Ecosystem-Scale Oxygen Manipulations Alter Terminal Electron Acceptor Pathways in a Eutrophic Reservoir

Lakes and reservoirs globally are experiencing unprecedented changes in land use and climate, depleting dissolved oxygen (DO) in the bottom waters (hypolimnia) of these ecosystems. Because DO is the most energetically favorable terminal electron acceptor (TEA) for organic carbon mineralization, its availability controls the onset of alternate TEA pathways (for example, denitrification, manganese reduction, iron reduction, sulfate reduction, methanogenesis). Low DO concentrations can trigger organic carbon mineralization via alternate TEA pathways in the water column and sediments, which has important implications for greenhouse gas production [carbon dioxide (CO₂) and methane (CH₄)]. In this study, we experimentally injected supersaturated DO into the hypolimnion of a eutrophic reservoir and measured concentrations of TEAs and terminal electron products (TEPs) in the experimental reservoir and an upstream reference reservoir. We calculated the electron equivalents yielded from each TEA pathway and estimated the contributions of each TEA pathway to organic carbon processing in both reservoirs. DO additions to the hypolimnion of the experimental reservoir promoted aerobic respiration, suppressing most alternate TEA pathways and resulting in elevated CO₂ accumulation. In comparison, organic carbon mineralization in the reference reservoir’s anoxic hypolimnion was dominated by alternate TEA pathways, resulting in both CH₄ and CO₂ accumulation. Our ecosystem-scale experiments demonstrate that the alternate TEA pathways that succeed aerobic respiration in lakes and reservoirs can be manipulated at the ecosystem scale. Moreover, changes in the DO dynamics of freshwater lakes and reservoirs may result in concomitant changes in the redox reactions in the water column that control organic carbon mineralization and greenhouse gas accumulation.

     

Fish Reliance on Littoral–Benthic Resources and the Distribution of Primary Production in Lakes

Pelagic, littoral, and terrestrial resources can all play a role in supporting consumers in lakes. The role of benthic algal-derived food web pathways in lakes is perhaps the least understood because limnologists have historically focused on pelagic (open-water) production and processes. We compiled carbon stable isotope data from 546 fish populations (75 lakes), and used a two end-member mixing model to calculate littoral–benthic reliance for each fish species in each lake. Fish littoral–benthic reliance values were averaged by lake to assess overall fish species benthic reliance for each lake. Lake-specific mean littoral reliance (BR_L; fish species not weighted according to production or biomass) averaged 57% and was independent of lake morphological and limnological attributes. For these same lakes, water column nutrients, light, and morphometry data were used to estimate whole-lake benthic algal and phytoplankton primary production. On average, benthic algae comprised 36% of whole-lake primary production (BP_f = 0.36). BP_f and BR_L were weakly correlated: BR_L tends to be high even in large/deep lakes in which benthic algae is a minor contributor to whole-lake primary production. The high littoral–benthic contribution to individual fish species appears to reflect the high concentration of fish species diversity in the littoral zone. Our work cannot be extrapolated to whole-lake fish production. However, the result is consistent with other work indicating that most fish species inhabit the littoral zone, whereas relatively few exclusively inhabit the pelagic. Our results suggest that it takes less primary production to support a single fish species in the littoral zone than is required to support a species in the pelagic.     

Processes affecting molecular and stable isotope compositions of sediment gas in estuarine waters along the southern Baltic coast (Poland)

This paper investigates the molecular and stable isotope compositions of sediment gases from seven coastal lakes along the southern Baltic coast in Poland. The aim is to extend the knowledge of the genesis and distribution of microbial gases in the zone of mixing of fresh and salt waters with special attention to the effect of salinity, climate-related seasonality, and vertical sediment mixing. We found differences in the compositions of gas between the studied lakes and within each lake. These differences are mainly controlled by lake water depth and the presence of macrophytes. Due to the dissolution of rising bubbles in highly oxygenated water, the concentrations of CH₄ and CO₂ show up to 67% decline along the water column in favor of N₂ and O₂. On the other hand, in vegetated parts of the lakes, the CH₄ is depleted in favor of CO₂, and the residual CH₄ and CO₂ are enriched in ¹³C. Despite the fact that the coastal lakes display highly oxidizing conditions in the water column and that the bottom sediments are mixed by wind waves the CH₄ reveals rather low oxidation. On the basis of the CH₄/N₂ ratio we established that there are differences in the intensity of ebullition throughout the lakes. Higher intensities of ebullition were found in shallower parts of the lakes. Salinity has no effect on the stable C and H isotope composition of sediment gas. It seems, however, that salinity affects the molecular composition of hydrocarbons via preferential oxidation of CH₄ under higher salinity conditions.     

