Ecology of mixotrophic flagellates with special reference to Chrysophyceae in Danish lakes

Abstract

Although intensively studied in Danish lakes, Chrysophytes constituted only a minor part of the phytoplankton in the lakes studied in the Danish Survey Programme of the Water Environment during 1989–94. However, in the lakes Holm Sø, Maglesø by Brorfelde, and Bastrup Sø, populations of naked and loricated (mixotrophic) Chrysophytes exhibited 2–3 maxima yr⁻¹ and contributed 2–36% to the yearly mean phytoplankton biomass. The mixotrophic Chrysophyte biomass in these lakes increased with increasing biomass of the entire phytoplankton community up to 5 mm³ 1⁻¹. Above this phytoplankton biomass, the mixotrophic Chrysophyte biomass became irregular and scarce.

Mixotrophic Chrysophytes were mainly found at TP concentrations below 0.050 mg l⁻¹ and at SRP concentrations below or at the detection limit (0.010 mg l⁻¹). There was a slight increase in the biomass when SRP climbed over the detection limit but above a concentration of 0.015 mg SRP l⁻¹, mixotrophic Chrysophytes disappeared. Mixotrophic Chrysophytes increased in the interval of 2–6 mg COD l⁻¹ and in the interval of 2.5–6 mg suspended matter l⁻¹. The mixotrophic Chrysophytes are hardly dependent on bacteria uptake for C but rather for P. Under substrate limitation, bacteria have a much lower C:P ratio than the minimum C:P ratio (Redfield ratio) of 106:1 for the optimum growth of algae. Under P-limitation, the C:P ratio of algae is normally higher. The difference in C:P ratio between bacteria and algae makes it possible for mixotrophic Chrysophytes to solve their demand of P by ingestion of bacteria.