Seasonal succession of phytoplankton and zooplankton along a trophic gradient in a eutrophic lake -: implications for food web management

The water quality and seasonal dynamics of phytoplankton and zooplankton in Lake Hiidenvesi were explored in order to evaluate the possibilites to improve the water quality by mass removal of fish. The four sampling stations were situated in basins representing a transition from hypertrophy to mesotrophy and maximum depths from 3 m to 33 m. The maximal phytoplankton biomasses ranged from 5 to 33 mg l(-1) and the maximal zooplankton biomasses from 160 to 520 mu g C l(-1). Cyanophytes dominated the phytoplankton biomass in summer and the copepod biomass was higher than that of cladocerans at all the stations. No spring peak in the zooplankton biomass occurred and the average size of daphnids was below 1 mm. At present, the top-down control of phytoplankton biomass is thus ineffective. However, the water quality and morphometry of L. Hiidenvesi seem favourable for biomanipulation. The nutrient concentrations and external loading fall in the limits that facilitate successful restoration and the morphometry of the lake is favourable for the recolonization by submerged macrophytes. However, since stations 2 and 3 receive their nutrient loading mainly from station 1, restoration of station 1 may be crucial for the whole lake. This may not be achieved through food web management alone, but requires a reduction of the nutrient flow from River Vihtijoki.