EFFECTS OF BENTHIVOROUS BREAM (ABRAMIS-BRAMA) AND CARP (CYPRINUS-CARPIO) ON SEDIMENT RESUSPENSION AND CONCENTRATIONS OF NUTRIENTS AND CHLOROPHYLL-A

1. The effect of benthivorous bream and carp on sediment resuspension and the concentrations of nutrients and chlorophyll a were studied in sixteen experimental ponds (mean depth 1 m, mean area 0.1 ha, sandy clay/clay sediment), stocked with bream or carp at densities varying from 0 to 500 kg ha-1. Planktivorous perch (Perca fluviatilis L.) were added to some ponds to suppress zooplankton. 2. Suspended sediment concentrations increased linearly with biomass of benthivorous fish. Bream caused an increase of 46 g sediment m-2 day-1 per 100 kg bream ha-1 and a reduction of 0.38 m-1 in reciprocal Secchi disc depth, corresponding to an increase in the extinction coefficient of 0.34 m-1. 3. No relationship was found between size of fish and amount of resuspension, but the effect of bream was twice as great as that of carp. Benthivorous feeding was reduced in May because alternative food (zooplankton) was available. 4. Assuming a linear relationship, chlorophyll a level increased by 9.0 mug l-1, total P by 0.03 mg l-1 and Kjeldahl-N by 0.48 mg l-1 per 100 kg bream ha-1. Silicate, chlorophyll a, total P and total N were all positively correlated with fish biomass, but orthophosphate showed no correlation.