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Picocyanobacterial abundance and their contribution to the total phytoplankton biomass, estimated as chlorophyll a, was investigated in 32 deep and shallow lakes. The lake series covered a wide range of natural and artificial lakes, from high altitude clear lakes and deep, large subalpine lakes through large shallow lakes, small reservoirs and fish ponds. The chlorophyll a concentrations ranged between 0.2 and 390 μg 1−1. No simple relation existed between trophic state and the abundance of picocyanobacterial, mainly represented by Synechococcus spp. Below 10 μg Chl a 1−1 the percentage contribution of picocyanobacteria biomass to total phytoplankton biomass exceeded 70% in some cases, whereas above 100 μg Chl a l−1, the highest contribution was only 10%. At low chlorophyll a concentrations phycoerythrin-rich picocyanobacteria (PE) dominated almost exclusively but their contribution never exceeded 10% of the total picocyanobacterial abundance when chlorophyll a concentration was higher than 50 μg 1−1. Above this value there was high light attenuation and a shift of the maximum light penetration from the blue-green towards the red portion of the spectrum. In this underwater light climate phycocyanin-rich picocyanobacteria (PC) dominated. In the lakes chosen for their representation of ranges in trophy and light quality, PC cells prevailed over the PE when vertical attenuation coefficient of PAR was higher than 2.25 m−1 and red light penetrated farthest. PE cells were 100% when the vertical attenuation coefficient of PAR was lower than 0.55 m−1 and the green and blue were the most penetrating lights.

In order to explore the changes in the community structure of Lake Balaton in response to progressive eutrophication, the historic plankton database (1965–1990) was analysed, using the Zipf-Mandelbrot (ZM) model of dominance and diversity. Our results confirm differences in community organisation between the less eutrophic eastern (Siofok) basin and the near hypertrophie western (Keszthely) basin: the ZM model shows there to be greater influence of the traditional limiting environmental factors in the less eutrophic basin. There is also a greater evenness in the total biomass in the less eutrophic basin, determined by the variability in the environmental control applied.

The N:P ratios (TN:TP, NO3-N:TP and DIN:SRP) in the shallow, hypertrophic Marcali reservoir varied about 1, far below the physiological optimum for the planktonic algae. Moreover, N2-fixing cyanobacteria were absent in the vegetation period. One possible explanation for this phenomenon could be the filtration of the stocked silver carp (Hypophthalmichthys molitrix Val.), a phytoplankton feeder, and bighead carp (Aristichthys nobilisRich.), an omnivorous plankton feeder population (600 kg ha–1) which strongly reduces the large sized algae.

A comparison of the composition of natural phytoplanktoncommunities with the gut content of silver carp (Hypophthalmichthys molitrix) showed that this fish speciescan nottake up algae smaller than 10 µm. The species compositionofalgal assemblages in the gut content of the silver carpusuallydiffers significantly from the composition of the >10 µmsizefraction of the natural phytoplankton assemblages. Thisdifferenceis mainly a result of the taxon selective digestion of silvercarp.In vitro experiments with digestive enzymes of thisspeciesresulted in a very fast (1–2 minutes) disintegration ofnon-mucilaginous cyanobacteria. The digestion of diatoms andcryptophytes was also effective, but mucilaginouscyanobacteria andgreen algae (Chlorococcales) proved practicallyindigestible.

Two reservoirs, constructed for pollution control in the catchment area of Lake Balaton, were studied. Both are hypertrophic due to high phosphorus loading, and both are rich in dissolved humic substances (colour in Pt units usually varies between 50–150 Pt, mg 1_1) as they were built on former wetland areas. We measured planktonic primary production with 14C technique, and bacterioplankton production using the [methyl-3H]thymidine incorporation method. The levels of dissolved organic carbon (DOC) and humic substances (as water colour) present were investigated. Temperature, pH and light intensity at different depths were recorded regularly. In both reservoirs studied the bacterioplankton production was as high as the phytoplankton production and in some cases significantly exceeded it. Dissolved humic substances proved to be an important substrate for planktonic bacteria in these water bodies.

Relationships between chlorophyll a content of the water, the shoreline-length: water area ratio and the annual total fish yield as catch per unit effort (CUE: kg ha−1 100 h−1 as annual mean values) have been calculated by multivariable regression. The determination coefficient (r 2 = 0.913) showed a significant dependence of fish yield on morphometry of different lake areas. Accordingly, fish carrying capacity of the open water areas, calculated from chlorophyll a content and S/A, ranged from 12 to 34%, but that of the littoral zone between 66 and 88%. These findings have also been supported by echo-sounding records of the horizontal distribution of fish.