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

1. B13E-0579 Ecosystem Metabolism and Air-Water Fluxes of Greenhouse Gases in High Arctic Ponds Igor Lehnherr‡*, Jason Venkiteswaran‡, Vincent St. Louis§, Sherry Schiff‡ and Craig Emmerton§ ‡Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, ON, *ilehnher@uwaterloo.ca §Department of Biological Sciences, University of Alberta, Edmonton, AB Introduction Pond Hydrology and Chemistry { evaporative • Background: • Freshwater ponds are ubiquitous features of many Arctic landscapes • These ponds are biogeochemical hotspots with a potentially large influence on regional carbon cycling • By contrast, Lake Hazen is ultra-oligotrophic and is the largest lake (by volume) north of the Arctic Circle. • These aquatic ecosystems are also sensitive to environmental change • changes in hydrology due increased melting of permafrost and glaciers • changes in growing season length due to longer ice-free season • Research Questions: • Are these ponds net sources or sinks of carbon (heterophic vs. autotrophic) and other GHGs (greenhouse gases; e.g., N2O)? • Can we use stable-isotope techniques1 (e.g.,changes in d18O of dissolved oxygen (DO)) to gain insights into whole-pond metabolism, despite obvious challenges? • Low signal (metabolism) to noise (air-water gas exchange) ratio in these shallow systems • Continuous daylight and therefore little amplitude in diel DO curves { meltwater receiving { shoreline stream July 5 A pond (Pond 16) receiving permafrost meltwater during the summer season. An evaporative pond (Pond 12), hydrologically isolated during the summer season. July 15 Abbreviations: DIC – dissolved organic carbon; DOC – dissolved organic carbon; DIN – dissolved inorganic nitrogen; TDN – total dissolved nitrogen; TDP – total dissolved phosphorus; Cond – conductivity; Chla – suspended chlorophyll a; n.d. – not detected • Ponds fall into three general categories based on hydrology: • Evaporative ponds: • receive no or very little inputs of water after snowmelt is over • Ponds 3, 7, 10 and 12 • Meltwater receiving: • receive inputs of water from ephemeral permafrost-fed streams • Ponds 11, 16 and Skeleton Lake • Shoreline ponds: • Exchange water with Lake Hazen depending on lake water level • Pond 1: unidirectional flow into the pond • Pond 2: bidirectional flow depending on relative water levels July 19 Lake Hazen Pond 1 • Water Chemistry • Ponds are characterized by: • Alkaline pH and high DIC • Low DIN but high dissolved organic nitrogen • Low suspended Chlaand presence of benthic mosses and macrophytes suggests benthic productivity is more important than pelagic productivity Study Area and location of ponds sampled. GHGs were monitored at a few select sites during June-July starting in 2005. A more extensive survey of ponds was conducted in July 2010 for quantifying aquatic ecosystem metabolic processes. A shoreline pond (Pond 1) flooding as a result of rising water levels in Lake Hazen. 1 Venkiteswaran et al. Oecologia 2007, 153, 385-398 Air-Water GHG Fluxes Pond Metabolism • CO2and CH4 fluxes: • Fluxes in Pond 1 correlated with water level: • CO2 sink during low water level years (2005, 2007) but source during flood years (2008-2010) • CH4 fluxes also higher in flood years • Skelton Lake: • CO2 concentrations higher during partial ice cover, decreasing during summer season • CH4 conc. have increased every year since 2007 during low water level years (2005, 2007) • Lake Hazen: • Episodic high GHG concentrations, possibly as a result of water column mixing • Use of Stable-Isotopes for Investigating Metabolism • d18O-DO and d13C-DIC in pond water showed significant deviations from atmospheric equilibrium as result of ecosystem metabolism, especially on 1st sampling day (calm) compared to 2nd sampling day (windy) • It is possible to obtain rates of production (P) and respiration (R) from the DO concentration and d18O data • However, DIC pools were too large to be able to detect small changes in concentration and d13C-DIC resulting from P and R • Dissolved oxygen model constrained by equations (1) and (2): • dO2/dt= P – R + (k/z)(O2(sat) – O2) • dd/dt = P(dw + ep) – R(d +eR) + (k/z)ak[(datm + eeq) – d] • Used continuous measurements of wind speed, and water temperature to estimate gas-exchange component • P scaled using measured PAR, R scaled based on water temperature and Q10 function a b a Equilibrium line Pond 1 not flooded b Volumetric rates of production, respiration and gas-exchange in ponds during July 2010. Equilibrium line Pond 2 N2O Saturation (%) • Are Ponds Heterotrophic or Autotrophic? • Based on the dissolved oxygen model a ratio of production to respiration (P:R) was calculated • Ponds exhibited a range from net autotrophic (P:R > 1, n = 2) to net heterotrophic (P:R <1, n = 2) with some ponds near-neutral (P:R ~1, n = 3) • However, model fit for ponds with P:R > 1 is relatively poor, casting doubt on these results • P:R did not appear to be a function of hydrology • Model could not be resolved for Pond 10 • Pond 1 model results are pending as model currently assumes constant water level which is not appropriate for a pond which becomes flooded c Pond 1 flooded Equilibrium point Pond 11 Dissolved N2O (% saturation) in pond waters on a calm day (first time point) and windy day (2nd time point) • N2O fluxes: • Ponds were N2O sinks and saturation ranged from n.d. to 97% • N2O consumption rates (based on estimated gas exchanges) ranged from n.d. to 13 mmol m-2 h-1 Example of modeled DO concentrations and d18O-DO compared to measured values Fluxes of CO2 (a) and CH4 (b) in Lake Hazen, Skeleton Lake and Pond 1. Note different scale for 2010 plots. Stable isotope ratios (d13C and d18O) of DIC (a) and DO (b) in pond waters; relationship of d18O vs. saturation (c) Take-Home: Ponds in the Lake Hazen area tended to be heterotrophic and a source of CO2 and CH4, despite little allochthonous C inputs. They were also sinks for atmospheric N2O Stable-isotope techniques hold promise for investigating metabolic processes in Arctic ponds, despite continuous summer daylight, and non steady-state conditions