine forest lakes have been sampled for 12 years in order to follow the long term variation of phytoplankton biomass and species composition together with environmental parameters. The lakes Iso Hietajärvi and Pieni Hietajärvi are shallow headwater lakes situated in the Patvinsuo National Park, eastern Finland. Lake Iso Hietajärvi was weakly stratified during the open water season in 1988–2001, in contrast to Lake Pieni Hietajärvi, which was always strongly stratified mainly because of its wind-sheltered location and dark water. During the long study period, the main changes in water quality have been slight increases of alkalinity and pH-value and also some decrease of the nitrate content. The phytoplankton (wet) biomass in surface water varied from 0.07 to 0.51 mg l⁻¹ in Lake Iso Hietajärvi and from 0.18 to 0.60 mg l⁻¹ in Lake Pieni Hietajärvi in 1991, during the intensive weekly sampling. The biomass was mainly composed of Cryptophyceae, Chrysophyceae and Bacillariophyceae in Lake Iso Hietajärvi, and Cryptophyceae in Lake Pieni Hietajärvi. In Lake Iso Hietajärvi, the differences in the vertical distribution of phytoplankton biomass and species composition were small and the productive layer reached down to 4 m, whereas in the dark coloured Lake Pieni Hietajärvi the productive layer was shallow, ≤1 m. When the whole productive water column is considered, the unstratified clear water Lake Iso Hietajärvi is more productive than the stratified dark coloured Lake Pieni Hietajärvi. The seasonal succession of phytoplankton species composition is different in these adjacent lakes; more variation was found in the unstratified Lake Iso Hietajärvi compared to the smaller stratified lake.