Hi Lars, we already know that in some places (like Mediterranean) some species of shrimp and fish (Mora moro) shown mercury level above the limit, but in other places (like deep Arctic Ocean) the organism have shown mercury levels below the limit. So I think it is very important to know more and to atualize what we have about mercury contamination. The approaches depends by the question you would like to answer. For example, fish and aquatic organisms are excellents markers to the point of view both human health and environmental pollution. Maybe you could study both, fishes or any organism and water/ sediments.

Lars-Eric, Look at the paper "Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models" by Elsie Sunderland et al. (Global Biogeochemical Cycles, 2009, http://onlinelibrary.wiley.com/doi/10.1029/2008GB003425/abstract). I suggest scanning some of Elsie Sunderland's other papers as well (you can view her profile here on Research Gate). Robert Mason from University of Connecticut (also on ResearchGate) would be another contact; start with his recent paper "Mercury biogeochemical cycling in the ocean and policy implications" (R.P. Mason et al, Environmental Research, 2012, http://www.sciencedirect.com/science/article/pii/S0013935112001132). Both of these individuals are interested in the question you pose, and would be useful for you to contact directly if you are doing research in this area.