Effects of nutrient load on species composition and productivity of phytoplankton in Lake Ladoga

As part of the joint Russian-Finnish evaluation of human impact on Lake Ladoga, we studied the phytoplankton species composition, biomass, chlorophyll a content and primary production in the lake in order to estimate the state of eutrophication. Samples were collected from 9–31 sampling stations in August 1992–95. In the surface water, the phytoplankton biomass varied from 0.3 to 6.6 g m–3 fresh weight, chlorophyll a from 1.7 to 18.6 mg m–3 and the primary productivity of phytoplankton from 33 to 471 mg C m–3 d–1. The biomass was lowest close to the western shore and highest in the Sortavala Bay. Blue-green and green algae were abundant inshore and small cryptophyceans (Rhodomonas lacustris, Cryptomonas spp.) offshore. In the most nutrient-rich parts of Lake Ladoga (Volkhov, Sortavala and Svir bays) the phytoplankton communities were dominated mainly by blue-greens and cryptomonads. Areas close to the Vuoksa and Burnaya rivers were dominated by diatoms (Diatoma tenuis, Asterionella formosa, Tabellaria fenestrata, Aulacoseira italica, A. granulata, Melosira varians, Fragilaria crotonensis, Stephanodiscus binderanus and Synedra actinastroides). In the 1970s and 1980s eutrophication was seen not only as changes in species composition of phytoplankton but also as growing biomass and algal primary productivity. The maximum mean total phosphorus content (26 mg m–3) was attained during the late 1970s when the mean biomass of algae was six times and the maximum values 20 times those in the 1960s. In the 1990s the tendency has been towards decreasing phosphorus content (1992–95 mean 18 mg m–3), but the changes in phytoplankton productivity between years were connected mainly with temperature. On the basis of phytoplankton biomass, chlorophyll a content and primary production, Lake Ladoga can presently be classified as a mesotrophic lake. The most eutrophicated areas are found in the northern archipelago and in areas influenced by large rivers.