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TECHNICAL REPORT
ATMOSPHERIC POLLUTANTS AND TRACE GASES

Greenhouse Gases in Non-Oxygenated and Artificially Oxygenated Eutrophied Lakes during Winter Stratification

^a Research and Development Unit of Environmental Health, Department of Environmental Sciences, Bioteknia 2, University of Kuopio, P.O. Box 1627, FIN-70211 Kuopio, Finland
^b North Savo Regional Environment Centre, P.O. Box 1049, FIN-70701 Kuopio, Finland
^c Department of Biology, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland

Corresponding author (jari.huttunen{at}uku.fi)

Received for publication September 14, 1999.

	ABSTRACT

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Concentrations of dissolved methane (CH₄), carbon dioxide (CO₂), and nitrous oxide (N₂O) were measured in the water columns of non-oxygenated and artificially oxygenated, ice-covered eutrophied lakes in the mid-boreal zone in Finland during late winter 1997 and 1999. Sampling was conducted during winter stratification, the critical period for oxygen (O₂) deficiency in seasonally ice-covered, thermally stratified lakes. Oxygen concentrations were maintained at least at a moderate level throughout the oxygenated water columns, whereas the non-oxygenated columns suffered anoxic hypolimnia. The mean concentrations of dissolved CH₄ exceeding the atmospheric equilibrium were greater in the non-oxygenated water columns (20.6–154 µM) than in the oxygenated ones (0.01–1.41 µM). In contrast, the mean excess CO₂ concentrations varied less between the non-oxygenated and oxygenated sites (0.28–0.47 and 0.25–0.31 mM, respectively). Oxygenated water columns had greater mean excess concentrations of N₂O (0.018–0.032 µM) than the non-oxygenated ones (0.005–0.024 µM). If the accumulated greenhouse gas stores in the water columns during winter are assumed to be released to the atmosphere during the spring overturn, the global warming potentials (GWP, time horizon 100 yr) of these potential emissions at the non-oxygenated, eutrophic study sites ranged from 177 to 654 g CO₂ equivalent (CO₂–e) m^-2 compared with 144 to 173 g CO₂–e m^-2 at the oxygenated sites. The increase in the accumulation of CH₄ was the main reason for the higher GWP of the non-oxygenated sites. Anthropogenic eutrophication of lake ecosystems can generate increased CH₄ emissions due to associated O₂ depletion of their sediment and water column.

Abbreviations: GWP, global warming potentials • NTOT, total nitrogen • ON, organic nitrogen • PTOT, total phosphorus

	INTRODUCTION

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NATURAL or human-induced nutrient loading of freshwater lakes (Rekolainen, 1989; Carpenter et al., 1998; Cooke and Prepas, 1998) may lead to increased accumulation of carbon (C) and nitrogen (N) in these ecosystems. It is clear that any increase in the turnover of organic matter would affect the lake biogeochemistry and accelerate the exchange of greenhouse gases between the lakes and the atmosphere. The most important radiatively active greenhouse gases (Rodhe, 1990) are CH₄, CO₂, and N₂O, which can be produced and consumed in various habitats, such as the sediments and water columns present in lakes. In lakes, the production and consumption of gases are controlled by several factors, for example, light and temperature conditions, nutrient status, O₂ availability, and the amount and quality of decomposable organic matter. These conditions can be changed during the eutrophication of lakes (Steenbergen et al., 1993).

