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Abstract

Knowledge about dissolved oxygen (DO) and carbon dioxide (CO2) distribution in lakes has increased considerably over the last decades. However, studies about high resolution dynamics of dissolved CO2 in different types of lakes over daily or weekly time scales are still very scarce. We measured summertime vertical DO and CO2 profiles at sub-hourly intervals during one week in eight Estonian lakes representing different lake types according to European Water Framework Directive. The lakes showed considerable differences in thermal stratification and vertical distribution of dissolved oxygen and CO2 as well as different diurnal dynamics over the measurement period. We observed a continuous CO2 supersaturation in the upper mixed layer of the alkalitrophic (calcareous groundwater-fed) lake and the dark soft-water lake showing them as CO2 emitting "chimneys" although with different underlying mechanisms. In three lake types strong undersaturation with CO2 occurred in the surface layer characterising them as CO2 sinks for the measurement period while in another three types the surface layer CO2 was mostly in equilibrium with the atmosphere. Factor analysis showed that DO% in the surface layer and the strength of its relationship with CO2% were positively related to alkalinity and negatively to trophic state and DOC gradients, whereas deeper lakes were characterised by higher surface concentration but smaller spatial and temporal variability of CO2. Multiple regression analysis revealed lake area, maximum depth and the light attenuation coefficient as variables affecting the largest number of gas regime indicators. We conclude that the trophic status of lakes in combination with type specific features such as morphometry, alkalinity and colour (DOC) determines the distribution and dynamics of dissolved CO2 and DO, which therefore may indicate functional differences in carbon cycling among lakes.

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... Similar summer clinograde profiles of gas concentrations were observed for the stratified lakes of our study. They were all characterised by an increase in [CH 4 ] aq and [CO 2 ] aq in the hypolimnion which was comparable to that reported in many studies (Rudd et al. 1976;Bastviken et al. 2003;Kortelainen et al. 2006;Miettinen et al. 2015;Rinta et al. 2015;Laas et al. 2016;Thalasso et al. 2020). In this water layer, gas concentrations ranged from 6.2 to 49.2 µmol/L for CH 4 and from 122.7 to 454.9 µmol/L for CO 2 , depending on the lake. ...
... Indeed, air water equilibrium concentration range between 0.002 to 0.004 µmol/L for CH 4 and 13 to 17 µmol/L for CO 2 depending on water temperature (according to solubility equations by Weiss (1974) and Wiesenburg and Guinasso (1979) and by considering atmospheric concentrations of 407 ppm of CO 2 and 1850 ppb of CH 4 (www.eea.europa.eu)). The strong summer hypolimnetic anoxia and the negative relationship between O 2 and CO 2 concentrations observed during the stratification period indicated sustained heterotrophic activities (Rantakari and Kortelainen 2005;Laas et al. 2016). The high concentration of CH 4 found in the hypolimnion of our lakes at the same time can be explain by the degradation of organic matter carried out by methanogenic Archea under such anoxic conditions (Rudd and Hamilton 1975;Rudd et al. 1976;Utsumi et al. 1998). ...
... In the 9 stratified lakes, although lower than in hypolimnion, epilimnetic [CO 2 ] aq were close or higher than air-water equilibrium. These results are in accordance with previous studies that evidenced sursaturation of CO 2 in the surface layer of stratified lakes (Laas et al. 2016;Pighini et al. 2018), reflecting high heterotrophic activities in the epilimnion during summer stratification. However, our results about epilimnetic CO 2 concentrations should be considered with caution, as they result from a balance between respiration and CO 2 fixation by primary producers, and are consequently possibly subject to large temporal variability ). ...
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Intensification of anthropogenic activities in many lake catchments during the twentieth century led to increased autochthonous organic matter sedimentation and degradation of hypolimnetic oxygen conditions due to the intensification of heterotrophic processes. These processes can be amplified by the effect of climate warming on thermal stratification in lakes. This study aimed to assess how metabolic disruptions affect carbon sources and pathways in lake pelagic food webs, focusing on methanogenic carbon. The studied lakes showed strong seasonal variations of carbon source availability and transfers to pelagic food webs, characterized by increased methanogenic carbon transfers to Daphnia populations in winter. The magnitude of these winter transfers seems to largely depend on the amount of methane stored in the hypolimnion during the stratification period, and thus on the amount of methane released with autumnal turnover. Methane production, storage and transfer mechanisms partly depend on thermal stratification intensity, but also on external factors such as land use. This study provides new insights into the impacts of global changes on the sources and pathways of carbon in pelagic food webs through their influence on lake metabolism and thermal regimes. These functional changes may lead to greater production and release of greenhouse gases into the atmosphere.
