Ecosystem Metabolism and Air-Water Fluxes of Greenhouse Gases in High Arctic Wetland Ponds

Freshwater lakes and wetlands can be very productive systems on the Arctic landscape compared to terrestrial tundra ecosystems and provide valuable resources to many organisms, including waterfowl, fish and humans. Rates of ecosystem productivity dictate how much energy flows through food webs, impacting the abundance of higher-level organisms (e.g., fish), as well as the net carbon balance, which determines whether a particular ecosystem is a source or sink of carbon. Climate change is predicted to result in warmer temperatures, increased precipitation and permafrost melting in the Arctic and is already altering northern ecosystems at unprecedented rates; however, it is not known how freshwater systems are responding to these changes. To predict how freshwater systems will respond to complex environmental changes, it is necessary to understand the key processes, such as primary production and ecosystem respiration, that are driving these systems. We sampled wetland ponds (n=8) and lakes (n=2) on northern Ellesmere Island (81° N, Nunavut, Canada) during the open water season for a suite of biogeochemical parameters, including concentrations of dissolved gases (O2, CO2, CH4, N2O) as well as stable-isotope ratios of dissolved inorganic carbon (δ13C-DIC), dissolved oxygen (δ18O-DO), and water (δ18O-H2O). We will present rates of primary production and ecosystem respiration, modeled from the concentration and stable isotope ratios of DIC and DO, as well as air-water gas exchange of greenhouse gases in these high Arctic ponds and lakes. Preliminary results demonstrate that ecosystem metabolism in these ponds was high enough to result in significant deviations in the isotope ratios of DIC and DO from atmospheric equilibrium conditions. In other words ecosystem rates of primary production and respiration were faster than gas exchange even in these small, shallow, well-mixed ponds. Furthermore, primary production was elevated enough at all sites except Lake Hazen, a large cold ultra-oligotrophic lake, to result in a shift in the δ18O of DO towards more depleted values. Two of the ponds also exhibited enriched δ13C-DIC indicative of high rates of DIC uptake and primary production. However most sites appear to be net heterotrophic systems, with the exception of a few net autotrophic ponds. Interestingly, due to the high DIC concentrations in pond waters, δ13C-DIC values are reflective of ecosystem metabolism on a somewhat longer time scale than δ18O-DO values, which might be useful for examining temporal changes in production and respiration. Finally, most ponds were sources of both CO2 and CH4 to the atmosphere, but were surprisingly under-saturated with respect to N2O, demonstrating that they are sinks for atmospheric N2O. Rates of N2O consumption (denitrification) were modeled from concentration-time data and ranged from <0.1 to 1.1 nmol L-1 h-1.