Methane is produced in lakes during anaerobic decomposition, and consumed in freshwater mainly through aerobic CH₄ oxidation (Kiene, 1991). Many temperate, boreal, and arctic lakes and reservoirs are supersaturated with CH₄ with respect to atmospheric equilibrium and during the open water period, they may release CH₄ into the atmosphere (Strayer and Tiedje, 1978; Kling et al., 1992; Duchemin et al., 1995; Striegl and Michmerhuizen, 1998; Huttunen et al., 1999; Riera et al., 1999; Casper et al., 2000). There are often large CH₄ emissions in the summertime from eutrophied lakes, where the production of CH₄ can be favored by high amounts of easily degradable organic matter in the O₂–deficient sediment (e.g., Huttunen et al., 1999; Casper et al., 2000). Carbon dioxide is bound in photosynthesis and released in both aerobic and anaerobic degradation of dead organic matter (Wetzel, 1975). Carbon dioxide has also exceeded its atmospheric equilibrium in many lakes and reservoirs during the open water period (Kling et al., 1992; Cole et al., 1994; Duchemin et al., 1995; Hope et al., 1996; Gelbrecht et al., 1998; Striegl and Michmerhuizen, 1998; Riera et al., 1999; Casper et al., 2000). Unlike the CH₄ emissions, CO₂ release from lakes tends to decrease with increasing lake trophy due to increasing photosynthesis, and the occasional undersaturation and uptake of atmospheric CO₂ have been measured in productive lakes (e.g., Schindler et al., 1972; Gelbrecht et al., 1998; Striegl and Michmerhuizen, 1998). High allochthonous C input has been suggested to maintain high CO₂ emissions from lakes (e.g., Kling et al., 1992; Hope et al., 1996; Striegl and Michmerhuizen, 1998; Riera et al., 1999). Most of the N₂O in lakes is formed in anaerobic denitrification and aerobic nitrification (Seitzinger, 1990). The N₂O emissions from lakes are poorly understood, but they are not considered to be extensive (Knowles et al., 1981; Mengis et al., 1997).

Winter is a critical period for O₂ deficiency and the accumulation of greenhouse gases in ice-covered eutrophic lakes. The stores of these gases that have accumulated in lakes are predominantly released into the atmosphere during the spring overturn following ice melt, which increases the annual pelagic greenhouse gas emissions (e.g., Michmerhuizen et al., 1996; Striegl and Michmerhuizen, 1998). Several methods have been employed for the restoration of eutrophied, O₂–deficient lakes (Lappalainen, 1982). One of them is artificial oxygenation, where O₂ concentrations of the hypolimnion and sediment are increased in order to decrease the internal nutrient loading (i.e., resuspension of nutrients from anaerobic sediment to the overlying waters) (Lappalainen, 1994). This possibility of enhancing the O₂ conditions in eutrophic water bodies has attracted worldwide interest (Cowell et al., 1987; Lappalainen, 1994), but it also can influence the production and consumption of greenhouse gases in lakes. Although the artificial oxygenation of eutrophic lakes is not an economically feasible way to affect the regional greenhouse gas fluxes, the comparison between the gas contents of oxygenated and non-oxygenated lakes could provide important information on the potential influence of increasing eutrophication and O₂ deficiency on the biogeochemical processes, especially greenhouse gas emissions going on in lake ecosystems. Therefore, we measured the concentrations of dissolved CH₄, CO₂, and N₂O in two hypereutrophic mid-boreal lakes with and without artificial oxygenation, and estimated the gas stores accumulated in the water column during two winter periods.