A comparison of diatom phosphorus transfer functions and export coefficient models as tools for reconstructing lake nutrient histories

1. We compared the baseline phosphorus (P) concentrations inferred by diatom‐P transfer functions and export coefficient models at 62 lakes in Great Britain to assess whether the techniques produce similar estimates of historical nutrient status. 2. There was a strong linear relationship between the two sets of values over the whole total P (TP) gradient (2–200 μg TP L^?1). However, a systematic bias was observed with the diatom model producing the higher values in 46 lakes (of which values differed by more than 10 μg TP L^?1 in 21). The export coefficient model gave the higher values in 10 lakes (of which the values differed by more than 10 μg TP L^?1 in only 4). 3. The difference between baseline and present‐day TP concentrations was calculated to compare the extent of eutrophication inferred by the two sets of model output. There was generally poor agreement between the amounts of change estimated by the two approaches. The discrepancy in both the baseline values and the degree of change inferred by the models was greatest in the shallow and more productive sites. 4. Both approaches were applied to two lakes in the English Lake District where long‐term P data exist, to assess how well the models track measured P concentrations since approximately 1850. There was good agreement between the pre‐enrichment TP concentrations generated by the models. The diatom model paralleled the steeper rise in maximum soluble reactive P (SRP) more closely than the gradual increase in annual mean TP in both lakes. The export coefficient model produced a closer fit to observed annual mean TP concentrations for both sites, tracking the changes in total external nutrient loading. 5. A combined approach is recommended, with the diatom model employed to reflect the nature and timing of the in‐lake response to changes in nutrient loading, and the export coefficient model used to establish the origins and extent of changes in the external load and to assess potential reduction in loading under different management scenarios. 6. However, caution must be exercised when applying these models to shallow lakes where the export coefficient model TP estimate will not include internal P loading from lake sediments and where the diatom TP inferences may over‐estimate TP concentrations because of the high abundance of benthic taxa, many of which are poor indicators of trophic state.     

Estimating oxygen exchange across the air–water interface of a hypereutrophic lake

Patterns of estimates of oxygen flux (J) across the air–water interface of hypereutrophic Onondaga Lake, NY, U.S.A., are characterized for time scales ranging from diel to seasonal for an 8-month period. The analysis is supported by a high frequency (most often hourly) monitoring program, conducted with a robotic buoy, that included measurements of dissolved oxygen (DO), temperature, and fluorometric chlorophyll a in the lake's surface waters, vertical profiles of DO through the epilimnion, and wind speed and solar radiance. The magnitude and direction of J is demonstrated to vary dramatically at diel, day-to-day, and seasonal time scales. Thus, large errors in estimates of J may result from extrapolating flux calculations made from short-term data to longer time periods. The variations in J were driven by variations in metabolic activity and meteorology, and were mediated by departures from equilibrium DO concentrations and wind-driven turbulence. Extended periods of high J values are shown to coincide with intervals of large departures from equilibrium DO concentrations, but day-to-day differences are driven mostly by variations in wind. A distinct diel pattern of J estimates is manifested for average conditions, with substantially higher J values during daylight hours. This pattern reflects the common diel patterns of the drivers of both higher DO oversaturation and wind speed over those hours. It is demonstrated that the magnitude of J is substantial relative to net changes in the epilimnetic DO pool, and thus must be accommodated accurately in estimates of primary production and community respiration that are to be based on diel monitoring of DO in the water columns of productive lakes.     

Deriving nutrient targets to prevent excessive cyanobacterial densities in U.S. lakes and reservoirs

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From Bacteria to Fish: Ecological Consequences of Seasonal Hypoxia in a Great Lakes Estuary

