Ninth International Conference on Permafrost ... - IARC Research

Carbon Gas Fluxes from Contrasting Boreal Lakes During Intensive Rain EventsJessica López BellidoUniversity of Helsinki, Department of Ecological and Environmental Science, Niemenkatu 73, FIN-15140 Lahti, FinlandAnne OjalaUniversity of Helsinki, Department of Ecological and Environmental Science, Niemenkatu 73, FIN-15140 Lahti, FinlandIntroductionThe variability in climate and weather are typicalcharacteristics in the Northern Hemisphere, which by itselfalready has a strong impact in the hydrology and ecology offreshwater ecosystems. Change in climate variables, such astemperature and precipitation, will definitely impact aquaticspecies at various trophic levels and alter the physical andchemical processes that act on and within the lake ecosystem.Nevertheless, not only will the extreme seasonality affectthe aquatic ecosystems, but also their surrounding areas. Forinstance, the Nordic region has a vast variety of terrains whichcontain a significant number and diversity of lakes. Thislinked pathway makes the lakes vulnerable to changes dueto the direct impact from the adjacent terrestrial ecosystem,leading to the reduction or abundance of organic matterwhich, in turn, significantly impacts aquatic production, thatis, agriculture, forestry, and urbanization. Under this processof human disturbance, the flux of gases between land, water,and atmosphere has been radically altered; thus, the increaseof greenhouses gases to the atmosphere, mainly of carbondioxide and methane.At high latitudes, wetlands (i.e., lakes, ponds, andpeatlands) are the key feature of the landscape. Amongwetlands, lakes are the most important areas where radicalchanges in water management or climate can significantlyaffect the quality of human life. On the other hand, lakesthemselves can affect the climate system. Globally, lakes aresupersaturated with CO 2and CH 4(Cole & Caraco 1998).Lakes in Finland have also been shown to be supersaturatedwith these greenhouse gases (Kortelainen 2000). They playan important role, therefore, in the exchange of CO 2and CH 4at the regional and global scale due to the amount of carbonstored in these ecosystems. This is actually a concern dueto the projections in global climate change in Nordic areas,where today’s sinks can be turned into sources of carbondioxide and methane.In this study, we explored CO 2and CH 4fluxes fromtwo lakes with contrasting limnological characteristics insouthern Finland: Lake Ormajärvi, a clear-water lake, andLake Pääjärvi, a humic brown–water system. The mainhypothesis was that the two lakes differ in their carbon gasfluxes, where the humic lake, which processes more organiccarbon of terrestrial origin, shows larger CO 2fluxes, whereasCH 4fluxes were higher from the more eutrophic clearwaterlake. Due to the limnological distinctions supposedlyresulting in differences in timing and intensity of mixingperiods, differences in seasonal flux patterns were examinedas well as their relation to biological carbon uptake andcarbon mineralization in the epilimnion of the pelagic zone.For instance, it was also hypothesized that fluxes from theclear water Lake Ormajärvi are more closely connected toautochthonous carbon uptake, whereas in the brown-waterLake Pääjärvi, the system is fueled more by allochthonouscarbon; thus fluxes and mineralization processes are tiedtogether. Finally, since during the study year 2004 thesummertime precipitation in the area doubled from theordinary 200–220 mm to 413 mm and the extra rain poureddown in four events June–July, the opportunity allowed thestudy of lake response to an extreme weather event.MethodsMeasurements of CO 2and CH 4fluxes were based onsurface water concentrations as well as gas accumulation infloating closed chambers.Primary production was measured by the radiocarbontechnique, using light/dark bottle incubations (Schindleret al. 1972). Plankton community respiration, that is,biological mineralization of organic carbon, was estimatedfrom the consumption of dissolved oxygen. For final results,the rates of oxygen consumption were converted to rates ofCO 2release using a value of one for the respiratory quotient(RQ) (Wetzel & Likens 2000). To facilitate the comparisonbetween the two different lakes, the data from respirationmeasurements as well as primary production were areallyintegrated. Pelagic CO 2net production due to biologicalprocesses was then calculated by subtracting the primaryproduction from the pelagic mineralization.Results and DiscussionBoth lakes presented seasonalities in fluxes, so thegases accumulated under ice and in the hypolimnion wereventilated out in spring and autumn. The annual CO 2fluxesfrom Lake Ormajärvi and Lake Pääjärvi were 3.6 mol m -2y -1 and 6.1 mol m -2 y -1 , respectively, and both lakes acted asa source of CO 2. The corresponding values for CH 4were24.5 mmol m -2 y -1 and 18.5 mmol m -2 y -1. In terms of globalwarming potential on annual basis, CH 4had contributed7.4% and 19.0% in Lake Pääjärvi and in Lake Ormajärvi,respectively.The most distinctive results of this study were theoverwhelming importance of rain events to gas fluxes. InLake Pääjärvi, the high precipitation resulted in a large peakin CO 2and CH 4fluxes which lasted for a couple of weeks andcontributed to 46% in CO 2and 48% in CH 4annual fluxes.In Lake Ormajärvi, the contribution of the rainy periodto carbon gas fluxes was 39% and 37% for CH 4and CO 2,respectively. The response of Lake Ormajärvi fluxes to high191