Effects of a waste stabilization lagoon discharge on bacterial and phytoplanktonic communities of a stream

To determine the effect of a waste stabilization lagoon discharge on the pl ankton in a receiving stream, bacterial and phytoplanktonic communities wer e studied above and below the outflow. The discharge apparently had seasona l and continuous effects on both the structure and function of plankton com munities. These effects below the lagoon resulted in a significant increase in the bacterial and phytoplanktonic biomasses and significant changes in the structure of these communities: increases of Euchlorophyceae (Chlorella and Scenedesmus), Euglenophyceae (Trachelomonas), Cyanobacteria biomasses, and the relative biomass of very large bacteria. Metabolic activities of t he plankton communities seemed to be significantly influenced; photosynthet ic and heterotrophic activities increased, while productivity (photosynthet ic assimilation and chlorophyll a) decreased. This decrease shows that allo chthonous phytoplankton photosynthetic efficiency is lower than that of the autochthonous community or that this efficiency is reduced in the receivin g stream. Discharge favored the development of mixotrophic microorganisms t o the detriment of photosynthetic microorganisms.