... In the deepest lake (StratMedalk), a chain of 12 HOBO Pendant temperature loggers was used reaching from 0.5 to 20 m depth. Detailed information on high frequency data collection is given in Laas et al., 2016. Automated stations were placed near the GHG flux measurement points. ...
... As expected, the correlation between DCO 2 in the surface layer and CO 2 fluxes was very strong (Table S2), suggesting DCO 2 as a very good predictor for the flux. Laas et al. (2016) has demonstrated clear negative correlation between the gradient of DCO 2 and the trophic state of the lake. Thus, DCO 2 and the associated emission of CO 2 in the studied lakes were likely affected by an interaction of many factors: the carbonate content of water (e.g. ...
... The studied Alk lake was intense CO 2 and moderate CH 4 emitter. As described in Laas et al. (2016), the main feature that makes Alk distinct is the predominant ground water feeding of this lake type while the watershed is located in carbonate-rich karstic area. Carbonate-rich ground water (> 300 mg L −1 of HCO 3 − , Information System of Environmental Monitoring) releases CO 2 via intense calcite precipitation (Marcé et al., 2015;Laas et al., 2016) that forms the most likely source of the outstanding CO 2 emissions in Alk which were comparable to the average emission of 6703 mg m −2 d −1 from an 1-year-old reservoir in the boreal Canada (Tadonléké et al., 2012) and surpassed 3-to-30-fold the average emissions measured for the other 15 lakes and 7-35 years old reservoirs in that study, as well as the other reports for boreal lakes Yang et al., 2015). ...
Article
Lakes are considered important regulators of atmospheric greenhouse gases (GHG). We estimated late summer open water GHG fluxes in nine hemiboreal lakes in Estonia classified under different lake types according to the European Water Framework Directive (WFD). We also used the WFD typology to provide an improved estimate of the total GHG emission from all Estonian lakes with a gross surface area of 2204 km 2 representing 45,227 km 2 of hemiboreal landscapes (the territory of Estonia). The results demonstrate largely variable CO 2 fluxes among the lake types with most active emissions from Alkalitrophic (Alk), Stratified Alkalitrophic (StratAlk), Dark Soft and with predominant binding in Coastal, Very Large, and Light Soft lakes. The CO 2 fluxes correlated strongly with dissolved CO 2 saturation (DCO 2) values at the surface. Highest CH 4 emissions were measured from the Coastal lake type, followed by Light Soft, StratAlk and Alk types; Coastal, Light Soft and StratAlk were emitting CH 4 partly as bubbles. The only emitter of N 2 O was the Alk type. We measured weak binding of N 2 O in Dark Soft and Coastal lakes, while in all other studied lake types, the N 2 O fluxes were too small to be quantified. Diversely from the common viewpoint of lakes as net sources of both CO 2 and CH 4 , it turns out from our results that at least in late summer, Estonian lakes are net sinks of both CO 2 alone and the sum of CO 2 and CH 4. This is mainly caused by the predominant CO 2 sink function of Lake Peipsi forming ¾ of the total lake area and showing negative net emissions even after considering the Global Warming Potential (GWP) of other GHGs. Still, by converting CH 4 data into CO 2 equivalents, the combined emission of all Estonian lakes (8 T C day −1) is turned strongly positive: 2720 T CO 2 equivalents per day.
... Calcifying Chara species grew to depths of 5 m and consumed 25-30% of the DIC pool in the surface waters of Williams Lake during summer stratification for 11 consecutive years, while input from tributaries and dissolution of carbonate in sediments and vertical mixing restored the surface water DIC pool from autumn to spring [27]. In comparison, the carbonate pump worked on a diel scale in the charophyte-rich shallow lakes examined here, and it can be absent in permanently mixed lakes [30]. ...
