Trophic state of the Great Masurian Lakes system in the past, present and future - causes, mechanisms and effects of changes

Lakes of the Great Masurian Lakes system (GML-system) have been the topic or model ecosystem in a large amount of studies, but a look at the recent history of ecological changes and evaluation of the present ecological state have not been previously performed in a comprehensive way. For this purpose, I utilised a database of published and unpublished ecological data from 1980-2008 collected by the Microbial Ecology Department. In addition, the present state of the GML-system was investigated during the spring and summer months of 2009, 2010 and 2011. These datasets were combined to evaluate changes in water quality from 1980 onwards. The ecological changes were compared to external input data from a governmental statistical office. The present ecological state of the system was analysed in more detail, and the best strategies to further decrease the trophic state of the lakes and to limit the biomass of phytoplankton and especially cyanobacteria were investigated.

The external nutrient inputs to the GML-system began to decrease in 1990. Signs of oligotrophication became clear after 1995 and continued up to 2005. The highest improvement occurred in Lake Niegocin, owing to highly improved wastewater treatment in the town of Giżyczko. After 2005 the oligotrophication of the GML-system stopped, which corresponds to increasing nutrient inputs from agriculture and tourism and possibly also from internal sources. The differences in trophic state index (TSI) between lakes have decreased significantly. During 2009-2011, mean TSIs ranged from 50 to 63, i.e. all lakes could have been classified as eutrophic. The trophic state of the GML-system was highly variable and dependent on weather conditions. In 2010, an extreme heat wave caused significant changes in multiple variables (such as limnetic nutrient concentrations and stratification structure) and possibly internal loading of phosphorus from sediments to epilimnion. Main effects of eutrophication of the GML-system were identified to be increased dominance of primary production over respiration (i.e. accumulation of organic matter), stronger nitrogen limitation and increased biomass of cyanobacteria. Phytoplankton growth was not found to be limited by a single nutrient, but rather showed co-limitation by nitrogen and phosphorus. Still the conducted analyses supported the classical view that decreasing external phosphorus inputs to lakes is the best lake management approach for controlling eutrophication and biomass of cyanobacteria. Possible solutions for decreasing external phosphorus inputs to the GML-system are, for example, the construction of vegetative riparian buffers and the improvement of wastewater treatment efficiency, especially in Ryn and Mikołajki.