David W. Schindler (1940-2021).

A world-renowned scientist who combined his formidable intellect with passionate advocacy to address problems affecting global freshwaters and the people who rely on them, while empowering generations of diverse peoples to action.

Oil sands development contributes elements toxic at low concentrations to the Athabasca River and its tributaries.

We show that the oil sands industry releases the 13 elements considered priority pollutants (PPE) under the US Environmental Protection Agency's Clean Water Act, via air and water, to the Athabasca River and its watershed. In the 2008 snowpack, all PPE except selenium were greater near oil sands developments than at more remote sites. Bitumen upgraders and local oil sands development were sources of airborne emissions. Concentrations of mercury, nickel, and thallium in winter and all 13 PPE in summer were greater in tributaries with watersheds more disturbed by development than in less disturbed watersheds. In the Athabasca River during summer, concentrations of all PPE were greater near developed areas than upstream of development. At sites downstream of development and within the Athabasca Delta, concentrations of all PPE except beryllium and selenium remained greater than upstream of development. Concentrations of some PPE at one location in Lake Athabasca near Fort Chipewyan were also greater than concentration in the Athabasca River upstream of development. Canada's or Alberta's guidelines for the protection of aquatic life were exceeded for seven PPE-cadmium, copper, lead, mercury, nickel, silver, and zinc-in melted snow and/or water collected near or downstream of development.

Oil sands development contributes polycyclic aromatic compounds to the Athabasca River and its tributaries.

For over a decade, the contribution of oil sands mining and processing to the pollution of the Athabasca River has been controversial. We show that the oil sands development is a greater source of contamination than previously realized. In 2008, within 50 km of oil sands upgrading facilities, the loading to the snowpack of airborne particulates was 11,400 T over 4 months and included 391 kg of polycyclic aromatic compounds (PAC), equivalent to 600 T of bitumen, while 168 kg of dissolved PAC was also deposited. Dissolved PAC concentrations in tributaries to the Athabasca increased from 0.009 microg/L upstream of oil sands development to 0.023 microg/L in winter and to 0.202 microg/L in summer downstream. In the Athabasca, dissolved PAC concentrations were mostly <0.025 microg/L in winter and 0.030 microg/L in summer, except near oil sands upgrading facilities and tailings ponds in winter (0.031-0.083 microg/L) and downstream of new development in summer (0.063-0.135 microg/L). In the Athabasca and its tributaries, development within the past 2 years was related to elevated dissolved PAC concentrations that were likely toxic to fish embryos. In melted snow, dissolved PAC concentrations were up to 4.8 microg/L, thus, spring snowmelt and washout during rain events are important unknowns. These results indicate that major changes are needed to the way that environmental impacts of oil sands development are monitored and managed.

Mercury in the Mackenzie River delta and estuary: concentrations and fluxes during open-water conditions.

Estimates of mercury (Hg) loadings to the Arctic Ocean from circumpolar rivers have not considered biogeochemical changes that occur when river water is temporarily stored in large deltas (delta effect). There are also few data describing Hg changes across the freshwater-saltwater transition zone (FSTZ) of these rivers. We assessed temporal changes in unfiltered total mercury (THg) and methylmercury (MeHg) concentrations during open-water 2004 in the Mackenzie River upstream of the Mackenzie River delta, and in 6 floodplain lakes across an elevation gradient. These data were used to calculate Hg fluxes from the Mackenzie River and to evaluate a delta effect on Hg using an estimate of delta river water storage and a mixing analysis. Mean THg concentrations were highest in river water (9.17+/-5.51 ng/L) and decreased up the lake elevation gradient. Mean MeHg concentrations were highest in lakes periodically connected to the river (0.213+/-0.122 ng/L) and MeHg concentrations in elevated lakes showed a mid-summer peak. Results from the mixing analysis showed that the delta effect may be large enough to affect Hg loadings to the Arctic Ocean. THg concentrations exiting the delta (10.2 ng/L) were 16% lower than those entering (12.1 ng/L), whereas MeHg showed little change. We calculated 2.5-month (open-water) THg and MeHg fluxes from the Mackenzie River of 1208 and 8.4 kg. These fluxes are similar in magnitude to previous annual estimates in the arctic literature suggesting that previously published annual Hg fluxes from the Mackenzie River may be large underestimates. We also assessed changes in Mackenzie River water THg and MeHg concentrations as it crossed the FSTZ during an open-water cruise. THg decreased non-conservatively across the estuary from 3.8-0.6 ng/L, possibly due to mixing and particle settling. MeHg concentrations were variable and near detection. Our results show that the Mackenzie River estuary is a dynamic environment and may have important controls on Hg delivered to the Arctic Ocean.

Organochlorine compounds in trout from lakes over a 1600 meter elevation gradient in the Canadian Rocky Mountains.

The effect of altitude on the concentration and composition of organochlorine compounds (OC) in troutwas investigated along an elevation gradient of 1600 m in the Canadian Rocky Mountains. The eight lakes sampled were within or adjacent to national parks in sparsely settled parts of Alberta and British Columbia, thus contaminants were assumed to have derived from long-range atmospheric transport. Concentrations of several OCs in trout increased significantly with lake elevation. In general, these increases were most pronounced for the higher K(ow) pesticides (i.e., dieldrin and DDTs), and less pronounced for lower K(ow) pesticides (e.g., HCHs and HCB) and PCBs. Most OC concentrations in trout were inversely correlated with fish growth rate. Growth rate explained more of the variation for some OCs (particularly PCBs) than lake elevation. Differences in trophic position (indicated by delta15N) explained little of the variation in OC concentration in comparison to other factors such as lake elevation and the growth rate and age of trout. Using principal component analysis (PCA), we identified the importance of lake elevation and octanol/water partition coefficient (K(ow)) to the OC composition of trout.

Forest fire increases mercury accumulation by fishes via food web restructuring and increased mercury inputs.

Recent findings indicate that fishes from lakes in partially burned catchments contain greater mercury (Hg) concentrations than fishes from reference catchments. Increased methyl Hg (MeHg) concentrations in fishes can result in serious health problems for consumers. Here we show that a forest fire caused a 5-fold increase in whole-body Hg accumulation by rainbow trout (Oncorhynchus mykiss) and smaller Hg increases in muscle of several fish species in a mountain lake. The enhanced Hg accumulation was caused primarily by increased nutrient concentrations in the lake, which enhanced productivity and restructured the food web through increased piscivory and consumption of Mysis. This restructuring resulted in increases to the trophic positions and Hg concentrations of fishes. Forest fire also caused a large short-term release of total Hg (THg) and MeHg to streams and the lake. This release initiated a small pulse of MeHg in invertebrates that contributed to enhanced Hg accumulation by fishes. Climate change and prescribed burning to compensate for past fire suppression are predicted to increase future forest fire occurrence in North America, and increased Hg accumulation by fishes may be an unexpected consequence.