The occurrence of bottom-water hypoxia is increasing in bodies of water around the world. Hypoxia is of concern due to the way it negatively impacts lakes and estuaries at the whole ecosystem level. During 2015, we examined the influence of hypoxia on the Muskegon Lake ecosystem by collecting surface- and bottom-water nutrient samples, bacterial abundance counts, benthic fish community information, and performing profiles of chlorophyll and phycocyanin as proxies for phytoplankton and cyanobacterial growth, respectively. Several significant changes occurred in the bottom waters of the Muskegon Lake ecosystem as a result of hypoxia. Lake-wide concentrations of soluble reactive phosphorus (SRP) and total phosphorus increased with decreasing dissolved oxygen (DO). Bacterial abundance was significantly lower when DO was less than 2.2 mg L^?1. Whereas there were no drastic changes in surface chlorophyll a concentration through the season, phycocyanin increased threefold during and following a series of major wind-mixing events. Phycocyanin remained elevated for over 1.5 months despite several strong wind events, suggesting that high SRP concentrations in the bottom waters may have mixed into the surface waters, sustaining the bloom. The fish assemblage in the hypolimnion also changed in association with hypoxia. Overall fish abundance, number of species, and maximum length all decreased in catch as a function of bottom DO concentrations. The link between hypoxia and wind events appears to serve as a positive feedback loop by continuing internal loading and cyanobacterial blooms in the lake, while simultaneously eroding habitat quality for benthic fish.     

Longer duration of seasonal stratification contributes to widespread increases in lake hypoxia and anoxia

The concentration of dissolved oxygen (DO) is an important attribute of aquatic ecosystems, influencing habitat, drinking water quality, biodiversity, nutrient biogeochemistry, and greenhouse gas emissions. While average summer DO concentrations are declining in lakes across the temperate zone, much remains unknown about seasonal factors contributing to deepwater DO losses. It is unclear whether declines are related to increasing rates of seasonal DO depletion or changes in seasonal stratification that limit re-oxygenation of deep waters. Furthermore, despite the presence of important biological and ecological DO thresholds, there has been no large-scale assessment of changes in the amount of habitat crossing these thresholds, limiting the ability to understand the consequences of observed DO losses. We used a dataset from >400 widely distributed lakes to identify the drivers of DO losses and quantify the frequency and volume of lake water crossing biologically and ecologically important threshold concentrations ranging from 5 to 0.5 mg/L. Our results show that while there were no consistent changes over time in seasonal DO depletion rates, over three-quarters of lakes exhibited an increase in the duration of stratification, providing more time for seasonal deepwater DO depletion to occur. As a result, most lakes have experienced summertime increases in the amount of water below all examined thresholds in deepwater DO concentration, with increases in the proportion of the water column below thresholds ranging between 0.9% and 1.7% per decade. In the 30-day period preceding the end of stratification, increases were greater at >2.2% per decade and >70% of analyzed lakes experienced increases in the amount of oxygen-depleted water. These results indicate ongoing climate-induced increases in the duration of stratification have already contributed to reduction of habitat for many species, likely increased internal nutrient loading, and otherwise altered lake chemistry. Future warming is likely to exacerbate these trends.     

Basin-Wide Considerations for Water Quality Management: Importance of Phosphorus Retention by Reservoirs

The relationship between phosphorus retention (R_P) and water residence time (R_T) was assessed for selected U.S. Army Corps of Engineers reservoirs and found to be influenced by areal phosphorus loading rate (P_L). For reservoirs with high P_L (>15 gm/m²/yr), R_P increased markedly with modest increases in R_T. Reservoirs with lower values of P_L exhibited less dramatic responses in R_P to changes in R_T. The water quality management implications of this are great since reservoirs for which marked changes in R_P can occur with modest changes in R_T are potential management loci within a drainage basin.     

Primary production in a tropical reservoir in Sri Lanka*

Primary production was measured every 2 weeks during 16 months (N = 33) in Tissawewa, a tropical shallow reservoir in the lowlands of south-east Sri Lanka. Results are interpreted in relation to selected environmental conditions such as oxygen concentrations, water temperature, Secchi-disc depth, wind force, conductivity, and morphoedaphic index and water level fluctuations. Because of regularly reoccurring high wind speeds the water column is well mixed. Daily gross primary production per unit area was plotted as a function of the algal biomass per unit area over the euphotic zone. Chlorophyll-a concentration in the euphotic zone was taken as measure for the algal biomass. The literature comparisons showed that the primary productivity in Tissawewa was in the same range as in 29 tropical lakes and reservoirs, of which 27 were from Africa. The productivity of these 30 tropical lakes and reservoirs was compared with: (a) 27 lakes of which 25 were temperate lakes, and (b) 49 North American temperate lakes. Firstly, comparisons were made on an annual basis for the tropical water bodies, but restricted to May–September, the growing season, for temperate water bodies. The gross primary productivity of tropical water bodies was ca. three times higher than that of temperate water bodies. These differences were even more dramatic if the two geographical regions are compared on an annual basis, i.e. the tropical systems are ca. six times more productive than their temperate counterpart.     

