Sedimentation losses of Scenedesmus in relation to mixing depth

Abstract

In Lake Nieuwe Meer, artificial mixing led to replacement of cyanobacterial dominance by a dominance of non-buoyant algae. In this study, sedimentation losses of Scenedesmus in the mixed lake are compared with those in the stratified lake. Sedimentation losses were much lower in the mixed lake and this can explain the higher number of cells of non-buoyant algae. Sedimentation losses of Scenedesmus were also investigated in experimental plankton towers with mixed layers of 2 and 10 metres depth. The sedimentation losses in the deep mixed tower were much lower than in the shallow mixed tower and this can explain the higher net increase of cells, despite the lower average irradiance experienced by the plankton in the deep mixed tower. A simple model was used to relate the growth rate and loss processes to mixing depth. This allowed a reconstruction of the phytoplankton population dynamics for the summer period for the stratified and mixed lake on basis of determined Z(eu)/Z(m) ratios. These calculations illustrate the decisive impact of the mixing depth on the phytoplankton population dynamics.