CYANOBACTERIA DYNAMICS IN A RESTORED FINNISH LAKE - A LONG-TERM SIMULATION STUDY

Several explanations have been presented for the formation of lacustri ne cyanobacteria blooms. These include elevated water temperature, nut rient enrichment, low N/P ratios, low light energy requirements, high pH and/or low carbon dioxide concentration, selective zooplankton graz ing, excretion of compounds which suppress the growth of competing alg ae, and migration. The impact of the first four were studied in Lake T uusulanjarvi, a eutrophic Finnish lake, using deterministic simulation . The study, which covered 15 years, included a sequence of cases on t he recovery of the lake after the implementation of two restoration me asures: aeration of the hypolimnion, and termination of municipal wast e discharge, both of which were done during the study period. The focu s was on Microcystis and Oscillatoria, and on the N-fixing cyanobacter ia, Anabaena and Aphanizomenon. Eukaryotic phytoplankton were also inc luded in the study. Monitoring data were used. The roles of the hypoth eses were discussed, from the point of view of the utility of the info rmation in water resources management.