Size variations of planktonic Aulacoseira Thwaites (Diatomae) in water and in sediment from Finnish lakes of varying trophic state

We measured siliceous frustules of diatoms belonging to the genus Aulacoseira Thwaites in phytoplankton samples of 33 lakes in Finland, and found some significant relationships between the frustule dimensions and the concentrations of total phosphorus, phosphate phosphorus, total nitrogen and nitrate nitrogen in lake water. The size alterations appeared to be species specific and the studied taxa showed different patterns of morphometic change along the nutrient gradients. Some taxa were clearly smaller in eutrophic and mesotrophic lakes. We also observed size reductions of diatoms in the upper parts of sediment cores from two lakes which have apparently become more nutrient rich during recent decades.