... Polymictic lakes undergo mixing more than twice a year [26] and take a position between the daily recurring stratified-mixing lakes and the dimictic lakes. In polymictic lakes, stratification may last for hours [30], days [28,31], weeks [32] or months [32]. Polymictic lakes may therefore share some of the features described here, though being less prominent. ...
... Polymictic lakes may therefore share some of the features described here, though being less prominent. For example, the large (270 km 2 ) and shallow (6.0 m) Lake Võrtsjärv, Estonia, was usually fully mixed, but occasionally stratified for few hours during summer accompanied by hypoxia and CO 2 build-up in the bottom waters [30]. The charophyte-dominated Lost Pond, Minnesota (1.2 m) stratified for several days accompanied by bottom hypoxia [31]. ...
Article
A common perception in limnology is that shallow lakes are homogeneously mixed owing to their small water volume. However, this perception is largely gained by downscaling knowledge from large lakes to their smaller counterparts. Here we show that shallow vegetated lakes (less than 0.6 m), in fact, undergo recurring daytime stratification and nocturnal mixing accompanied by extreme chemical variations during summer. Dense submerged vegetation effectively attenuates light and turbulence generating separation between warm surface waters and much colder bottom waters. Photosynthesis in surface waters produces oxygen accumulation and CO2 depletion, whereas respiration in dark bottom waters causes anoxia and CO2 accumulation. High daytime pH in surface waters promotes precipitation of CaCO3 which is re-dissolved in bottom waters. Nocturnal convective mixing re-introduces oxygen into bottom waters for aerobic respiration and regenerated inorganic carbon into surface waters, which supports intense photosynthesis. Our results reconfigure the basic understanding of local environmental gradients in shallow lakes, one of the most abundant freshwater habitats globally. © 2017 The Author(s) Published by the Royal Society. All rights reserved.
... However, all of the studied lakes were highly productive, according to nutrient (nitrogen and phosphorus) concentrations and chlorophyll-a values (Table A1). Each of these lakes has been extensively studied and described in detail elsewhere [31][32][33][34]. ...
... The time series for the measured parameters in the Estonian lakes are presented in the Appendix figures (Appendix Figures A2-A4). Measured water temperature and air pressure data were used to calculate the real CO2 partial pressure (pCO2) and dissolved CO2 concentration from the signals captured by the sensors, according to the manufacturer manuals [31]. ...
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Pelagic calcification shapes the carbon budget of lakes and the sensitivity of dissolved inorganic carbon (DIC) responses to lake metabolism. This process, being tightly linked to primary production, needs to be understood within the context of summer eutrophication which is increasing due to human stressors and global change. Most lake carbon budget models do not account for calcification because the conditions necessary for its occurrence are not well constrained. This study aims at identifying ratios between calcification and primary production and the drivers that control these ratios in freshwater. Using in situ incubations in several European freshwater lakes, we identify a strong relationship between calcite saturation and the ratio between calcification and net ecosystem production (NEP) (p-value < 0.001, R2 = 0.95). NEP-induced calcification is a short-term process that is potentiated by the increase in calcite saturation occurring at longer time scales, usually reaching the highest levels in summer. The resulting summer calcification event has effects on the DIC equilibria, causing deviations from the metabolic 1:1 stoichiometry between DIC and dissolved oxygen (DO). The strong dependency of the ratio between NEP and calcification on calcite saturation can be used to develop a suitable parameterization to account for calcification in lake carbon budgets.
... Lakes with similar characteristics and settings are common throughout the world, and the temperature and stratification-mixing dynamics described here are likely to represent a very large number of small lakes worldwide (Downing, 2010). Our study differs from most other studies in that we investigated nine lakes simultaneously over a full year as opposed to studies of single lakes and/or shorter durations (Branco & Torgersen, 2009;Laas et al., 2016;Andersen et al., 2017b). Our approach allowed for statistical empirical evaluation with focus on occurrence and drivers of stratification in small lakes, as an alternative to mechanistic physical modeling. ...
... This finding was in contrast to previous studies on small, shallow lakes dominated by charophytes in which the formation of a strong thermocline was followed by oxygen supersaturation in surface waters and anoxia in the shaded bottom waters (Andersen et al., 2017a;Martinsen et al., 2017). Similar patterns have been observed in turbid lakes (Laas et al., 2016). Bottom anoxia is likely a result of the interplay between stratification and steep light attenuation through the water column. ...
