Hydrology and Nutrient Biogeochemistry of Shallow Pond-Peatland Complexes, Hudson Bay Lowlands

Abstract

Across the circumpolar north, the degradation of permafrost in tundra and peatland landscapes has resulted in significant changes to land cover, including an increase in the extent of thermokarst landforms. Climatically-driven changes to soil hydrology and temperature have the potential to impact nutrient cycling biogeochemical processes, which have implications for plant productivity, greenhouse gas fluxes, and surface water quality. Terrestrial and aquatic ecological productivity are often nutrient-limited in subarctic permafrost environments, linking the cycling of bioavailable nutrients to the capacity of the landscape to take in and store carbon. Within these permafrost peatland catchments, the fate of small (< 1 km2) freshwater ponds and lakes has been the subject of scientific interest due to their ubiquity in the landscape, capacity to exchange carbon and energy with the atmosphere, and their potential to inform researchers about past climates through sediment records. High latitude regions are experiencing significant climatic change, including rapid warming and changing precipitation patterns, which may result in changes in nutrient dynamics within terrestrial and aquatic systems and hydrochemical transport dynamics between them. With climate warming, thermokarst lake expansion has the potential to modify the ability of these aquatic systems to maintain these functions through changes to nutrient inputs and cycling. Through seasonal hydrometric and hydrochemical monitoring, laboratory experimentation, and paleolimnological methods, a set of pristine and thermokarst-impacted peatland catchments in the Hudson Bay Lowlands served as the study site to design research questions and methodologies to address several major themes across the pond-peatland interface. Within the peatland catchment, experimental laboratory work was used to examine the climatic controls on mineralization rates across landscape units, which represent potential nutrient contributions along runoff flow paths. Next, seasonal variability in runoff quantity and quality from catchments to pond was explored, as well as the relationship of hydrologic drivers to how temporal patterns in pond chemistry varied spatially in the landscape among ponds. Finally, the impacts of thermokarst shoreline collapse on hydroecological functioning of a pond were studied, (including all aforementioned mineralization and runoff processes) inferred through the sediment record of a recently impacted pond in the Hudson Bay Lowlands. This work has (1) improved scientific understanding of the combined controls of hydrologic inputs and ground frost on runoff and nutrient transport between peatlands and ponds, which represent up to 60% of all snow-free water input to ponds, (2) demonstrated the importance of understanding hydrologically driven chemodynamics in permafrost ponds on multiple scales (seasonal and event scale), in particular for a set of hydrologically-driven ions (Cl-, Na+, K+, Mg2+, dissolved organic nitrogen), (3) provided novel insight to nutrient cycling processes in northern peatland landscapes along two spatial gradients (landscape unit and depth), including the potential for rapid nitrification of thermokarst material under oxic conditions (4.6 μg NO3-N g-1 dry peat d-1) and (4) furthered understanding of the hydroecological response of small surface water bodies to thermokarst shoreline expansion under a changing climate, showing that nutrient fluxes increased dramatically both prior to and following shoreline slumping, with isotopic signatures indicating a shift towards increased supply of allochthonous carbon and atmospheric nitrogen. Overall, this thesis addresses multiple and cross-cutting themes of the hydrology and nutrient biogeochemistry of shallow pond-peatland complexes in the Hudson Bay Lowlands.