69 Cyanobacterial blooms in shallow lakes of the Iławskie Lake District Limnological Review (2011) 11, 2: 69-79 DOI 10.2478/v10194-011-002 - Introduction Algal blooms in inland waters are a frequently observed phenomenon occurring constantly in lakes with extremely high trophy (poly- and hypertrophy), or periodically in less fertile lakes. Special attention is given to water blooms of Cyanobacteria. he suc- cess of blue-green algae in many aquatic ecosystems is possible due to the high lexibility of this group in relation to changes in environmental parameters, such as temperature, pH or salinity (Reynolds et al. 1987). his group of organisms is particularly distinctive due to the cosmopolitan distribution and potential toxic- ity of some species, which create problems for human health and the environment (Berg et al. 1987; Car- michael 1994, 1996; Codd 2000). Phytoplankton blooms are a typical phenom- enon of shallow and over-eutrophicated water bod- ies. According to the concept of “alternative stable states” by Schefer & Jeppesen (1998), such lakes are Cyanobacterial blooms in shallow lakes of the Iławskie Lake District Ewa Dembowska Department of Hydrobiology, Institute of Ecology and Environment Protection, Nicolaus Copernicus University, Gagarina 9, 87-100 Toruń, Poland, e-mail: dembow@umk.pl Abstract: he dominance of blue-green algae observed in many lakes is related to a high trophic level. Shallow eutrophic lakes are particularly oten abundant in blue-green algae. he research on phytoplankton, the results of which are presented in this paper, was carried out between 2002 and 2005 in six lakes. hese lakes difered 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 inluence on the mixing. Sporadic mixed algal blooms were typical of lakes situated in forest catchment areas. Key words: Blue-green algae, cyanobacteria, water bloom, phytoplankton, shallow lakes, turbulent species characterised by little water transparency. Plankton algae are the main primary producer, and their usu- al mass development results in high water turbidity, which consequently leads to a self-shading efect. In a turbid environment, the development of submerged vegetation is limited by poor light availability. Wind mixing disturbs short-term stagnation. Sediments are more frequently resuspended here, mainly as a result of wave motion, which brings about a further increase in turbidity. Resuspension of bottom sediments can lead to sudden, but short-term (pulsatory) enrich- ment of the pelagic zone with nutrients. Due to the lack of rooted vegetation, zooplankton is exposed to strong pressure from ish, and this in turn afects the further development of phytoplankton. According to Wallace et al. (2000), Mischke (2003), as well as Mis- chke & Nixdorf (2003), phytoplankton communities of very shallow (z max <5m) eutrophic lakes are oten dominated by blue-green algae. Cyanobacteria have several, potentially important adaptations, which fa- 8y