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Small lakes are understudied compared to medium- and large-sized lakes, but have recently received increased attention due to their abundance and importance for global scale biogeochemical cycles. They have close terrestrial contact, extensive environmental variability, and support high biodiversity among them. Temporal and spatial variability of water temperature, oxygen, and stratification–mixing dynamics were examined during a year in nine small Danish lakes. We found that diel mean surface water temperatures were similar among lakes while the diel range decreased with increasing water depth. Vertical temperature stratification occurred on 47% of the days during the entire year and 64% of summer days, usually with daytime stratification and nocturnal convective mixing. The probability of daytime stratification increased with higher incident irradiance, higher air temperature, and lower wind speed. During spring, daytime stratification caused differences in oxygen saturation between surface and bottom waters. These findings offer new insights on the high variability of water temperature and oxygen in time and space in small temperate shallow lakes. The variable water temperature and the regular stratification–mixing processes will have a pronounced influence on biogeochemical cycles. Also, these features are expected to affect the performance and evolutionary process of organisms associated with small lakes.
... According to the WFD system of classification, the lake has been classified predominantly as having a moderate status. The annual mean total phosphorus concentration is around 40 µg L −1 and Chl-a about 35.7 mg L −1 [59]. Cyanobacteria and diatoms dominate the algal groups, whereas green algae, cryptophytes, and dinoflagellates are scarce. ...
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The frequency of heatwave events in Europe is increasing as a result of climate change. This can have implications for the water quality and ecological functioning of aquatic systems. We deployed three spectroradiometer WISPstations at three sites in Europe (Italy, Estonia, and Lithuania/Russia) to measure chlorophyll-a at high frequency. A heatwave in July 2019 occurred with record daily maximum temperatures over 40 °C in parts of Europe. The effects of the resulting storm that ended the heatwave were more discernable than the heatwave itself. Following the storm, chlorophyll-a concentrations increased markedly in two of the lakes and remained high for the duration of the summer while at one site concentrations increased linearly. Heatwaves and subsequent storms appeared to play an important role in structuring the phenology of the primary producers, with wider implications for lake functioning. Chlorophyll-a peaked in early September, after which a wind event dissipated concentrations until calmer conditions returned. Synoptic coordinated high frequency monitoring needs to be advanced in Europe as part of water management policy and to improve knowledge on the implications of climate change. Lakes, as dynamic ecosystems with fast moving species-succession, provide a prism to observe the scale of future change.
... First, log(O2) appeared as the main explanatory positively correlated variable in the lasso regression for log(BdgT) (Figure 8). High oxygen concentrations are common in the epilimnion of autotrophic lakes where the primary production releases dissolved oxygen in the layer of photosynthesis active radiation (PAR) [55]. As the samples in this study were collected from the surface, elevated O2 concentration likely indicates high primary production in the raw water. ...
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Dissolved Natural Organic Matter (DNOM) is a heterogeneous mixture of partly degraded, oxidised and resynthesised organic compounds of terrestrial or aquatic origin. In the boreal biome, it plays a central role in element cycling and practically all biogeochemical processes governing the physico-chemistry of surface waters. Because it plays a central role in multiple aquatic processes, especially microbial respiration, an improved understanding of the biodegradability of the DNOM in surface water is needed. Here the current study, we used a relatively cheap and non-laborious analytical method to determine the biodegradability of DNOM, based on the rate and the time lapse at which it is decomposed. This was achieved by monitoring the rate of oxygen consumption during incubation with addition of nutrients. A synoptic method study, using a set of lake water samples from southeast Norway, showed that the maximum respiration rate (RR) and the normalised RR (respiration rate per unit of carbon) of the DNOM in the lakes varied significantly. This RR is conceived as a proxy for the biodegradability of the DNOM. The sUVa of the DNOM and the C:N ratio were the main predictors of the RR. This implies that the biodegradability of DNOM in these predominantly oligotrophic and dystrophic lake waters was mainly governed by their molecular size and aromaticity, in addition to its C:N ratio in the same manner as found for soil organic matter. The normalised RR (independently of the overall concentration of DOC) was predicted by the molecular weight and by the origin of the organic matter. The duration of the first phase of rapid biodegradation of the DNOM (BdgT) was found to be higher in lakes with a mixture of autochthonous and allochthonous DNOM, in addition to the amount of biodegradable DNOM.
