Seasonality of phytoplankton in subarctic Lake Saanajärvi in NW Finnish Lapland

Abstract

We studied the phytoplankton seasonality in the subarctic Lake Saanajarvi, Finnish Lapland, in two successive years with slightly different weather conditions. The total number of taxa studied during the period was 148. Characteristic phytoplankton species were chrysophytes Uroglena sp., Chrysococcus spp., Dinobryon spp and diatoms Cyclotella spp. The results were analysed in relation to weather patterns and physico-chemical variables measured from the lake during the 2 years. The seasonal dynamics of phytoplankton were characterized by (1) maxima in total densities during autumn and minima in winter; (2) different species reaching maximum and minimum densities during different seasons; (3) close to equilibrium state during strong thermal stratification in 1997 with dominance of only a few taxa; and (4) two annual maxima in species diversity at the beginning of the thermal stratification and during the autumn overturn. According to canonical ordinations, calcium buffer capacity, nutrients and temperature all play a role in regulating algal biomass and species compositions. With regard to physical factors, the length of the mixing cycle, thermal stability of the water column and water temperature seem to have a major control over the plankton dynamics. The length of the ice-free season seems to be more decisive for biomass production than the thermal stability during this period, which, in turn, appears to affect the algal biodiversity.