Study finds high mercury levels, simplified food chain in prairie reservoir

A new study from the University of Lethbridge discovers exceedingly high mercury concentrations in northern pike (Esox lucius) residing in a newly constructed reservoir. In addition, the study suggests the reservoir’s food web is extremely simplified, a factor that could be further exacerbating the elevated levels of mercury.

Several studies have observed mercury trends in recently created reservoirs where concentrations dramatically peak at construction and gradually decline to background levels within 15-30 years. The rise of mercury availability is caused from methylation, where inorganic mercury is microbially converted to a more dangerous and biomagnifiable, methylmercury.   

Southern Alberta, Canada has a large water demand due to its semi-arid climate and irrigation-based agriculture. Although many studies have focused on mercury problems in Canadian wetland ecosystems, this is the first published work to look at contamination in prairie reservoirs.

Researchers Lars Brinkman and Joseph Rasmussen selected the 2 yr old Twin Valley Reservoir (TVR) for data collection and included the Oldman River as a reference site for further comparisons. They collected fish, invertebrates, macrophytes, shallow and deep water substrate, and zooplankton for mercury and stable isotope analysis.

A majority of the fish sampled, all of which were either pike or white suckers, exceeded the Health Canada Consumption guideline of 0.5 ppm with an average of 0.89 ppm. This was also 3 times higher than similar-sized pike in the reference site at Oldman River. TVR’s amphipods also contained mercury content higher than most other systems in Eastern Canada (0.11 to 0.42 ppm).

The stable isotope analysis revealed a complete absence of piscivorous fish. The lack of fish-eaters drastically simplifies the food web and suggests that pike are solely feeding on invertebrates. Young pike are able to thrive on amphipods, but it is not sustainable for large pike. Thus, the fish must consume several hundred a day, resulting in stunted growth. The large pike’s metabolic activity is now spent on somatic growth, while mercury is allowed to build in the body tissue more quickly.

Reservoir construction can have potentially severe mercury effects with levels 3.5-fold than nearby rivers. Prairie reservoirs support soils that are rich in phosphate, sulfate, and organic matter; all of which are necessary components in creating anoxic substrates for microbial mercury methylation. In addition to these prime conditions, the simplified food web also increases mercury levels within inhabiting fish.

Researchers suggest that food web management, like the introduction of large prey species, could be helpful in alleviating the disturbed systems, but warn that more research must first be executed.

--Reviewed by Evyan Borgnis

Brinkmann, L., & Rasmussen, J. (2010). High levels of mercury in biota of a new Prairie irrigation reservoir with a simplified food web in Southern Alberta, Canada Hydrobiologia DOI: 10.1007/s10750-009-0050-0