	MATERIALS AND METHODS

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Measurements were conducted during late winter stratification on 13 Mar. 1997 and 10 Mar. 1999 in two seasonally ice-covered, mid-boreal hypereutrophic freshwater lakes of the same watercourse, Lake Kevätön and Lake Pöljänjärvi, in east-central Finland (Fig. 1) . The lakes have become more eutrophic as a result of intensive agriculture in their catchment areas. Lake Kevätön has also received a sewage load from a hospital from the 1930s up until 1975. Lake Kevätön consists of one non-oxygenated basin, whereas Lake Pöljänjärvi has two basins, one of which has been artificially oxygenated since December 1995 by the Mixox hypolimnetic oxygenation method (Lappalainen, 1994; K. Matti Lappalainen, Water-Eco Ltd., Kuopio, Finland, personal communication, 2000). In this method, the oxygen-rich epilimnetic water is pumped down to the oxygen-deficient hypolimnion. The stratification of the water column is maintained by appropriate pumping power in order to prevent the mixing of the nutrient-rich hypolimnion with the epilimnion. The Mixox apparatus is a special submersible electrical pump that is placed 1 to 3 m below the water surface at the deepest part in the lake, leaving only a signal buoy visible above the water surface. There are four models of the Mixox apparatus available: MC-500, MC-750, MC-1000, and MC-1100, with power inputs of 0.6, 1.1, 2.1, and 2.5 kW, respectively. The Mixox oxygenators have been designed to oxygenate lakes of 30, 100, 200, or 400 ha in area. Model MC-750 was used in Lake Pöljänjärvi during this study. The O₂ transfer capacity of the Mixox apparatus is greater (6–12 kg O₂ kWh^-1) than that of traditional aerators, which introduce air bubbles in the hypolimnion (about 0.2 to 5 kg O₂ kWh^-1). All the study lakes were totally ice-covered during the two winter periods, thus dissolved gases were not stripped out of the water column from the oxygenated and the non-oxygenated basins during the winter. Study site A1 was located on the non-oxygenated Lake Kevätön, and site A2 on the non-oxygenated basin of Lake Pöljänjärvi (Fig. 1). Sites B1 and B2 were close to each other on the oxygenated basin of Lake Pöljänjärvi. Study sites A1, A2, B1, and B2 were located at the deepest points in the basins at depths of 9.7, 9.8, 12.9 to 13.0, and 13.0 to 13.1 m, respectively.

View larger version (17K): [in this window] [in a new window]   Fig. 1. Study sites on the non-oxygenated basin of Lake Kevätön (A1), and on the non-oxygenated (A2) and oxygenated (B1 and B2) basins of Lake Pöljänjärvi in the mid-boreal zone of Finland. Streams are indicated by arrows, and the location of the Mixox oxygenator by X. The earlier hospital sewage outlet is indicated by L

Water samples for the determination of dissolved CH₄, CO₂ , and N₂O were taken at 1-m intervals from a depth of 1 m below the water surface down to 1 m above the sediment. Some samples were also taken closer to the sediments. The samples (30 mL) were carefully withdrawn by means of a hypodermic needle (diameter 0.9 mm) from the Limnos sampler (equipped with silicone tubing) into 50-mL polypropylene syringes (Terumo Europe, Leuven, Belgium) equipped with three-way stopcocks (Codan Steritex, Hoejvangen, Denmark). Samples were acidified in the field with sulfuric acid (1 mL H₂SO₄, 20% v/v) for preservation and for the measurement of CO₂ in laboratory. The gas concentrations were quantified using a headspace equilibration technique (McAuliffe, 1971) within 24 h of sampling in the Laboratory of Environmental Microbiology, National Public Health Institute (Kuopio, Finland). Nitrogen-filled syringes (30 mL water + 30 mL N₂) were equilibrated by shaking vigorously for 3 min. The headspace gas concentrations were analyzed on two gas chromatographs (Hewlett–Packard [Palo Alto, CA] 5890 Series II) using FI, TC, and EC detectors for CH₄, CO₂, and N₂O, respectively (for details see Nykänen et al., 1995). The gas concentrations in the water samples were calculated from the headspace gas concentrations with Henry's law (Sander, 1999) using the values from Lide and Fredrikse (1995). The CO₂ concentration in situ was calculated from the measured CO₂ concentration based on actual water temperatures and pH (Butler, 1982). The effect of temperature on the free CO₂–carbonate CO₂–bicarbonate CO₂ equilibrium was also taken into account (Butler, 1982).

Excess greenhouse gas concentrations accumulated below the ice during winter were calculated from the concentrations in water exceeding the atmospheric equilibrium (equilibrium concentrations were 3.6–4.0 nM for CH₄, 22–26 µM for CO₂, and 14–17 nM for N₂O at the measured water temperatures). The equilibrium gas concentrations in water were calculated assuming the atmospheric mixing ratios of 1.72 µL L^-1 for CH₄ (from 1994 data), 358 µL L^-1 for CO₂ (from 1994 data), and 312 nL L^-1 for N₂O (estimated from 1984–1994 data) and taking into account their annual increases of 0.01 µL L^-1 yr^-1, 1.5 µL L^-1 yr^-1, and 0.8 nL L^-1 yr^-1, respectively (Intergovernmental Panel on Climate Change, 1996). Potential greenhouse gas emissions associated with spring overturn following ice melt were calculated by summing the excess gas stores at different depths. The GWP of the emissions were calculated in CO₂–e by multiplying the emissions by their GWP values: 1 for CO₂, 21 for CH₄, and 310 for N₂O, with a time horizon of 100 yr (Intergovernmental Panel on Climate Change, 1996).

