It is the goal of this paper to discuss the more salient recent AdvancCll in the understanding of the controls of net CH3Hg formation in Datural systems. The discussion lIighlights the gaps in knowledge and the areas where progress in understanding has occurred. In particular, this chapter focuses on recent developments in Hg bioavailability and uptake by methylating bacteria, on the competing roles of sulfate and sulfide in the control of methylation, and in pathways for demethylation. The role of sulfide in influencing methylation is discussed in detail. In addition, the impact of other environmental variables such as pH, dissolved organic carbon and temperature on mercury methylation are discussed.Lastly, we provide a synthesis of the variability in the methylation response to Hg inputs across ecosystems. We suggest that although methylation is a function of Hg concentratioD, the range of methylation rates across ecosystems is larger than the range in Hg deposition rates. Overall, we conclude that factors in addition to the amount Hg deposition playa large role in controlling CH3Hg production and bioaccumulation in aquatic ecosystems. 262to

The octanol-water partioning of inorganic mercury decreased with increasing sulfide, supporting a model that predicts decreased fractions of neutral Hg-S species with increasing sulfide. These results help explain the decreased availability of Hg to methylating bacteria under sulfidic conditions, and the inverse relationship between sulfide and methylmercury observed in sediments.

Aquatic systems are increasingly exposed to multiple stressors from anthropogenic sources. These stressors can vary in the consistency and magnitude of responses they elicit in biota and in how the presence of additional stressors modifies their effects. Understanding how the biological environment and temporal dynamics influence responses to stressors, and how stressors interact, is important to predicting their effects in the natural environment. We examined temporal variability in responses of an experimental estuarine food web to elevated trace elements and nutrients, as well as non-additive effects of the combination of these two stressors. Experiments were conducted four times during spring through autumn 1996 in 20 l-m 3 mesocosms. We measured a range of system-, population-, and individual-level parameters to quantify responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, copepods, fish, and benthic invertebrates to trace element and nutrient additions.The response to trace element additions was more variable both temporally and among phytoplankton and higher trophic level taxa than was the response to nutrient additions. Most taxa increased, either significantly or showed a trend toward increasing, in response to nutrient additions in all four mesocosm runs. In contrast, the direction as well as the magnitude of responses to trace element additions varied considerably among taxa and experimental runs. Two distinct types of nutrientϫtrace element interactions were important. First, temporal dynamics of nutrient ratios appeared to affect the temporal pattern of toxicity of trace elements to phytoplankton. Second, in the June mesocosm run when trace element additions reduced production, abundance, or growth of many organisms, these reductions were often proportionately greater in nutrient addition tanks than where no nutrients were added. Our results suggest that considerable temporal and taxonomic variation in responses to trace element loadings are likely to be seen in field settings even under constant loadings to the system and that trace elements may mask the magnitude of the response to high nutrient loadings in eutrophic systems. More generally, the presence of multiple stressors may either increase or dampen the temporal and spatial variability seen in aquatic systems, depending on the interactions among stressors and the influence of background environmental conditions and sensitive species on the expression of stressor effects.Stressors vary considerably in the specificity of their effects; some stressors may affect nearly all organisms within AcknowledgmentsWe thank B. Albright, D. Butera, L. Cole, D. Connell, R. DeKorsey, T. Huber, A. Imirie, S. Sellner, J. Smallwood, M. Ward, M. Weinstein, T. Wiegner, and W. Yates for their help in running experiments and data analysis; S. Nixon, S. Grainger, and E. Buckley for suggestions on mesocosm design and unpublished data from their seagrass mesocosm experiments; M. Bundy for advice on copepod feeding strategies; E. Perry f...

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Microbial communities in water from Baltimore Harbor and from the mainstem of Chesapeake Bay were examined for sensitivity to mercuric chloride, monomethyl mercury, stannic chloride, and tributyltin chloride. Acute toxicity was determined by measuring the effects of [3H]thymidine incorporation, [14C]glutamate incorporation and respiration, and viability as compared with those of controls. Minimum inhibitory concentrations were low for all metals (monomethyl mercury, <0.05 ,ug liter-1; mercuric chloride, <1 ,ug liter-1; tributyltin chloride, <5 ,ug liter-) except stannic chloride (5 mg liter-'). In some cases, mercuric chloride and monomethyl mercury were equally toxic at comparable concentrations. The Chesapeake Bay community appeared to be slightly more sensitive to metal stress than the Baltimore Harbor community, but this was not true for all treatments or assays. For culturable bacteria the opposite result was found. Thymidine incorporation and glutamate metabolism were much more sensitive indicators of metal toxicity than was viability. To our knowledge, this is the first use of the thymidine incorporation method for ecotoxicology studies. We found it the easiest and fastest of the three methods; it is at least equal in sensitivity to metabolic measurements, and it likely measures the effects on the greater portion of the natural community.