Changes of phosphorus and nitrogen concentration and of phytoplankton in enriched lake enclosures1

Enclosures isolating 17 m3 water with the underlying sediments in the mesotrophic area of Lake Balaton were used to simulate the main processes taking place in its hypertrophic area. In an enclosure with both P and N added the production and the biomass of phytoplankton increased immediately. Algae rapidly took up the added nutrients, transporting a third of the loadings from the water to the sediments and supplying a large amount of organic substrate for denitrification. As a result of N loss from the system, Aphanizomenon flos‐aquae formed a second bloom. In an enclosure with only N added the production and the biomass of algae remained similar to those in the open water. NO3− accumulated in the water for a long time. In an enclosure with only P added the algal production and biomass showed a moderate increase, followed by an outburst of A. flos‐aquae. PO43− accumulated in the water during the first part of the experiment and was used up during the second.