Trophic levels methyl mercury

Although the entry of MeHg into the base of the food web and its subsequent transfer in the lowest trophic levels are poorly understood, it is evident that the concentration of MeHg in all trophic levels is strongly correlated with its supply from methylating environments. In fish, for example, much of the modem spatial variation in mercury concentrations (within a given trophic level) can be attributed to variation in factors and processes that affect the microbial production of MeHg and its entry into oxic waters. 

Arnold BS. 2000. Distribution of mercury within different trophic levels of the Okefenokee swamp, within tissues of top level predators, and reproductive effects of methyl mercury in the Nile tilapia (Oreochromis niloticus). PhD dissertation. University of Georgia. 

Jackson, T.A. 1986. Methyl mercury levels in a polluted prairie river-lake system seasonal and site-specific variations, and the dominant influence of trophic conditions. Canad. Jour. Fish. Aquat. Sci. 43 1873-1887. 

Mercury is rarely amplified between invertebrates and small fish (Knauer and Martin 1972, Leatherland et al. 1973), but it is sometimes amplified in large fish, where there are effects linked to both trophic level (Rat-kowsky et al. 1975) and the age of the animals (Jackson 1998). Bacteria are important in the transformation of mercury to methyl mercury. Such conversion is probably the major source of methyl mercury present in the aquatic food chains (see Part III, Chapter 17 Jackson 1998). 

The accumulation of nnacceptable concentrations of mercury in fish occurs largely as a result of bioaccumulation of methyl mercnry np the food chain to the top predator fish, which are the most desired species. Methyl mercnry forms largely in the anaerobic sediments of water bodies and then moves up through several trophic levels. 

Methylmercury is produced by methylation of inorganic mercury present in both freshwater and saltwater sediments, and accumulates in aquatic food chains in which the top-level predators usually contain the highest concentrations. The percent of total mercury accounted for by methylmercury generally increases with higher trophic levels, confirming that methylmercury is more efficiently transferred to higher trophic levels than inorganic mercury compounds. 

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