Cyanobacterial blooms in shallow lakes of the Iławskie Lake District
The dominance of blue-green algae observed in many lakes is related to a high trophic level. Shallow eutrophic lakes are particularly often abundant in blue-green algae. The research on phytoplankton, the results of which are presented in this paper, was carried out between 2002 and 2005 in six lakes. These lakes differed considerably in their size and management methods applied in the catchment (drainage) area. A few types of water blooms were distinguished, which is related to the catchment area management, the intensity of mixing and the trophic level. Algal blooms of the Planktothrix type appeared in lakes situated in an open area of agricultural catchment basins. Algal blooms of the Limnothrix type were characteristic of lakes with a forest-agricultural catchment area but surrounded by high shores, which reduced the wind influence on the mixing. Sporadic mixed algal blooms were typical of lakes situated in forest catchment areas.
The routine evaluation of water quality is limited to lakes with the largest area. In Poland, only lakes with an area exceeding 50 hectares are monitored by the State Environmental Monitoring System. For many local communities, however, small lakes are more important. This applies mainly to areas with a small number of lakes, where even the smallest lakes are used for various purposes. This paper presents the results of phytoplankton analysis in a small and shallow lake used for recreation. The study was conducted at three sites located in different parts of the lake. A total of 122 algae taxa were identified in the phytoplankton, mainly diatoms and green algae. The most constant taxa in the lake were: Stephanodiscus hantzschii, Desmodesmus communis, Pediastrum tetras and Crucigenia tetrapedia. The average phytoplankton biomass was 37 mg l−1. The maximum biomass, almost 140 mg dm−3, was recorded in late July at the site located near the beach. At that time, there was a massive cyanobacterial bloom composed of Microcystis wesenbergii and Aphanizomenon issatschenkoi. Based on these studies, the lake should be classified as hypertrophic with bad ecological status. This lake should not be used for recreational purposes in the current state.