Journal of Plankton Research Vol.25 no.8 pp.873-883, 2003
© Oxford University Press 2003
Potential effects of abiotic factors on the abundance of autotrophic picoplankton in four boreal lakes
Institute of Ecology, Department of Hydrobiology, Polish Academy of Sciences, 05-092 
omianki, Poland and
1 Lammi Biological Station, Helsinki University, 16900 Lammi, Finland
* Corresponding Author: iwonaj65{at}hotmail.com
The seasonality of autotrophic picoplankton (APP) in four boreal lakes of varying trophic state and light availability was studied. Red-fluorescing cyanobacteria (CyAPP) dominated the APP in three lakes, but in the fourth, most humic lake, Valkea Kotinen, eukaryotic picoalgal cells (EuAPP) predominated. The most productive, least humic, and stratified lake (Ormajärvi) was the only one in which low numbers of orange-fluorescing cyanobacteria were present in winter (7 x 102 cells ml-1) and early spring (1 x 102 cells ml-1) under the ice cover. The numbers of CyAPP did not correspond to the trophic status of the lakes. The highest average and maximal numbers of CyAPP were recorded in shallow and moderately productive Jylisjärvi (6.2 x 105 cells ml-1) while the lowest number (8 x 104 cells ml-1) was recorded in the deepest lake, Pääjärvi, with fairly similar water quality. Light availability and water temperature seemed to be more important abiotic factors in regulating CyAPP and EuAPP density than nutrient concentrations. In the deepest lakes, light and temperature correlated very strongly with density of CyAPP in spring, while in the shallowest lake this correlation only occurred in autumn. Although light and temperature were occasionally significantly correlated with EuAPP cell density, no common trend throughout all the seasons (as seen for prokaryotic APP) was found.