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A new technique, delayed fluoresence, developed within the EC-project ”Phytoplankton on line”, was used to follow the phytoplankton succession under the ice and through the ice-breakup in Lake Erken, Sweden. The monitoring was done automatically with high frequency measurements. A complementary weekly monitoring of chlorophyll a, phytoplankton composition and nutrient concentrations was performed and used to interprete the results.

An interesting diurnal pattern of delayed fluorescence was shown under the ice and getting stronger as the light conditions improved through the melting of the snow and ice cover. Peaks in fluorescence occured in early morning and in late afternoon-evening.