Abstracts of papers read at the winter meeting at the University of Bristol

Summaries of changes since 1965 in the phytoplankton fora of Lake Mälren, the third largest lake in Sweden, are presented. The phosphorus load was reduced by 90% in 1970–1973 and a fertilizer factory on the lake decreased its N-outlet by 80%. The phosphorus concentrations in the water decreased by up to 40% and the total nitrogen concentration by 50% between 1970 and 1983. But in one part of the lake the N-concentration increased by 50% due to drainage from surrounding land and atmospheric deposition. With changes in the ratios of N/P and Si/P as well as the turbidity of the water there have been drastic alterations in species composition and total phytoplankton biomasses.

There was a rise in the number of species over the period of study at all times of the year. Colonial species decreased while small and large flagellates of chrysophytes, cryptomonads and dinoflagellates increased. In the most polluted basins cyanobacterial blooms have halved their peak biomasses or have almost disappeared.

At times during the 19 years of the study the summer phytoplankton communities have been tested to fits of a log-normal distribution, assuming such a distribution to be valid in well-balanced ecosystems. The general trend indicates an increasing adjustment to a log-normal distribution. Phytoplankton changes in the lake are associated with effects known to cause reversed eutrophication, although climatically induced variations may also be identified.