Determination of phytoplankton losses by comparing net and gross growth

By comparing primary production (¹⁴C) and biomass variation it is possible to calculate the total losses of phytoplankton. For the mesotrophic drinking water reservoir Saidenbach, average loss rates of -0.31 d^-1 for the total phytoplankton and -0.99 d^-1 for nanoplankton were determined from September 1980 to May 1981. The greater the share of nanoplankton in the total phytoplankton, the less the real activity as reflected in biomass changes observed. The considerable (mainly nanoplankton) losses, however, cannot be explained by grazing or sedimentation. They are assumed to be caused by high mortality of flagellates due to a relatively high depth of mixing and their retention in the aphotic layer.