Phytoplankton of two Danish lakes, with special reference to seasonal cycles of the nannoplankton

Summary

The phytoplankton of the Tystrup-Bavelse Lakes and of Lake Esrom has been examined, especially as regards the composition, quantity, and seasonal cycles of the nannoplankton.

Nannoplankton percentages of the total cell number were very high (about 80%) most of the year, with lower values in summer and early autumn. In the Tystrup-Bavelse Lakes diatoms were dominant in spring, Chlorophyceae in summer, and Cryptophyceae in autumn and winter. In Lake Esrom Chrysophyceae and Chlorophyceae were dominant in winter and spring, Cryptophyceae in summer and autumn.

On an average, the nannoplankton constituted 25% of the phytoplankton biomass in both lakes. There was a prominent vernal maximum (50–80%), and during the rest of the year very low values. During this maximum the activity coefficient reached its highest values, an indication of the importance of nannoplankton for the primary production. The seasonal cycles of nannoplankton biomass were on the whole due to the diatoms. Of the total phytoplankton only the nannoplankton directly serves as food for zooplankton, and so grazing effects on the phytoplankton will only be of importance at high nannoplankton biomass percentages.