DYNAMICS OF PHYTOPLANKTON DETRITUS IN A SHALLOW, EUTROPHIC LAKE (LAKE LOOSDRECHT, THE NETHERLANDS)

A study was made of the mortality and aerobic decomposition of light- and phosphorus-limited cultures of Oscillatoria limnetica, a dominant phytoplankton species in shallow, eutrophic Lake Loosdrecht (The Netherlands). When placed in the dark at 20-degrees-C, most cells died and lysed within twelve days. The labile organic matter was completely decomposed within three weeks. Absorbance spectra indicated that blue green algae may contributed significantly to the refractory dissolved substances in the lake. Refractory particulate matter constituted from 7 to 24 % of the biomass of 0. limnetica, depending on the growth rate before incubation in the dark. The decomposition rate of this fraction was 0.005 d-1. On a basis of a steady-state model of the dynamics of phytoplankton detritus, the areal organic dry weight concentration of the detritus in the lake is ca. 60 g m-2. This means the quantities of detritus in the seston and epipelon are about equal.