Thus, any changes in the MeHg levels in aquatic ecosystems, particularly in fish and wildlife populations, should be documented and compared with reductions in mercury emissions and deposition. As a result, how effective emissions reductions will be in decreasing biotic MeHg levels in freshwater, estuarine, and coastal ecosystems is not clear. The problem is that the terrestrial–aquatic mercury cycle is complex, with many nonlinear processes that link atmospheric mercury emissions and MeHg bioaccumulation in fish and wildlife (7 Figure 1). Some intensive studies and syntheses of regional databases have been conducted, but their overall applicability to different ecosystems or at the continental scale is uncertain. In addition, no broad-scale data sets are available to test model predictions. Current computer models and other assessment tools provide widely divergent estimates for the effectiveness of emissions controls at reducing MeHg levels in fish (6–8). Yet, many questions about the environmental benefits of emissions reductions remain unanswered. Because advisories have been posted for so many water bodies against consumption of fish with elevated concentrations of potentially dangerous methylmercury (MeHg), regulations limiting mercury emissions have been promulgated in many countries or are likely to be put forward in the near future (1–5).
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