Evaluating Mercury Deposition over Space and Time in Critical Nesting Habitat for Endangered Whooping Crane (Wood Buffalo National Park)

Abstract

The only remaining wild, self-sustaining population of endangered Whooping Crane (the Aransas-Wood Buffalo population of Grus americana) nests and breeds in a remote boreal landscape of northwestern Canada known as the Whooping Crane Summer Range (WCSR), a Ramsar Wetland of International Importance. Although geographically isolated from industrial development, ponds in the WCSR may receive deposition of substances of concern, including mercury (Hg), from far-field sources via direct atmospheric transport and deposition and locally via remobilization of legacy Hg stored in soils and catchments. Mercury is of particular concern because it is a potent neurotoxin capable of long-range transport and accumulation in aquatic ecosystems. However, there are no data available to evaluate present-day Hg concentrations in aquatic sediment, the enrichment of Hg relative to naturally occurring baseline concentrations, or long-term accumulation of Hg within WCSR pond sediment. This study investigates total mercury (THg) concentrations over space and time in pond sediments of the WCSR to determine whether the concentrations pose risk of adverse biological effects on whooping crane and other aquatic biota and to quantify the enrichment and storage of THg in pond sediment. Total Hg concentrations were measured in surface sediments collected in 2024 from 63 ponds and in radiometrically dated sediment cores spanning the past ~370 years from three ponds within the Sass–Klewi Nesting Area (SKNA), a subregion of the WCSR. Sediment cores were used to establish a multi-site pre-industrial baseline to quantify enrichment using organic matter–normalized Enrichment Factors (EFs), and to calculate cumulative inventories of excess THg for comparison with other Canadian lakes at near- and far-field distances from major sources of emissions. Concentrations of THg ranged from 2.7 – 45.4 ng g⁻¹ (mean 20.0 ± 9.9 ng g⁻¹, n = 63) in the surface sediments and from 5.4 – 72.0 ng g⁻¹ in the sediment cores. All concentrations are well below the Canadian Interim Sediment Quality Guideline of 170 ng g⁻¹ and indicate that adverse biological effects on aquatic life are unlikely to occur at present and in the past. Strong, positive linear relations between THg concentrations and organic matter (OM) content in sediment deposited before ~1900 at three ponds allowed for construction of a multi-site pre-industrial OM-normalized baseline capable of estimating the amount of enrichment in samples deposited since 1900 in sediment cores each of the three pond and the surface sediment samples from 63 ponds. Peak THg enrichment occurred between ~1960 and ~2010 at the three ponds, when EFs range from 1.5-2.6 and signify ‘minimal enrichment’ based on widely used categorization. After ~2010, EFs decline rapidly to around 1.0, indicating a return to concentrations that existed before 1900. The mean EF for the surface sediment samples collected from 63 ponds in 2024 is 1.03, indicating no enrichment relative to pre-1900 sediment, and EFs exceed the 1.5 threshold for ‘minimal enrichment’ at fewer than 10% of the ponds. The cumulative inventory of excess THg at pond SK 43 falls within a narrow range reported for other lakes in Canada at far-field distances from industrialization and is 700 times lower than a lake at near-field distance from a major point source of Hg emissions (mine and smelter at Flin Flon, Manitoba). These findings suggest that mercury poses little risk of harm to whooping crane and other aquatic organisms. The pre-1900 baselines generated during the research can be used in future sediment quality monitoring to detect whether ongoing climatic and environmental changes begin to elevate THg concentrations above natural levels.