Large mammals in subsidies' dark side: contaminant ecology of mammal-mediated aquatic-terrestrial linkages

Background/Question/Methods

Transboundary linkages have proven critical for supplementing recipient-system food webs and nutrient supplies, with affects that reach multiple trophic levels. However, less understood is the potential of mammals that serve as aquatic-terrestrial vectors to transfer harmful contaminants that accumulate in the donor system. The strength of aquatic-terrestrial contaminant transfer may be mediated by organisms, climate change, human water and land use, and other factors that alter the extent and condition of water bodies on the landscape.  We studied whether aquatic macrophytes may be an important contributor to mercury consumption by moose, who may then move this toxin to terrestrial systems. We assessed whether the levels of Hg in moose aquatic forage may warrant concern with respect to mercury's potential effects on the consumer’s reproductive system, in light of the current, unexplained trends in declining birth rates amongst moose populations in the Great Lakes region. We further assessed whether inter-species interactions may alter the extent of aquatic-terrestrial contaminant transfer, by asking whether beaver populations mediate mercury transport by altering the extent of shallow, dystrophic lakes on the landscape.

Results/Conclusions

In an island wilderness area separated by Lake Superior from anthropogenic watershed mercury transfer, the non-rooted macrophyte species Utricularia vulgaris displays mercury levels as high as 0.08ppm. In Northeast Minnesota, where moose populations have declined 62% since 2010, mercury levels in U. vulgaris are in excess of 0.09ppm. A rooted macrophyte species, Nuphar variegata, has much lower levels of mercury, and appears to exclude the toxin. Moose and beaver are known to consume both of these macrophyte species. Thus the species composition of a given aquatic plant community may have an important effect on the amount of toxins that herbivores consume and move from aquatic to terrestrial systems.  Inter-system toxin transfer may also be affected by the types of lakes on the landscape. Non-rooted macrophytes from beaver-expanded lakes and ponds have significantly higher mercury levels than the same species have from glacial lakes (p=0.03). Natural and anthropogenic mechanisms that increase the relative amount of dystrophic lakes and lakes of higher trophic status may affect the extent of mercury toxicity in aquatic and terrestrial food webs.