What controls the levels of methylmercury in fish?
Food chain length and trophic level (position in the food chain) are important controls on methylmercury bioaccumulation and biomagnification. The biggest step in biomagnification occurs from water to algae, where methylmercury concentrations can increase from 100,000 to 1,000,000 times. In contrast, increases for subsequent steps in the food chain (from algae to macroinvertebrates, from macroinvertebrates to forage fish, and from forage fish to predator fish) are individually much smaller (5 to 10 times for each step). Among streams with large gradients in aqueous methylmercury concentrations, the amount of methylmercury that is available for uptake by algae at the base of the food web (rather than trophic level) can be the most important factor controlling methylmercury concentrations in predator fish. For example, trout in Oregon streams (where aqueous methylmercury concentrations were low) actually had lower mean methylmercury tissue concentrations than macroinvertebrates in Florid
