Denitrification in the river estuaries of the northern Baltic Sea.

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 micromol N m(-2) d(-1). The estuary bays at the Archipelago Sea and the Gulf of Bothnia had denitrification rates from 90 micromol N m(-2) d(-1) to 910 micromol N m(-2) d(-1) and from 230 micromol N m(-2) d(-1) to 320 micromol 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.

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.

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.

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.

Gas dynamics in eutrophic lake sediments affected by oxygen, nitrate, and sulfate.

In many freshwater ecosystems, the contents of NO3- and SO4(2-) have increased, whereas O2 has been depleted due to the increased acid and nutrient loads. These changes may affect carbon turnover and the dynamics of the major greenhouse gases CO2, CH4, and N2O. We studied the effects of O2, NO3-, and SO4(2-) availability on carbon mineralization, and fluxes of CO2, CH4, and N2O in the sediments of hyper-eutrophic Lake Kevätön, Finland. Undisturbed sediment cores from the deep (9 m) and shallow (4 m) profundal were incubated in a laboratory microcosm with oxic and anoxic water flows with NO3- or SO4(2-) concentrations of 0, 30, 100, 300, and 2000 microM. The carbon mineralization rate (i.e., the sum of released CO2-C and CH4-C) was not affected by the oxidants. However, the oxidants did change the pathways of carbon degradation and the release of CH4. All of the oxidants depressed CH4 fluxes in the shallow profundal sediments, which had low organic matter content. In the deep profundal sediments rich in organic matter, the CH4 release was reduced by O2 but was not affected by SO4(2-) (the effect of NO3- was not studied). There was an increase in N2O release as the overlying water NO3- concentration increased. Anoxia and highly elevated NO3- concentrations, associated with eutrophication, increased drastically the global warming potential (GWP) of the sedimentary gases in contrast to the SO4(2-) load, which had only minor effects on the GWP.

Phosphorus removal in a wetland constructed on former arable land.

Phosphorus in surface runoff water may cause eutrophication of recipient water. This study clarifies the mechanisms of P removal in the wetland of Hovi, Finland, constructed on arable land in 1998. Before the construction, the surface soil (removed in the construction) and subsoil (the current wetland bottom) were analyzed for Al and Fe oxides (Al(ox) and Fe(ox)) reactive in P sorption, and for the distribution of P between various pools as well as for P exchange properties. Retention of P from runoff water within the wetland was studied from 1999 to 2001 in situ and factors affecting the P removal (O2 availability and P concentration in water) were investigated in a laboratory microcosm. The processes taking place in the wetland diminished by 68% the total P load and by 49% the dissolved reactive P load. Desorption-sorption tests indicated that without removal of the surface soil, there would have been a risk of the wetland being a source of P, since the equilibrium P concentration of the soil removed was high compared with the mean P concentration of the inflowing water. The subsoil contained less P and high amounts of reactive oxides, which could bind P. Evidently, the P sorption by Al(ox) played an important role in a first phase removal of P, since the wetland retained P efficiently even under anoxic conditions, where Fe tends to be reduced. Fine-textured, mineral soil on the bottom of the wetland (subsoil of the former arable land) seemed to be very efficient in retaining P from agricultural runoff.