Influence of diet shifts in underyearling fish on phosphorus recycling in a hypertrophic biomanipulated reservoir

1. In addition to effects of direct predation by planktivorous fish, nutrie nt recycling by fish may also contribute to structuring foodwebs in lakes. There is little evidence, however, about whether underyearling fish undergo ing several ontogenetic diet shifts may have a comparable bottom-up impact. 2. This study examined seasonal patterns of phosphorus (P) concentration an d external load, phytoplankton, zooplankton and benthos, and diet shifts in three underyearling fish [perch (Perca fluviatilis), roach (Rutilus rutilu s) and ruffe (Gymnocephalus cernuus)] in the shallow, hypertrophic biomanip ulated Bautzen reservoir, Germany. Phosphorus metabolism of fish was calcul ated by a balanced bioenergetics model on the basis of fish diet, growth an d water temperature. 3. The fish showed several shifts from planktivory to other food sources du ring the sampling period from May to September. These shifts were probably caused by the seasonal succession of the zooplankton community, mainly the midsummer decline of Daphnia galeata. 4. The diet shifts in fish also had consequences for the amount of P consum ed and released. During periods of dominant zooplanktivory, the excretion o f P did not exceed the removal of P stored in pelagic prey. By contrast, if benthivory dominated, fish subsidized the pelagic P pool by excreting more P from benthic prey than had been removed from the pelagic area. This occu rred predominantly in perch and ruffe during periods of low zooplankton bio mass, whereas the roach ate more algae and therefore excreted less P of ben thic origin. 5. Phosphorus release by underyearling fish was estimated at a maximum of 0 .1 mg m(-3) day-l. This value was negligible! compared with both the extern al load of P to Bautzen reservoir and the concentration of P in the pelagic area during summer. It is therefore concluded that both the predominance o f underyearling zooplanktivorous fish and the high Daphnia biomass during c ertain periods of the year in the Bautzen reservoir may be the reason that nutrient release by the fish structured the foodweb only marginally. 6. This study suggests that biomanipulation has altered both top-down and b ottom-up impacts of fish in Bautzen reservoir. The highest efficiency of fo odweb manipulations may be obtained after reduction of the external P loadi ng below a certain threshold. In turn, if external restoration of eutrophie d lakes is not accompanied by changes in fish community, then the combined forces of strong zooplanktivory and high P recycling of dense stocks of zoo planktivorous and benthivorous fish may hold the water in a eutrophic-like stage, even if external load has been significantly reduced.