	RESULTS

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During sampling, the water columns were thermally stratified (Table 1; Fig. 2 and 3) . However, thermal stratification was weaker in the oxygenated than in the non-oxygenated water columns both in winter 1997 and 1999. The non-oxygenated sites had anoxic hypolimnion, whereas oxygenation maintained at least moderate oxygen concentrations (3.8–8.6 mg L^-1) throughout all oxygenated water columns. Epilimnetic O₂ concentrations ranged from 4.0 to 9.6 mg L^-1 among the study sites. The NTOT and PTOT concentrations in the anoxic hypolimnion of the non-oxygenated water columns exceeded the concentrations near the bottom in the oxygenated water columns. The epilimnetic NTOT levels were rather similar in the non-oxygenated and oxygenated water columns. The epilimnetic PTOT concentrations were slightly higher in the oxygenated water columns. The ON concentration varied randomly both in the epilimnion and hypolimnion among the sites. The NH⁺₄–N concentrations near the bottom were much higher in the non-oxygenated water columns than in the oxygenated water columns, while the epilimnetic concentrations were relatively similar at all the sites. The –N concentrations were higher in the oxygenated than in the non-oxygenated waters. The differences in water characteristics between the non-oxygenated and oxygenated sites remained rather constant in 1997 and in 1999. All the study sites showed lower O₂ concentrations in winter 1999.

View larger version (35K): [in this window] [in a new window]   Fig. 2. The measured in situ concentrations of dissolved CH4, CO2, N2O, and O2, and temperature in the water columns of non-oxygenated sites (a) A1 and (b) A2 and oxygenated sites (c) B1 and (d) B2 during winter stratification on 13 Mar. 1997. The concentrations in equilibrium with the atmosphere were 3.6 to 4.0 nM for CH4, 22 to 26 µM for CO2, and 14 to 17 nM for N2O at the measured water temperatures

View larger version (33K): [in this window] [in a new window]   Fig. 3. The measured in situ concentrations of dissolved CH4, CO2, N2O, and O2, and temperature in the water columns of non-oxygenated sites (a) A1 and (b) A2 and oxygenated sites (c) B1 and (d) B2 during winter stratification on 10 Mar. 1999. The concentrations in equilibrium with the atmosphere were 3.6 to 4.0 nM for CH4, 22 to 26 µM for CO2, and 14 to 17 nM for N2O at the measured water temperatures. Those CH4 concentrations below the detection limit are indicated with an asterisk

The non-oxygenated water columns had a greater excess of dissolved CH₄ than the oxygenated waters (Table 2). Therefore, the potential springtime CH₄ emissions from the non-oxygenated sites were 14 to 16000 times the emissions from the oxygenated sites. The difference in the excess CO₂ content between the non-oxygenated and oxygenated sites was minor. Overall, the N₂O concentrations in the oxygenated water columns were greater than in the non-oxygenated ones. The potential N₂O emissions, expressed as GWP, were low compared with the GWP of CH₄ and CO₂ (Table 2). The total GWP for the emissions from the artificially oxygenated water columns were 29 to 65% of those from the non-oxygenated waters in 1997, and 26 to 98% in 1999.