... High-resolution pCO 2 datasets have been used to explore lake and reservoir C processing dynamics in boreal (Laas et al. 2016, Denfeld et al. 2018), temperate (Morales-Pineda et al. 2014, and tropical (Junger et al. 2019) climates. To the best of our knowledge, no similar studies have been published for peatland lakes within temperate maritime climate zones. ...
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Humic lakes play a key role in the processing of organic carbon (OC) mobilised from their catchments, but knowledge of OC dynamics in lakes within maritime temperate climates is limited. Climate exerts a significant influence on mechanisms of OC capture, storage, and processing on the wet and cloudy west coast of Ireland. We examined a high-frequency dataset of partial pressure of CO2 (pCO2) in the surface waters of Lough Feeagh collected over 1 year. The annual pattern in pCO2 ranged between 491 and 1169 µatm and was strongly related to allochthonous riverine OC inputs. In contrast to observations in colder climates, a single peak in pCO2 occurred in Lough Feeagh in early September. Generalised additive mixed modelling revealed that 2 variables, inflow water colour concentration (a reliable proxy for DOC concentrations) and lake Schmidt stability, together explained 68% of pCO2 variability. Both the statistical analysis and timing of the peaks in inflow DOC and pCO2 strongly suggested that catchment carbon export drove pCO2 supersaturation in the lake, and hence CO2 emissions. We estimated that between 217 and 370 t CO2-C (0.55–0.94 t/ha) was emitted during the study period. These results highlight the interplay between catchment OC fluxes and climate in determining pCO2 dynamics in maritime temperate lakes.
... Bevelhimer et al. [20] quantified GHG emissions from six hydropower reservoirs in the southwestern United States via selected emission mechanisms, namely diffusive emissions, ebullition, and tailwater degassing, and found diffusive emissions to be dominant. Under increased rainfall, flooding may transport large amounts of organic matter into the artificial reservoir from the upper watershed; this organic matter is accumulated in the lake and decomposed to emit final products such as gaseous CO 2 and CH 4 , which maintain the CO 2 supersaturation in the hypolimnion [21,22]. ...
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Temporal and spatial variations of temperature and dissolved oxygen were measured in Triangle Lake in Oregon. The lake was relatively uniform in the horizontal direction and significantly stratified vertically. Diel dissolved oxygen variations were less than 0. 5 mg/l in surface regions. Diel temperature variation resulted in a nocturnal density overturning extending to about 9 ft in depth.
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A series of vertical profiles of dissolved oxygen (DO) collected periodically over two consecutive ice-free seasons in an oligotrophic high-elevation lake (Emerald Lake, California) were used to investigate volumetric and areal rates of gross primary production (GPP), community respiration (CR), and net ecosystem production (NEP). Diel patterns in DO did not weaken with depth in this lake, where the entire 10-m water column was within the euphotic zone and where a deep chlorophyll a (Chl a) maximum was common during periods of thermal stratification. During stratification, both GPP and CR increased with depth, and heterotrophy (NEP < 0) tended to occur below the thermocline in association with higher Chl a and particulate matter concentrations. With the onset of autumn mixing each year, vertical gradients in metabolism weakened or disappeared and the entire water column was autotrophic. Net autotrophy over the growing season was confirmed using three methods of estimating whole-lake metabolism. During periods of stratification, flux across the thermocline, where eddy diffusivities were near molecular, was small (4% of total epilimnetic fluxes), while within the hypolimnion, where stratification was weaker and eddy diffusivities larger, fluxes between strata were more substantial (12% of total fluxes). For this lake and other small lakes with low wind speeds and Lake numbers near 10, mixing due to turbulence should be included in computations of metabolism within the hypolimnion. However, single-station measurements from within the epilimnion provide a reasonable estimate of seasonal metabolism, especially in the autumn when the lake is mixing on a diel basis.
