Anu Liikanen

Fluxes of methane, carbon dioxide and nitrous oxide in boreal lakes and potential anthropogenic effects on the aquatic greenhouse gas emissions.

We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.

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.

Effect of ammonium and oxygen on methane and nitrous oxide fluxes across sediment-water interface in a eutrophic lake.

Eutrophication has decreased the O(2) content and increased the NH(4)(+) availability in freshwaters. These changes may affect carbon and nitrogen transformation processes and the production of CH(4) and N(2)O, which are important greenhouse gases. We studied release of CH(4) and N(2)O from a eutrophic lake sediment under varying O(2) and NH(4)(+) conditions. Intact sediment cores were incubated in a laboratory microcosm with a continuous anoxic or oxic water flows containing 0, 50, 500, 5,000, or 15000 microM NH(4)(+). With the anoxic flow, the sediment released CH(4), up to 7.9 mmol m(-2)d(-1). With the oxic flow, the CH(4) emissions were small indicating limited CH(4) production and/or effective CH(4) oxidation. Addition of NH(4)(+) did not affect sediment CH(4) release, evidence that the CH(4) oxidizing bacteria were not disturbed by the extra NH(4)(+). The release of N(2)O from the sediment was highest, up to 7.6 micromol m(-2)d(-1), with the oxic flow without NH(4)(+) addition. Oxygen was the key factor regulating the production of NO(3)(-), which enabled denitrification and production of N(2)O. However, the highest NH(4)(+) addition increased nitrification and associated O(2) consumption causing a decrease in sediment O(2) content and in accumulation of NO(3)(-) and N(2)O, which were effectively reduced to N(2) in denitrification. In summary, sediment CH(4) and N(2)O dynamics are regulated more by the availability of O(2) than extra NH(4)(+). Anoxia in eutrophic lakes favouring the CH(4) production, is the major contributor to the atmospheric consequences of water eutrophication.

A new gypsum-based technique to reduce methane and phophorus release from sediments of eutrophied lakes:: (Gypsum treatment to reduce internal loading)

The release of nutrients from the sediment into the water column, i.e. internal nutrient loading, is an important problem in the restoration of eutrophied lakes. The ebullition of gases, mainly methane, has been identified as the key process in nutrient transportation in the internal loading. We studied whether methane production in lake sediment and the release of nutrients into the water column could be prevented by the addition of gypsum (CaSO4 x 2H2O). Three different gypsum products were examined in a controlled laboratory microcosm with a continuous water flow. The addition of gypsum (especially Fe-gypsum) into the sediment increased the redox potentials and reduced methane production and release of phosphorus from the sediments during an incubation period of 3 months. Gypsum evidently acted as a slow-releasing source of sulphate in sediment, which likely enhanced the activity of sulphate-reducing bacteria and improved the overall mineralisation rate of organic matter. The liberation of H2S and phosphate, which might be an undesired consequence of increased sulphate reduction rates was prevented by the application of Fe-containing gypsum likely as a result of the formation of pyrite and apatite minerals.

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.

Denitrification in the River Estuaries of the Northern Baltic Sea

Abstract Estuaries have been suggested to have an important role in reducing the nitrogen load transported to the sea. We measured denitrification rates in six estuaries of the northern Baltic Sea. Four of them were river mouths in the Bothnian Bay (northern Gulf of Bothnia), and two were estuary bays, one in the Archipelago Sea (southern Gulf of Bothnia) and the other in the Gulf of Finland. Denitrification rates in the four river mouths varied between 330 and 905 μmol N m−2 d−1. The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 μmol N m−2 d−1 to 910 μmol N m−2 d−1 and from 230 μmol N m−2 d−1 to 320 μmol N m−2 d−1, respectively. Denitrification removed 3.6–9.0% of the total nitrogen loading in the river mouths and in the estuary bay in the Gulf of Finland, where the residence times were short. In the estuary bay with a long residence time, in the Archipelago Sea, up to 4.5% of nitrate loading and 19% of nitrogen loading were removed before entering the sea. According to our results, the sediments of the fast-flowing rivers and the estuary areas with short residence times have a limited capacity to reduce the nitrogen load to the Baltic Sea.