	DISCUSSION

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The differences in the concentrations of dissolved greenhouse gases between the oxygenated and non-oxygenated sites showed that oxygenation played a significant role in limiting the anaerobic microbial–chemical processes in the water column and sediment. The concentrations of CH₄, produced in strictly anaerobic methanogenesis and consumed in the presence of O₂ by methanotrophic bacteria (Kiene, 1991), were higher in the hypolimnion of the non-oxygenated sites with O₂ depletion. Lower CH₄ concentrations in the oxygenated water columns can be attributed to the decreased CH₄ production and enhanced CH₄ oxidation in the O₂–rich conditions. It has been suggested that CH₄ oxidation can enhance anoxia in ice-covered lakes (Rudd and Hamilton, 1978). The true CH₄ oxidation rates of our eutrophic, CH₄–enriched waters remain unknown. The higher CH₄ concentration in the non-oxygenated water columns could reflect a greater internal nutrient loading, and subsequently a higher autochthonous load of organic matter (primary production) and a higher rate of decomposition.

Oxygenation did not seem to influence markedly the CO₂ content of the water column, as there were rather similar CO₂ concentrations at the non-oxygenated and oxygenated sites. This observation suggests that, in spite of the O₂ deficiency, the decomposition was efficient. This supports the conclusion of Jones and Simon (1980) that in the profundal zone of freshwaters, as much as 60% of the accumulated CO₂ can originate from anaerobic microbial processes such as denitrification and sulfate reduction. The slightly higher water temperature near the sediments of the non-oxygenated sites than the oxygenated sites also favors microbial activities at the non-oxygenated sites.

The higher N₂O concentrations in the oxygenated water columns could probably be due to more active nitrification and a coupling of nitrification and denitrification in the water column and sediment. The accumulation of –N, the products of nitrification, was higher in the oxygenated than in the non-oxygenated waters. In general, the highest N₂O concentrations in lakes have been found within the zones of high concentrations of O₂ and NO^-₃, reflecting nitrification activity in these zones, or at the oxic–anoxic interface, where both nitrification and denitrification can take place (Knowles et al., 1981; Seitzinger, 1990; Mengis et al., 1996, 1997). The low N₂O concentrations in our study, especially in the anoxic hypolimnion of the non-oxygenated sites, suggested a low nitrification activity or the reduction of N₂O to N₂ in complete denitrification, or both. The accumulation of NH⁺₄–N in the non-oxygenated waters also indicated low levels of nitrification processes (see Knowles et al., 1981; Seitzinger, 1990; Mengis et al., 1997).

A buildup of CH₄ and CO₂ in the ice-covered water columns during winter led to a supersaturation of CH₄ and CO₂ with respect to the atmospheric equilibrium, as reported for many lakes in temperate, boreal, and arctic regions (Kling et al., 1992; Michmerhuizen et al., 1996; Gelbrecht et al., 1998; Striegl and Michmerhuizen, 1998; Huttunen et al., 1999; Riera et al., 1999; Kortelainen et al., 2001). The observed supersaturation of CH₄ and CO₂ suggests that these gases will be released into the atmosphere during the spring overturn after ice melt. In this study, the possible consumption of CH₄ by CH₄ oxidation (Rudd and Hamilton, 1978; Michmerhuizen et al., 1996) and reduction of CO₂ concentration by photosynthesis (Miller et al., 1986), as well as the production of these gases between the sampling and the spring overturn, were assumed to be negligible. The N₂O concentration was also assumed to be unchanged up until the overturn. In contrast with CH₄ and CO₂, the N₂O concentrations were generally low in these lakes. Undersaturation of N₂O can occur in highly anoxic water columns during stratification periods (Knowles et al., 1981; Mengis et al., 1997; Kortelainen et al., 2001). Some studies have shown that artificial oxygenation can increase N₂O supersaturation (Mengis et al., 1996, 1997; Kortelainen et al., 2001). Here, both the oxygenated and non-oxygenated water columns were predominantly supersaturated with N₂O, the concentrations being slightly higher in the oxygenated waters.

On the basis of differences in the greenhouse gas stores between the oxygenated and non-oxygenated water columns, oxygenation decreases the springtime CH₄ emissions, increases the N₂O emissions, but has less effect on the CO₂ release. According to the GWP approach, artificial oxygenation had a positive climatic effect by reducing the potential springtime greenhouse gas emissions. The total GWP values (time horizon 100 yr) for the potential emissions from the oxygenated sites ranged from 26 to 98% of the emissions from the non-oxygenated sites, mainly due to the decreased accumulation of CH₄ during winter. Therefore, the CH₄ emissions from lakes can be expected to increase due to eutrophication and associated O₂ depletion.