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Recent advances in open-water measurements suggest significant temporal and spatial variability of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) with implications for understanding carbon cycling in lakes. This study applied high-frequency depth profiles in three stratified lakes of different trophic status to investigate (1) the importance of vertical variations in metabolic rates, (2) the effects of changes in the depth of the mixed layer (Zmix) and the photic zone (Zeu), and (3) the photoacclimative responses of the aquatic autotrophs to changes in these conditions. Taking account of vertical differences in metabolism improved the reliability of whole-areal NEP estimates during stratification. Whereas the hypolimnion was always heterotrophic, and the epilimnion was mostly autotrophic, the metalimnion had NEP > 0 when Zeu > Zmix. Although most of GPP and R occurred in the epilimnion, between 0% and 20% of GPP and 4% and 37% of R took place in the metalimnion. Areal metabolic estimates based on surface measurements deviated up to 60% for GPP and 80% for R when Zeu > Zmix. The vertical variability in metabolism was driven by available light in both the epi- and metalimnion. Coupling between GPP and R was low in all layers and indicated increasing background R with depth. Light utilization efficiency was significantly higher under low light conditions, indicating photophysiological acclimation of phytoplankton to decreasing light in the metalimnion.
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Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67 mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.
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Carbon dioxide (CO2) and methane (CH4) concentrations and evasion rates were measured in surface waters draining Mer Bleue peatland (Ontario, Canada) between spring and autumn 2005. All sites exhibit a consistent pattern of supersaturation throughout the year, which is broadly related to hydrological and temperature changes between spring snowmelt and autumn freezing. Both measurements and estimates of CO2 and CH4 evasion from open water to the atmosphere suggest that parts of the catchment (including beaver dams) are significant degassing hot spots. We present data showing how vertical gaseous carbon fluxes compare with lateral carbon fluxes and make an initial estimate of the importance to the overall carbon budget of CO2 and CH4 evasion to the atmosphere from water surfaces at Mer Bleue. Copyright © 2007 John Wiley & Sons, Ltd.
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Sensors for the partial pressure of CO2 (pCO2) and dissolved O2 (DO) were deployed near the surface and bottom of a freshwater lake (Placid Lake, Montana) during the period from ice cover to seasonal stratification. Sources of variability were examined using one-dimensional physical and biogeochemical models. Model predictions for pCO2 and DO were compared to further constrain model parameters. A number of transient processes were documented that have not been well characterized in previous studies. The models made it possible to link these short-term events to specific forcings. We found that (1) 11 d of the 13-d turnover period occurred under ice through light- driven convective mixing, (2) phytoplankton biomass increased to its highest seasonal level under ice, (3) weak stratification set up immediately after ice-out, causing bottom water pCO2 and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO2 and DO back toward surface levels, and (5) before stable stratification, vertical entrainment of CO2-rich water, net production, and air-water exchange drove 100-200 matm daily changes in pCO2, but, because of their counterbalancing effects, surface pCO2 remained .1,000 matm for nearly 1 month after ice-out. Upon stable stratification, net production and air-water exchange overcame pCO2 gains from mixing and heating and reduced pCO2 to near atmospheric levels within 20 d. Net production and gas exchange accounted for ;75% and 25%, respectively, of the decrease in surface pCO2 observed after ice-out. Diel convection was the dominant mixing process both under ice and after ice-out and may be an important underrep- resented mechanism for CO2 and DO exchange between surface and bottom water.
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We traced the origin of dissolved organic matter (DOM) in the large, shallow, eutrophic Lake Võrtsjärv in Estonia. Allochthonous DOM (Al-DOM) had higher d13C values than autochthonous DOM (Au-DOM). The d13C of inflow DOM varied from 228.2% to 225.4% (mean 226.7%) and in-lake DOM varied from 228.4% to 226.1% (mean 227.2%). Low stable isotope (SI) signatures of Au-DOM were caused by relatively 13C-depleted values of its precursors (mainly phytoplankton) with mean d13C of 228.9%. SI signatures of dissolved inorganic carbon (DIC) in the inflows and in the lake were also relatively low (from 215.1% to 23.28%). SI values of DOM were lower during the active growing season from May to September and higher from October to April, with the corresponding estimated average proportions of Al-DOM 68% and 81%. The proportion of Al-DOM decreased with increasing water temperature, chlorophyll a, and pH and increased with increasing water level and concentration of yellow substances and DIC. The high proportion of Al-DOM in Võrtsjärv shows that, even in this highly productive ecosystem, the labile Au-DOM produced is rapidly utilized and degraded by microorganisms and thus makes a relatively small contribution to the instantaneous in-lake DOM pool.
