Environmental Drivers of Rare Earth Element Bioaccumulation in Freshwater Zooplankton.

Human activities have resulted in significant release of rare earth elements (REEs) into the environment. However, the pathways of REEs from waters and soils into freshwater food webs remain poorly understood. Recent studies suggest that aquatic invertebrates may be good biomonitors for REEs, yet there is little information on factors that control REE bioaccumulation in these organisms. Our goal was to study the environmental drivers of REE levels in zooplankton, a key component in plankton food webs, across lakes from geographic areas with different bedrock geology. From 2011 to 2014, bulk zooplankton samples were collected for REE analysis from 39 lakes in eastern Canada. We observed a more than 200 fold variation in surface water REE concentrations and a 10-fold variation in sediment REE concentrations. These concentration gradients were associated with a range of more than an order of magnitude in zooplankton REE concentrations (∑REEY 3.2-210 nmol g-1). We found higher REE bioaccumulation in zooplankton from lakes with lower pH and higher REE to dissolved organic carbon ratios. Bioaccumulation was also strongly linked to the free ion concentrations of REEs (REE3+) in surface waters. Our study suggests that zooplankton REE bioaccumulation is an excellent predictor of bioavailable REEs in freshwaters.

Fate and Trophic Transfer of Rare Earth Elements in Temperate Lake Food Webs.

Many mining projects targeting rare earth elements (REE) are in development in North America, but the background concentrations and trophic transfer of these elements in natural environments have not been well characterized. We sampled abiotic and food web components in 14 Canadian temperate lakes unaffected by mines to assess the natural ecosystem fate of REE. Individual REE and total REE concentrations (sum of individual element concentrations, ΣREE) were strongly related with each other throughout different components of lake food webs. Dissolved organic carbon and dissolved oxygen in the water column, as well as ΣREE in sediments, were identified as potential drivers of aqueous ΣREE. Log10 of median bioaccumulation factors ranged from 1.3, 3.7, 4.0, and 4.4 L/kg (wet weight) for fish muscle, zooplankton, predatory invertebrates, and nonpredatory invertebrates, respectively. [ΣREE] in fish, benthic macroinvertebrates, and zooplankton declined as a function of their trophic position, as determined by functional feeding groups and isotopic signatures of nitrogen (δ15N), indicating that REE were subject to trophic dilution. Low concentrations of REE in freshwater fish muscle compared to their potential invertebrate prey suggest that fish fillet consumption is unlikely to be a significant source of REE to humans in areas unperturbed by mining activities. However, other fish predators (e.g., piscivorous birds and mammals) may accumulate REE from whole fish as they are more concentrated than muscle. Overall, this study provides key information on the baseline concentrations and trophic patterns for REE in freshwater temperate lakes in Quebec, Canada.

Effects of experimental thermocline and oxycline deepening on methylmercury bioaccumulation in a Canadian shield lake.

Environmental disturbances like deforestation or climate change may influence lake thermal and oxic stratification, thereby modifying cycles of contaminants such as mercury (Hg). In a lake naturally separated into three basins, the thermocline and oxycline of an experimental basin were deepened by 4 and 3 m, respectively, to study the effect on the methylmercury (MeHg) accumulation. This treatment decreased hypolimnetic MeHg concentration by approximately 90%, zooplankton concentrations by 30 to 50%, and in some fish by 45%. A multiple linear regression indicated that oxycline depth significantly influenced hypolimnetic MeHg concentrations, with no significant effect of thermocline depth, anoxic water volume, interface area of oxic-anoxic water, and sediment area in contact with anoxic water. Fish MeHg decline varied, with a greater response by low oxygen-tolerant bullhead. Increased pelagic primary and secondary production likely caused zooplankton and fish MeHg decreases via algal and growth dilution. Environmental changes leading to oxycline deepening are therefore predicted to cause a decrease in MeHg bioaccumulation in similar Canadian Shield lakes. If associated ecosystem impacts related to the deepening treatment are deemed acceptable, then this experiment provides a potential remediation method for small lakes confronted with MeHg accumulation.

Factors affecting methylmercury biomagnification by a widespread aquatic invertebrate predator, the phantom midge larvae Chaoborus.

MeHg biomagnification by the phantom midge Chaoborus in relation to MeHg concentrations in their prey and its migratory behavior was investigated in two Canadian Precambrian Shield lakes. Three Chaoborus species with contrasted migratory behavior were collected in a fishless and a fish-inhabited lake. All species accumulated MeHg through their ontogenic development. In the lake inhabited by fish, all instars of Chaoborus punctipennis displayed a marked migratory behavior and were unable to biomagnify MeHg, whereas in the fishless lake, Chaoborus americanus and Chaoborus trivittatus biomagnified MeHg. Reduced biomagnification capacity of C. trivittatus, the coexisting species living with C. americanus, was also ascribed to a progressive vertical segregation with age. Growth dilution, amount and type of prey items or trophic position could not explain the different patterns of biomagnification. Our findings demonstrate that the most common invertebrate predator of temperate planktonic food webs can biomagnify mercury, contrarily to previous reports.