Spatial and temporal variation of phytoplankton in two subtropical Brazilian reservoirs

The purpose of this study was to verify the longitudinal distribution of phytoplankton biomass in two subtropical Brazilian reservoirs in the State of Paraná and investigate intervening factors on changes in phytoplankton biomass according to functional groups. In the Capivari and Segredo reservoirs, samples were obtained every 3 months during 2002, along a longitudinal axis (fluvial, transition, and lacustrine zones) at different depths. One hundred and eighteen taxa were identified, with Chlorophyceae as the most specious group. During the study period, both reservoirs had mostly low biomass values (less than 1 mm³ l⁻¹). The short retention time of these reservoirs constituted the principal limiting factor to phytoplankton development. Biomass values above 1 mm³ l⁻¹ were observed in the Capivari fluvial zone in March and in the Segredo lacustrine zone in December, with dominance by Microcystis aeruginosa Kütz (L_M) and Anabaena circinalis Rab. (H₁), respectively. Vertical and horizontal gradients of analyzed abiotic variables and phytoplankton biomass were observed. Considering the phytoplankton biomass values, both reservoirs were oligotrophic for the duration of the study. The Canonical Correspondence Analysis (CCA) evidenced temporal and spatial gradients of phytoplankton biomass; nevertheless, it did not follow the classic model proposed for deep reservoirs, since higher biomass was registered in the lacustrine zone during some months and in fluvial zones during other months. Distinct functional groups of phytoplankton characterized both studied reservoirs. Capivari Reservoir was best characterized by L_M and Y groups, indicative of its greater water column stability and higher phosphorus concentration, whereas Segredo Reservoir was principally characterized by the MP functional group, indicative of its greater mixing zone extension and higher nitrate concentration. The obtained results also evidenced the influence of morphometric conditions and watershed purposes as important structuring factors of phytoplankton biomass in these reservoirs. Handling editor: L. Naselli-Flores     

Effect of Thermal Effluents and Retention Time on Lake Functioning and Ecological Efficiencies in Plankton Communities

The following changes were observed in three lakes as a result of their heating and decrease of retention time (data for summer periods): increase of mixing rate, active bottom area, and nutrients (P_inorg., N_inorg.) concentrations; significant changes in gross primary production, phytoplankton biomass and production of filtering cladocerans. These changes seem to indicate the higher efficiency of utilization of autochthonous trophic resources and energy transfer in plankton.     

Long‐term phytoplankton community changes in a deep subalpine lake: responses to nutrient availability and climatic fluctuations

1. In natural lakes, modifications in the species composition and abundance of phytoplankton communities may ultimately be responses to changes in nutrient availability and climatic fluctuations. Phytoplankton and associated environmental factors were collected at monthly intervals from the beginning of the 1990s to 2007 in the large subalpine Lake Garda (z_max = 350 m, V = 49 × 10⁹ m³). In this study period, the lake showed a slight and continuous increase of total phosphorus (TP) in the water column, up to concentrations of 18–20 μg P L^?1. This increase represented the last stage of a long‐term process of enrichment documented since the 1970s, when concentrations of TP were below or around 10 μg P L^?1. 2. At the community level, annual phytoplankton cycles underwent a unidirectional and slow shift mainly due to changes in the species more affected by the nutrient enrichment of the lake. After a first and long period of dominance by conjugatophytes (Mougeotia) and diatoms (Fragilaria), phytoplankton biomass in recent years was sustained by cyanobacteria (Planktothrix). Other important modifications in the development of phytoplankton were superimposed on this pattern due to the effects of annual climate fluctuations principally mediated by the deep mixing events at spring overturn and, secondarily, by temperature and thermal stability of the water column during the growing season. 3. Interannual variations in the stability and temperature of the water column appeared to influence the development of a few subdominant flagellates (dinophytes and cryptophytes). Nevertheless, the major impact of climate on phytoplankton was indirect, and mediated through the effects of winter climatic conditions on deep mixing dynamics. Winter climatic fluctuations proved to be a key element in a linked chain of causal factors including cooling of hypolimnetic waters, deep vertical mixing and epilimnetic nutrient replenishment. The process of fertilisation was measurable both for TP and dissolved inorganic nitrogen, although only the first had a large effect, reinforcing the seasonal growth of a few dominant groups. The degree of nutrient replenishment further increased the spring development of large diatoms and the increase of Planktothrix in summer and autumn. 4. Currently, changes in nutrient concentrations have the greatest effect on the phytoplankton community, while direct effects due to the interannual variations in the thermal regime are of secondary importance compared with the indirect effects mediated through deep water mixing and spring fertilisation. Overall, the results demonstrate that the consequences of climatic fluctuations and climate warming on phytoplankton communities need to be studied at different levels of complexity and integration, from the direct effects of temperature and thermal regime, to the indirect effects mediated by the physiographic characteristics of water bodies.     

