Trophic levels 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. 

Cizdziel JV, Hinners TA, Pollard JE, Heithmar EM, Cross CL. 2002. Mercury concentrations in fish from Lake Mead, USA, related to fish size, condition, trophic level, location, and consumption risk. Arch Environ Contam Toxicol 43 309-317. 

Kidd KA, Hesslein RH, Fudge RJP, Hallard KA. 1995. The influence of trophic level as measured by 5 N on mercury concentrations in freshwater oiganisms. Water Air Soil Pollut 80 1011-1015. 

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. 

Additionally, the determination of THg levels in muscle of some fish species within the Flix reservoir has been studied (Diez, unpublished results). As expected, THg concentrations increased with trophic level. A comparison between mercury levels in muscle tissue of the individual fish species (p < 0.05) produced the following order European catfish > common carp > rudd > roach. Average THg concentrations (in ww) in muscle of European catfish (n = 9), common carp (n = 30), rudd (n = 20), and roach (n = 5) were 0.848 0.476 ig/g 0.333 0.155 pg/g 0.283 0.163 pg/g and 0.217 0.199 pg/g, respectively. The relative trophic level of the analyzed species is based on a generalized knowledge of the species and stomach contents of the studied adult fish populations [56, 60, 62, 71],... 

Methylmercury can be bioconcentrated in organisms and biomagnified through food chains, returning mercury directly to man and other upper trophic level consumers in concentrated form... 

O2 concentrations, such as found in marine wetlands. High biomethylation rates have also been observed in coastal sediments. Because methylmercury is transferred up the food chain, the marine fish that occupy high trophic levels have very high mercury concentrations. In some cases, such as for tuna and swordfish, concentrations are high enough to pose human health risks. 

Medium or trophic level Methylmercury Inorganic mercury % Methylmercury... 

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). 

Mercury is known to bioconcentrate, bioaccumulate and biomagnify. In fact, mercury is one of the few metals that is known to biomagnify in aquatic food webs. Calculations of the US EPA (1997) for BCFs and BAFs for the aquatic food chain are summarized in Table 17.5. The BMFs (PPFs) for the trophic levels 2 to 4 are calculated to an order of 2.6 to 15.5 (US EPA 1997 Vol. Ill, Appendix D). 

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. 

The efficiency of mercury transfer through natural marine food chains among lower levels was comparatively low however, higher trophic levels including teleosts and fisheating birds and mammals, show marked mercury amplification. The variability in concentrations is explainable, in part, by collection locale wherein samples were taken... 

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