Indirect effect of different fish communities on nutrient chlorophyll relatonship in shallow hypertrophic water quality reservoirs

摘要

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years(1992-1993) in three Hungarian shallow water reservoirs 9Cassette and outer reservoir of the Kis-Balaton Water Protection System,and Marcali reservoir).Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examiend.Considerable differences in the proportions of different nutrient forms were observed between the three test sites,which could be explained by the presence of different fish stocks in these reservoirs.In the Cassette,the fish biomass necessary for a watr quality improvement was around 50 kg ha~-1.Phytoplankton biomass was ocntrolled by the zooplankton,consequently chlorophyll a concentrations decreased considerably,while those of dissolved nutrients significnatly increased.In the outer reservoir,phytoplankton was controlled bottom-up,since the 250 g ha~-1 fish biomass waslarger than the critical value due to the high proportion o fplanktivorous species.Chlorophyll a concnetrations were high,and nutrients were mainly in particulate form (in algal cells).In the Marcali reservoir,the reservoir,the recently introduced silver carp population could not control fully the phytoplankton.The biomass of phytoplankton decreased only slightly,while its composition changed considerably.although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms,reduction of the amount of planktivorous fish seems to be a more adequate method for increasing watr transparency,rather than introduction of phytoplankton feeding fish.