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Peatland streams potentially represent important conduits for the exchange of gaseous carbon between the ter- restrial ecosystem and the atmosphere. We investigated how gaseous evasion of carbon from the stream surface compared with downstream carbon transport at three locations on a Scottish headwater stream. Carbon dioxide was consistently above atmospheric saturation in the stream, with mean concentrations of 159.1, 81.8, and 29.5 mmol L 21 at the lower, middle, and upper sites, respectively (i.e., 7.6, 3.9, and 1.2 times in excess of atmospheric equilibrium concentrations). Methane concentrations in stream water were much lower but showed a similar pattern. Rates of gaseous evasion from the stream surface to the atmosphere, determined experimentally using direct mea- surement of dissolved gas concentrations in conjunction with coinjection of conservative solute and volatile gas tracers, also declined downstream. Combined stream losses of all forms of carbon from the entire catchment (i.e., degassing from the stream surface and exports downstream) totaled 54,140 kg C yr 21 . Evasion of carbon dioxide from the stream surface accounted for 34% of this total, compared to 57% lost as dissolved organic carbon via export downstream. When expressed per unit area of watershed, the gaseous C evasion from the stream represents a loss of 14.1 g C m 22 yr 21 , which equals 28-70% of the estimated net carbon accumulation rate for such peatlands. This study shows that gaseous carbon loss from the surface of temperate headwater streams can be both spatially variable and significant in terms of rates of net annual land surface-atmosphere exchange at the catchment scale.
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On the basis of data collected in Quebec lakes, del Giorgio and Peters (1994) and Carignan et al. (2000) have come to opposite conclusions regarding the metabolic balance between heterotrophy and autotrophy in lakes in general. In the present study, epilimnetic oxygen and carbon dioxide saturation was measured in 33 lakes from the St. Lawrence Lowlands region of Quebec to examine the extent of epilimnetic net heterotrophy (i.e., O2:CO2 balance) in lakes of different characteristics. We found that ∼ 75% of the lakes were under-saturated with oxygen and supersaturated with CO2. There was a strong negative relationship between the departures of O2 and CO2 from saturation. What has not been noted elsewhere is that oxygen concentrations were negatively related to dissolved organic carbon (DOC) concentration, and, therefore, metabolic gas balances could be predicted from DOC; a value between 4 and 6 mg L-1 DOC corresponds to metabolic equilibrium. Because most of the lakes in del Giorgio and Peters (1994) had DOC concentrations above this threshold and most lakes in Carignan et al. (2000) were below, their apparently contradictory conclusions can be reconciled within a larger general theory. Contrary to studies elsewhere, however, we found that the degree of oxygen undersaturation increased with lake trophic status, expressed either as total phosphorus or nitrogen concentrations.
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Respiratory CO 2 release from inland waters is a major process in the global carbon cycle, retaining more than half of the carbon flux from terrestrial sources that otherwise would reach the sea. The strongly lake type-specific balance between primary production and respiration determines whether a lake acts regionally as a net sink or source of CO 2 . This study presents two-year (2009, 2010) results of high-frequency metabolism measurements in the large and shallow polymictic eutrophic Lake Võrtsjärv (area 270 km 2 ; mean depth 2.8 m). We estimated the net ecosystem production (NEP), com-munity respiration (R) and gross primary production (GPP) from continuous measurements of oxygen, irradiance, wind and water temperature. A sinusoidal model fitted to the calculated metabolic rates showed the prevalence of net autotrophy (mean GPP:R [ 1) from early spring until August/September, whereas during the rest of the year heterotrophy (mean GPP:R \\ 1) prevailed, characterizing the lake as CO 2 neutral on an annual basis. Community respira-tion lagged behind GPP by approximately 2 weeks, which could be explained by the bulk of the phyto-plankton biomass accounted for by filamentous cya-nobacteria that are considered mostly inedible to zooplankton, and the seasonally increasing role of sediment resuspension. In the warmer year 2010, the seasonal peaks of GPP, R and NEP were synchro-nously shifted nearly 1 month earlier compared with 2009. The strong stimulating effect of temperature on both GPP and R and its negative effect on NEP revealed by the multiple regression analysis suggests increasing metabolic rates and increasing heterotro-phy in this lake type in a warmer climate.
