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Accumulation of Hg in aquatic organisms is influenced not only by the contaminant load but also by various environmental variables. We compared biomagnification of Hg in aquatic organisms, i.e., the rate at which Hg accumulates with increasing trophic position, in three lakes differing in trophic state. Total Hg (THg) concentrations in food webs were compared in an oligotrophic, a mesotrophic and a eutrophic lake with naturally elevated levels of Hg associated with geothermal water inputs. We explored relationships of physico-chemistry attributes of lakes with Hg concentrations in fish and biomagnification in the food web. Trophic positions of biota and food chain length were distinguished by stable isotope 15N. As expected, THg in phytoplankton decreased with increasing eutrophication, suggesting the effect of biomass dilution. In contrast, THg biomagnification and THg concentrations in trout were controlled by environmental physico-chemistry and were highest in the eutrophic lake. In the more eutrophic lake frequent anoxia occurred, resulting in favorable conditions for Hg transfer into and up the food chain. The average concentration of THg in the top predator (rainbow trout) exceeded the maximum recommended level for consumption by up to 440%. While there were differences between lakes in food chain length between plankton and trout, THg concentration in trout did not increase with food chain length, suggesting other factors were more important. Differences between the lakes in biomagnification and THg concentration in trout correlated as expected from previous studies with eight physicochemical variables, resulting in enhanced biomagnification of THg in the eutrophic lake. [ABSTRACT FROM AUTHOR]

Picophytoplankton biomass and its contribution to total phytoplankton biomass were investigated in relation to the nutrient concentration and total N: total P ratio of the epilinnetic waters of 42 Japanese lakes during the warm season in 1991 (April-October). Picophytoplankton biomass (as chlorophyll a) in meso-, eu-, and hypertrophic lakes was significantly higher than those observed in oligotrophic lakes. However, picophytoplankton biomass increased significantly with increased total P concentrations in all systems excluding hypertrophic lakes. Picophytoplankton contribution to total chlorophyll a content was significantly higher in oligo- and mesotrophis lakes than in eu- and hypertrophic lakes and was inversely correlated with total P concentrations in lake water. Picophytoplankton contribution to the total phytoplankton biomass was positively (r = 0.54, n = 42, n P = 0.0003) correlated with the total N : total P ratio of lake waters. Each lake trophic type with the exception of hypertrophic lakes, showed this trend, although the correlation was not significant. We suggest that picophytoplankton contribution is influenced by the total N : total P ratio rather than lake trophic state; however, picophytoplankton were of little importance in hypertrophic lakes. [ABSTRACT FROM AUTHOR]