The monitoring of mercury in aquatic food webs supporting the production of fish and wildlife is directly relevant to concerns about health and ecological risks of methylmercury (MeHg) exposure. We present a framework for monitoring concentrations of mercury in aquatic biota, with emphasis on assessing responses to changes in loadings of mercury from atmospheric deposition and other sources. In this chapter, we (1) identify specific attributes (criteria) of indicators that would be useful for discerning temporal trends and spatial patterns in the concentration of mercury in aquatic biota, (2) critically evaluate and rank candidate biological indicators useful for monitoring trends in mercury, (3) outline approaches for sampling and analysis of recommended biological indicators, (4) identify ancillary data needs and potential confounding factors that should be considered or documented to ensure the defensible interpretation of data on monitored biological indicators, and (5) consider the environmental settings (waterbody type and geographic location) that would be most sensitive for detecting changes in atmospheric deposition of mercury. Criteria were applied to ensure that the biological indicators selected are useful, relevant, and sufficiently diagnostic to detect a change in mercury bioaccumulation in response to altered mercury loadings. Toxicological problems with mercury in aquatic ecosystems result from biotic exposure to MeHg, a highly toxic compound that readily accumulates in exposed organisms and can biomagnify to high concentrations in organisms atop aquatic food webs. Biotic monitoring should, therefore, focus on assessing trends in bioaccumulation of MeHg; in samples from trophic levels below fish, this requires the determination of MeHg. We considered six general groups of

aquatic biological indicators: piscivorous fish, prey fish, benthic invertebrates, zooplankton, phytoplankton, and periphyton. Piscivorous fish and 1-year-old prey fish, all analyzed individually, are considered the preferred aquatic biological indicators for trend monitoring. For piscivorous fish, total-mercury determinations on axial muscle (preferably without skin), sampled annually, would indicate gradual (multiyear) trends in MeHg that are directly relevant to humans who eat sport fish. For prey fish, annual sampling and analysis of either whole fish or axial muscle for total mercury should indicate annual changes in exposure to MeHg. In North America, the historical record for MeHg in piscivorous fish (from data on total mercury in filets or axial muscle) extends about 35 years, much longer than the comparatively sparse historical record for MeHg in water and aquatic biota of lower trophic levels. The analytical method (cold vapor atomic absorption spectrophotometry) that produced most of the historic data on total mercury in piscivorous fish is valid, and the potential utility of these existing data for trend analysis merits careful consideration. Benthic invertebrates have been monitored and analyzed for mercury more extensively in estuarine systems than in fresh waters. The consumption of estuarine macroinvertebrates, such as oysters, clams, shrimp, and crabs, is also a direct pathway for human exposure to MeHg. Determination of MeHg in shellfish and macrocrustaceans could, therefore, be useful for trend monitoring in estuaries. The importance of MeHg uptake and transfer at the base of the food web is recognized. However, the utility of periphyton, phytoplankton, zooplankton, and freshwater benthic invertebrates for trend monitoring is diminished by interpretational complexities associated with large temporal variation in the biotic composition and MeHg content among samples, and by our limited understanding of the processes and variables that affect concentrations of MeHg in these groups. Many anthropogenic and natural factors, independent of the bulk loading of mercury from atmospheric deposition, can strongly influence the concentrations of MeHg in aquatic biota. Our ability to discern linkages between MeHg concentrations in aquatic biota and changing external loadings of mercury to aquatic systems will depend on knowledge of such factors and on the minimization of their confounding effects in biotic monitoring programs.