Tero Väisänen

Effects of temperature and oxygen availability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH4, CO2, N2O) and nutrients(NH4+, NO2−+NO3−, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO2increased with increasing temperature in both oxic (Q10 3.2 ±0.6) and anoxic (Q10 2.3 ± 0.4) flows. The release ofCH4 increased with temperature in anoxic conditions (Q102.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH4+ increased with temperature with the oxic and anoxic flows(Q10 2.4 ± 0.1). There was a net production of NO2−, NO3− and N2O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q102.9 ± 0.5) and oxic (Q10 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH4 production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH4productionin sediments.

Exchange of CO2, CH4 and N2O between the atmosphere and two northern boreal ponds with catchments dominated by peatlands or forests

Concentrations of carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) in the water column and their exchange at the water/air interface were studied during the open water period in two freshwater ponds with different catchment characteristics in the northern boreal zone in Finland; either peatlands or coniferous upland forests dominated the catchment of the ponds. Both ponds were supersaturated with dissolved CO2 and CH4 with respect to the equilibrium with the atmosphere, but were close to the equilibrium with N2O. The mean CO2 efflux from the pond was higher in the peatland-dominated catchment (22 mg m−2 h−1) than in the forested catchment (0.7 mg m−2 h−1), whereas the mean CH4 emissions were similar (7.6 and 3.5 mg m−2 d−1, respectively). The fluxes of N2O were generally negligible. The higher CO2 concentrations and efflux in the pond with the peatland-dominated catchment were attributed to a greater input of allochthonous carbon to that pond from its catchment due to its higher water colour and higher total organic carbon (TOC) concentration. The water pH, which also differed between the ponds, could additionally affect the CO2 dynamics. Since the catchment characteristics can regulate aquatic carbon cycles, catchment-scale studies are needed to attain a deeper understanding of the aquatic greenhouse gas dynamics.

A novel sediment gas sampler and a subsurface gas collector used for measurement of the ebullition of methane and carbon dioxide from a eutrophied lake

A novel sediment bubble gas sampler and a subsurface bubble gas collector were designed to measure the ebullition of gases from profundal sediments of aquatic ecosystems. The sediment gas sampler was constructed to collect bubble gas samples directly from the uppermost sediment layers for gas composition analysis. The floating subsurface gas collector, designed to trap the bubbles released naturally from sediments, permitted the measurement of both the volume and the composition of the bubble gas. Due to its low cost, light weight and rapid sampling capability, the gas collector is ideal for studies requiring many replicate collectors. These devices were used for measurement of the ebullition of methane (CH4) and carbon dioxide (CO2) during an open water period from hypereutrophic Lake Postilampi, situated within the midboreal zone in Finland. The bubble gas obtained from the sediment with the sediment gas sampler had higher concentrations of CH4 and CO2 than the bubbles trapped in the gas collectors. This indicated that the bubble gas composition changed, either naturally during the migration of the bubbles from the sediment through the water column to the gas collectors, and/or during their storage in the collectors prior to sampling. The mean CH4 ebullition from Lake Postilampi was estimated to be in the range from 36 to 46 mg m(-2 d(-1), based on the bubble gas CH4 concentrations measured from the gas collectors and sediment, respectively. The bubbles contained only 0.02-0.57% of CO2 and thus, the ebullition had no significance in the release of CO2 from the lake.

Spatial and seasonal variation in greenhouse gas and nutrient dynamics and their interactions in the sediments of a boreal eutrophic lake

Dynamics of greenhouse gases, CH4, CO2 and N2O, and nutrients, NO2− + NO3−, NH4+ and P, were studied in the sediments of the eutrophic, boreal Lake Kevätön in Finland. Undisturbed sediment cores taken in the summer, autumn and winter from the deep and shallow profundal and from the littoral were incubated in laboratory microcosms under aerobic and anaerobic water flow conditions. An increase in the availability of oxygen in water overlying the sediments reduced the release of CH4, NH4+ and P, increased the flux of N2O and NO2− + NO3−, but did not affect CO2 production. The littoral sediments produced CO2 and CH4 at high rates, but released only negligible amounts of nutrients. The deep profundal sediments, with highest carbon content, possessed the greatest release rates of CO2, CH4, NH4+ and P. The higher fluxes of these gases in summer and autumn than in winter were probably due to the supply of fresh organic matter from primary production. From the shallow profundal sediments fluxes of CH4, NH4+ and P were low, but, in contrast, production of N2O was the highest among the different sampling sites. Due to the large areal extension, the littoral and shallow profundal zones had the greatest importance in the overall gas and nutrient budgets in the lake. Methane emissions, especially the ebullition of CH4 (up to 84% of the total flux), were closely related to the sediment P and NH4+ release. The high production and ebullition of CH4, enhances the internal loading of nutrients, lake eutrophication status and the impact of boreal lakes to trophospheric gas budgets.