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.

No abstract

Diel changes in bacterial ingestion by a mixotrophic flagellate, Cryptomonas sp., and heterotrophic nanoflagellates (HNF) were examined in situ at 4-h intervals for 2 d in the epilimnion and metalimnion of Lake Biwa using bacteria-sized fluorescent microspheres as a tracer food. Clearance rates of HNF for the microspheres ranged between 1.3 and 4.5 nl cell Ϫ1 h Ϫ1 , but the average rate did not differ between day and night. In contrast, clear diel changes were observed in the clearance rate of Cryptomonas sp. in the epilimnion from Ͻ0.5 nl cell Ϫ1 h Ϫ1 at midnight to Ͼ3 nl cell Ϫ1 h Ϫ1 at noon. In the metalimnion where light intensity was lower, however, the clearance rate of Cryptomonas sp. was always Ͻ0.5 nl cell Ϫ1 h Ϫ1 through the study period. Thus, bacterial ingestion of Cryptomonas sp. is not to acquire supplementary energy or carbon at low phototrophic activities. During the study period, both inorganic phosphorus and nitrogen concentrations were less than or close to the detection limits (10 nM P and 1 M N) in the epilimnion, but much higher in the metalimnion. The results strongly support the idea that Cryptomonas sp. utilizes N and P from bacteria as substitutable nutrients when photosynthesis takes place under conditions of nutrient depletion. To assess the grazing effect of mixotrophic algae on bacterial populations, it is essential to consider diel changes in their phagotrophic mode of nutrition that are induced by light regime and nutrient concentrations in ambient water.

Culture experiments were conducted with the alga Scenedesmus acutus and heterotrophic bacteria to examine if the nature of their relationship changes according to the balance of light and nutrient supplies. Mixtures of algae and bacteria were grown in vanous combinations of 6 light intensities and 4 phosphorus (P) concentrations at high N:P ratio (80:l). We used an artificial medium composed of inorganic nutrients so that bacteria relied on organic matter released by algae as carbon (C) source. Every 2 d, 25 % of the culture suspension was replaced by fresh medium. At the end of incubation when both bacterial and algal densities were stabilized, bacteria were separated from algae. Bioassays with glucose and/or inorganic P enrichment were then performed to assess the extent to which bacterial growth rate was limited by organic C or inorganic P. The algal density in the semibatch culture was low under the light intensity 1 5 5 FE rn-'s-' regardless of P concentrations, while it was higher at higher light and P supply rate above that light intensity. The bacterial density was higher in the cultures where algal density was higher. The bioassay revealed that bacteria were C limited at the light intensity <55 pE m-2 S-', indicating a commensal relationship between algae and bacteria.Above that light intensity, bacteria suffered from deficiency of organic carbon rather than P at lowest P supply rate, because of low algal biomass due to a shortage in P supply. At moderate P supply rates and light intensities 255 PE m-2 S-', however, bacterial growth was limited by P rather than organic C, because supply of organic C from algae exceeded P supply relative to bacterial demand. Further increase in P supply released both algae and bacteria from P limitation. Thus, con~petitive interaction for P was most intense at a moderate P supply rate. These results demonstrate that there is a shift between comrnensalism for C and competition for P depending on light intensity and nutnent supply rate.

To clarify the roles of zooplankton in the sedimentation of seston from the epilimnion, the sinking flux of particulate carbon was measured along with primary production rate and zooplankton biomass from July 1996 to October 1997 at a pelagic site in the north basin of Lake Biwa. During the study period, the flux varied seasonally from 66 to 510 mg C m Ϫ2 day Ϫ1 and was low in summer when zooplankton, composed mainly of Eodiaptomus japonicus and Daphnia galeata, were abundant. Simple correlation analysis revealed that the sinking flux correlated neither with the primary production rate nor with the amount of sestonic carbon above the sediment trap. However, the particle elimination rate, estimated as the difference between the primary production rate and the sinking flux, correlated positively with the zooplankton biomass. These results suggest that zooplankton play a substantial role in decreasing the sinking flux in Lake Biwa.