The dark side of the hyporheic zone: depth profiles of nitrogen and its processing in stream sediments

1. Although it is well known that sediments can be hot spots for nitrogen transformation in streams, many previous studies have confined measurements of denitrification and nitrate retention to shallow sediments (<5 cm deep). We determined the extent of nitrate processing in deeper sediments of a sand plains stream (Emmons Creek) by measuring denitrification in core sections to a depth of 25 cm and by assessing vertical nitrate profiles, with peepers and piezometers, to a depth of 70 cm. 2. Denitrification rates of sediment slurries based on acetylene block were higher in shallower core sections. However, core sections deeper than 5 cm accounted for 68% of the mean depth‐integrated denitrification rate. 3. Vertical hydraulic gradient and vertical profiles of pore water chloride concentration suggested that deep ground water upwelled through shallow sediments before discharging to the stream channel. The results of a two‐source mixing model based on chloride concentrations suggested that the hyporheic zone was very shallow (<5 cm) in Emmons Creek. 4. Vertical profiles showed that nitrate concentration in shallow ground water was about 10–60% of the nitrate concentration of deep ground water. The mean nitrate concentrations of deep and shallow ground water were 2.17 and 0.73 mg NO₃‐N L^?1, respectively. 5. Deep ground water tended to be oxic (6.9 mg O₂ L^?1) but approached anoxia (0.8 mg O₂ L^?1) after passing through shallow, organic carbon‐rich sediments, which suggests that the decline in the nitrate concentrations of upwelling ground water was because of denitrification. 6. Collectively, our results suggest that there is substantial nitrate removal occurring in deep sediments, below the hyporheic zone, in Emmons Creek. Our findings suggest that not accounting for nitrate removal in deep sediments could lead to underestimates of nitrogen processing in streams and catchments.     

Nitrous oxide emissions from inland waters: Are IPCC estimates too high?

Nitrous oxide (N₂O) emissions from inland waters remain a major source of uncertainty in global greenhouse gas budgets. N₂O emissions are typically estimated using emission factors (EFs), defined as the proportion of the terrestrial nitrogen (N) load to a water body that is emitted as N₂O to the atmosphere. The Intergovernmental Panel on Climate Change (IPCC) has proposed EFs of 0.25% and 0.75%, though studies have suggested that both these values are either too high or too low. In this work, we develop a mechanistic modeling approach to explicitly predict N₂O production and emissions via nitrification and denitrification in rivers, reservoirs and estuaries. In particular, we introduce a water residence time dependence, which kinetically limits the extent of denitrification and nitrification in water bodies. We revise existing spatially explicit estimates of N loads to inland waters to predict both lumped watershed and half‐degree grid cell emissions and EFs worldwide, as well as the proportions of these emissions that originate from denitrification and nitrification. We estimate global inland water N₂O emissions of 10.6–19.8 Gmol N year^?1 (148–277 Gg N year^?1), with reservoirs producing most N₂O per unit area. Our results indicate that IPCC EFs are likely overestimated by up to an order of magnitude, and that achieving the magnitude of the IPCC's EFs is kinetically improbable in most river systems. Denitrification represents the major pathway of N₂O production in river systems, whereas nitrification dominates production in reservoirs and estuaries.     

