Interpreting Linkages among Landscape, Water Chemistry, and Diatom Communities to Better Understand Subarctic Paleoenvironmental Records

Arctic ecosystem response to climate warming will likely be complex, with important linkages among terrestrial, wetland, and aquatic systems set within the context of geologically unique landscapes. Ecosystem responses to warming include: increased lake productivity, permafrost thaw, shrub expansion, and northward shifts in subarctic tree line. There have been many studies on freshwater aquatic responses to climate change in the Arctic, but few consider the role of the terrestrial landscape. As part of a three-year project to study the Holocene history of lakes and landscapes and their response to climate, we undertook a hydrobiological survey along the northern Manitoba boreal forest-tundra ecotone, with the aim of linking lake water chemistry with surrounding landscape and with the algal (diatom and chrysophyte) community composition in the lakes. Fossil algal assemblages from sediment cores can be better interpreted in terms of complex relationships between climate change, landscape change, and lake response if an understanding of modern linkages is developed. Using this modern study to develop quantitative calibration, sediment cores were used to focus on landscape-level biogeochemical changes in these systems at multiple time scales over the past 8,000 years. Forty-five lakes across the boreal-tundra transition were sampled for diatoms and physico-chemistry in 2008-2009 and their watersheds characterized using GIS. Diatoms were collected from the surface (top 1-cm) of a sediment core. Water quality measures included nutrients, anions and cations, pH, conductivity, Secchi depth, and dissolved carbon. Landscape variables including watershed area, watershed to lake area ratios, wetland area within 100 m of lake shore and percent cover of different vegetation types were calculated using GIS with on the ground verification. Primary gradients among the lakes are related to pH and water clarity. Among the landscape variables, distance to treeline, slope, wetland areas and areas of exposed till were strongly correlated to water quality variables, suggesting that landscape position and development across the tundra-boreal ecotone exert strong influence on water quality. Diatom communities showed strong relationships with both water chemistry and landscape variables. By applying diatom-based inference models, in conjunction with multi-proxy evidence from magnetics, pollen, and isotope data, we see relative stability in the recent record of algal community composition in some lakes, despite known climate changes in the region. Other core sections show lake response to shifts in vegetation; this suggests that certain settings buffer lakes from rapid climate changes. Understanding these processes has implications for how we understand linkages between terrestrial and aquatic ecosystems (e.g. coupled biogeochemical cycling of carbon and nitrogen) and how we interpret recent (~300 years) biological changes in the context of a lake’s entire history.