Analyzing only the dissolved CH₄ in water below the ice cover leads to an underestimation of the CH₄ produced during the winter. Artificial oxygenation can decrease the CH₄ saturation in the sediment, and thus, decrease the ebullition of CH₄, which is an important mechanism for transporting CH₄ from organic-rich, CH₄–saturated sediments to the atmosphere (Strayer and Tiedje, 1978; Huttunen et al., 1999; Casper et al., 2000). The high accumulation of dissolved CH₄ below the ice cover in the non-oxygenated sites (see Fig. 2 and 3) suggests that ebullition took place during the winter, and that CH₄ has redissolved from the bubbles in the surface water. Therefore, the reduction in the net CH₄ efflux due to oxygenation may be higher than that shown by the results for dissolved CH₄. Furthermore, if the reduction in CH₄ production and ebullition continues during the open-water season, in some cases oxygenation can reduce the internal nutrient loading in lakes by decreasing the nutrient flow transported by CH₄ bubble convection from the sediment to the epilimnion (see Itkonen and Olander, 1997). This can reduce primary production and the associated production of greenhouse gases, especially CH₄.

Springtime CH₄ emissions may constitute a considerable proportion of the pelagic CH₄ emissions, because as much as 40% of the CH₄ emitted annually from the pelagic regions of lakes can be released during spring overturn (Michmerhuizen et al., 1996). The springtime CH₄ emissions from north-temperate lakes (0.006–2.97 g m^-2) have been partially related to sediment type by Michmerhuizen et al. (1996). They demonstrated that the emissions were greater from small lakes with soft organic sediments. Our sites, which had a large supply of organic matter to the sediment, also showed high potential springtime CH₄ emissions under anaerobic conditions. The pelagic CH₄ emission from Lake Kevätön was 8 to 20 Mg during the open water period (Huttunen et al., 1999; J.T. Huttunen, unpublished results, 1997–1998). The potential springtime CH₄ emission from its deepest region (6–10 m deep, this study) was 2 to 3 Mg even though that region constitutes only 3.4% of the total lake area of 4.07 km². The shallower pelagic area also had probably a great importance in the springtime CH₄ release. The contribution of littoral regions with macrophyte vegetation to the CH₄ emissions from lakes can be considerable (Alm et al., 1996; Hyvönen et al., 1998). The importance of the littoral regions in the gas exchange of boreal lakes is poorly understood. It must be stressed, however, that oxygenation cannot reduce greenhouse gas emissions from lake shores. Using the summer (Nykänen et al., 1998) and winter (Alm et al., 1999) CH₄ emissions estimated for minerogenous peatlands (fens) in Finland, it was calculated that the vegetated littoral region of Lake Kevätön (24% of the lake area) would release 2 to 47 Mg of CH₄ during the summer and 0.3 to 6 Mg during the winter.

The effect of lake oxygenation on springtime CH₄ release, expressed here as the difference between the emissions from oxygenated and non-oxygenated water bodies, has important implications in the future atmospheric forcing of boreal lakes. The CH₄ emissions from lakes would be anticipated to increase due to eutrophication and associated O₂ depletion of the sediment and water column. The predicted increase in winter precipitation (Intergovernmental Panel on Climate Change, 1996) may also lead to accelerated eutrophication due to increased allochthonous nutrient loading in lakes, O₂ deficiency, and CH₄ enrichment in water bodies, which will subsequently increase the CH₄ emissions.

	ACKNOWLEDGMENTS

 
We express our appreciation to Eija Konttinen for the gas analyses and to Pirjo Punju, Hannu Partanen, and Reijo Heikkinen for their help in the field. K. Matti Lappalainen is acknowledged for his comments on an early version of the manuscript. The study was financed by The Maj and Tor Nessling Foundation, The North Savo Regional Environment Centre, and The Academy of Finland. The English text was revised by John Derome and Ewen MacDonald.

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