Lakes as nitrous oxide sources in the boreal landscape

Estimates of regional and global freshwater N₂O emissions have remained inaccurate due to scarce data and complexity of the multiple processes driving N₂O fluxes the focus predominantly being on summer time measurements from emission hot spots, agricultural streams. Here, we present four‐season data of N₂O concentrations in the water columns of randomly selected boreal lakes covering a large variation in latitude, lake type, area, depth, water chemistry, and land use cover. Nitrate was the key driver for N₂O dynamics, explaining as much as 78% of the variation of the seasonal mean N₂O concentrations across all lakes. Nitrate concentrations varied among seasons being highest in winter and lowest in summer. Of the surface water samples, 71% were oversaturated with N₂O relative to the atmosphere. Largest oversaturation was measured in winter and lowest in summer stressing the importance to include full year N₂O measurements in annual emission estimates. Including winter data resulted in fourfold annual N₂O emission estimates compared to summer only measurements. Nutrient‐rich calcareous and large humic lakes had the highest annual N₂O emissions. Our emission estimates for Finnish and boreal lakes are 0.6 and 29 Gg N₂O‐N/year, respectively. The global warming potential of N₂O from lakes cannot be neglected in the boreal landscape, being 35% of that of diffusive CH₄ emission in Finnish lakes.     

Long‐term limnological changes in the Ecuadorian páramo: Comparing the ecological responses to climate warming of shallow waterbodies versus deep lakes

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Nitrous oxide in brackish Lake Nakaumi, Japan II: the role of nitrification and denitrification in N2O accumulation

In order to understand the role of nitrification and denitrification in the accumulation of nitrous oxide (N₂O) in the hypolimnetic water of brackish Lake Nakaumi, the effects of dissolved oxygen (DO) concentration on these activities were investigated by incubation experiments. N₂O was produced during the oxidation of NH₄ ⁺ to NO₂ ⁻ in nitrification and during the reduction of NO₃ ⁻ to N₂ in denitrification. N₂O-producing activity by nitrification (N₂O_N) increased markedly with decreasing concentrations of DO. Low DO (10%–30% saturation) induced high N₂O_N. In contrast to nitrification, N₂O-producing activity by denitrification (N₂O_D) decreased with decreasing concentrations of DO. Little N₂O was accumulated during denitrification under low-level conditions of DO (10%–30%), because of further reduction of N₂O to N₂. It can therefore be assumed that N₂O produced as the by-product of nitrification is concurrently reduced to N₂ by denitrification under low-DO conditions. This would result in no substantial accumulation of N₂O during active nitrification in the hypolimnetic water of Lake Nakaumi. Received: July 6, 2001 / Accepted: December 10, 2001     

A Proposed Framework for the Identification of Habitat Utilisation Patterns of Macrophytes in River Po Catchment Basin Lakes (Italy)

The main aim of this study, which is based on cover-abundance values of 20 species of floating-leaved and submersed macrophytes collected in 18 lakes located within the River Po catchment basin (Italy), is to investigate the relationships between lake-scale environmental features (including morphology, hydrology, trophic state and water quality variables) and the changes in species composition and species richness in macrophyte communities. The findings reveal that the main changes in community composition of the pondweed communities, identified by principal component analysis, could be effectively explained by a newly introduced morpho-hydrological parameter (the theoretical laminar water renewal rate – R _L) and by the trophic state of the lakes, expressed as Carlson’s trophic state index (TSI). The results of the multivariate analyses performed also show that the grouping of species is strictly related to growth-form traits. In particular, floating-leaved and submerged macrophytes appeared mutually exclusive in dominating different habitat types defined on the basis of the R _L–TSI framework. An increasing trend in species richness was also observed in relation to R _L. The possibility of using the bivariate R _L–TSI framework for the identification of habitat utilisation patterns by single species was also investigated, and cover-abundance prediction maps based on the R _L–TSI framework were produced for several macrophyte species. Observations in another system characterised by high spatial heterogeneity for hydrological and trophic conditions, support the predictions in terms of growth-form prevalence, species richness and single species cover-abundance (where available), and also suggest a more refined application of the proposed approach.     

Sources and sinks of nitrous oxide (N2O) in deep lakes

As reported from marine systems, we found that also in15 prealpine lakes N₂O concentrations werestrongly correlated with O₂ concentrations. Inoxic waters below the mixed surface layer, N₂Oconcentrations usually increased with decreasingO₂ concentrations. N₂O is produced in oxicepilimnia, in oxic hypolimnia and at oxic-anoxicboundaries, either in the water or at the sediment-waterinterface. It is consumed, however, incompletely anoxic layers. Anoxic water layers weretherefore N₂O undersaturated. All studied lakeswere sources for atmospheric N₂O, including thosewith anoxic, N₂O undersaturated hypolimnia.However, compared to agriculture, lakes seem not tocontribute significantly to atmospheric N₂Oemissions.