Contribution of metazoan plankton to the cycling of nitrogen and phosphorus in Lake Biwa

To clarify the contribution of metazoan plankton to N and P fluxes in Lake Biwa, we examined particle elimination, net production, and nutrient release rates simultaneously with primary production from June to November 1992. Throughout the study period, the zooplankton eliminated 70% of the elements fixed as particulate forms by primary producers. However, because the N: P ratio of the seston was higher than that of the zooplankton on most dates, the zooplankton selectively accumulated P from seston with higher efficiency, while a larger fraction of N was released to maintain elemental balance. As a result, the mass ratio of N to P released by the zooplankton changed markedly from 19 to 123 according to the magnitude of deviation in the N:P ratio of seston from that of the zooplankton. On average, the zooplankton released 50% of N but only 15% of P fixed by primary production during the study period. The results demonstrate that whether zooplankton act as source or sink for a bioelement depends on the relative abundance of that element in both the food and the zooplankton. In Lake Biwa, the zooplankton can promote P limitation for phytoplankton by fixing a large fraction of the P in the system into their own mass and recycling a substantial amount of the N to a dissolved form.