Transport and Burial as a Cause of Whitefish (<i>Coregonus</i> sp.) Egg Mortality in a Eutrophic Lake

Abstract

Predictions based on the Shields diagram and confirmed by experiments conducted in eutrophic Lake Sempach imply that bottom currents associated with winter storm events are responsible for the simultaneous transport of coregonid eggs and fine silt and clay (grain size [Formula: see text]) from the spawning grounds into deeper lake regions. The critical shear stress required to initiate egg transport is estimated to lie in the range 0.02–0.04 N∙m −2 , corresponding to mean current speeds of 10–15 cm∙s −1 at a reference height of 0.5 m above the sediment surface. On settling out, egg burial is likely. This will increase egg mortality not only by physically hindering oxygen transport to the egg, but also, in POC-rich eutrophic lake sediment, by relocating the egg in a zone of steep oxygen gradients and low mean oxygen concentrations. Microelectrode measurements and computation of the thickness of the oxygen diffusive boundary layer over the sediment reveal that even eggs that escape interment are likely to be subjected to ambient oxygen concentrations insufficient for development to hatching. It is suggested that transport and burial may in general be important factors determining coregonid egg mortality in eutrophic lakes.