Eeva Huitu

Fish catch in relation to zooplankton community structure in lakes of varying trophic status

The nutrient concentration (especially phosphorus) of a lake is one of the most important determining factors for zooplankton and fish community structure. The correlation between nutrient concentrations and abundance of fish has been established in many types o f lakes around the world (HANSON & LEGGETI 1982, QUIROS 1990, BACHMANN et al. 1996), and the increasing proportion of cyprinids along the phosphorus (P) gradient has also been documented (JEPPESEN et al. 1990, ÜLIN et al. 2002). This increase of cyprinids has been found to cause changes in crustacean zooplankton community, toward smaller cladocerans (JEPPESEN et al. 2000). Fish stock surveys were carried out on 16 lakes in southern Finland during 1998, 2001-2002 (ÜLIN et al. 2002, NYHOLM et al. 2003). Plankton sampling and water chemistry measurements were done during the same period (NYHOLM et al. 2003, HuiTu & MÃKELÃ unpubl.). The purpose of this study was to determine changes in crustacean zooplankton community structures in lakes of varying trophic status, including the possible effects o f varying abundance o f fish and the structure o f fish assemblage.

Control mechanisms of cyanobacterial blooms in lakes of varying trophic status and morphometry

The positive relationship between the nutrient concentration and algal biomass in freshwater lakes has been cJearJy proved (VOLLENWEIDER !970, SCHINDLER 1977). Likewise, the increase o f the proportion of cyanobacteria with progressive eutrophication is well documented (PICK & LEAN 1987, TRIMBEE & PREPAS 1987). Nutrient concentration, however, is not the only factor affecting the cyanobacterial biomass or taxonomic composition of the phytoplankton (WATSON et al. 1997). In fact, previous studies have concluded that physical factors may have a greater influence on the cyanobacteria than any single chemical factor. For example, the conditions of thermal stratification, and to some extent nutrient ratios, may play a comprehensive ro le in the cyanobacterial occurrence in natural conditions (VARIS 1990, ÜJALA et al. 2003 ). In addition, high content ofhumic matter, a characteristic of Finnish lakes, a1so affects the availability of nutrients (ARVOLA et al. 1999) and thus can change the algal community structure. In this study, an extensive data set was used to study cyanobacterial occurrence over a range o f nutrient status and organic matter content. Ihe aim of the study was to clarify the role of chemical and/or physical factors that control the dynamics of cyanobacterial blooms.