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We show that sediment respiration is one of the key factors contributing to the high CO2 supersaturation in and evasion from Finnish lakes, and evidently also over large areas in the boreal landscape, where the majority of the lakes are small and shallow. A subpopulation of 177 randomly selected lakes (<100 km2) and 32 lakes with the highest total phosphorus (Ptot) concentrations in the Nordic Lake Survey (NLS) data base were sampled during four seasons and at four depths. Patterns of CO2 concentrations plotted against depth and time demonstrate strong CO2 accumulation in hypolimnetic waters during the stratification periods. The relationship between O2 departure from the saturation and CO2 departure from the saturation was strong in the entire data set (r2=0.79, n=2 740, P<0.0001). CO2 concentrations were positively associated with lake trophic state and the proportion of agricultural land in the catchment. In contrast, CO2 concentrations negatively correlated with the peatland percentage indicating that either input of easily degraded organic matter and/or nutrient load from agricultural land enhance degradation. The average lake-area-weighted annual CO2 evasion based on our 177 randomly selected lakes and all Finnish lakes >100 km2 (Rantakari & Kortelainen, 2005) was 42 g C m−2 LA (lake area), approximately 20% of the average annual C accumulation in Finnish forest soils and tree biomass (covering 51% of the total area of Finland) in the 1990s. Extrapolating our estimate from Finland to all lakes of the boreal region suggests a total annual CO2 evasion of about 50 TgC, a value upto 40% of current estimates for lakes of the entire globe, emphasizing the role of small boreal lakes as conduits for transferring terrestrially fixed C into the atmosphere.
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Photosynthetic activity and respiration in Lake Sonachi (Kenya), a meromictic soda lake lying in a volcanic crater, were measured through diel cycles during a 15-month period. A pattern of thermal stratification in the morning and mixing in the afternoon and night occurred in the mixolimnion. Diel variations in dissolved oxygen at 50 cm were 2.2–7.5 mgO2 1⁻¹1% of the incident photosynthetically available irradiance (PAR) reached a depth of 1.3–2.4 m and, as a consequence, the steepest thermal gradients and highest oxygen concentrations occurred in the top 1–2 m.
Article
We studied the interannual variation of surface water partial pressure of CO2 (pCO2) and the CO2 emissions from the 37 large Finnish lakes linking them to the water quality, catchment and climate attributes in 1996–2001. The lake water CO2 was measured three times a year in the study lakes in 1998 and 1999 and for the rest of the years the CO2 was modeled by measured alkalinity. The median annual CO2 emission to the atmosphere ranged between 1.49 and 2.29 mol m−2 a−1. The annual CO2 emission followed closely the annual precipitation pattern with the highest emission during the years when the precipitation was highest (r2=0.81–0.97, P<0.05). There was a strong negative correlation (r2=0.50–0.82, P<0.001) between O2 and CO2 saturation in the lake water during stratification suggesting effective decomposition of organic matter in the lakes. Furthermore, total phosphorus and the proportion of agricultural land in the catchment had significant positive correlations with CO2 saturation.
Article
Surface waters associated with peatlands, supersaturated with CO2 and CH4 with respect to the atmosphere, act as important pathways linking a large and potentially unstable global repository of C to the atmosphere. Understanding the drivers and mechanisms which control C release from peatland systems to the atmosphere will contribute to better management and modelling of terrestrial C pools. We used non-dispersive infra-red (NDIR) CO2 sensors to continuously measure gas concentrations in a beaver pond at Mer Bleue peatland (Canada); measurements were made between July and August 2007. Concentrations of CO2 in the surface water (10 cm) reached 13 mg C l−1 (epCO2 72), and 26 mg C l−1 (epCO2 133) at depth (60 cm). The study also showed large diurnal fluctuations in dissolved CO2 which ranged in amplitude from ∼1·6 mg C l−1 at 10 cm to ∼0·2 mg C l−1 at 60 cm depth. CH4 concentration and supersaturation (epCH4) measured using headspace analysis averaged 1·47 mg C l−1 and 3252, respectively; diurnal cycling was also evident in CH4 concentrations. Mean estimated evasion rates of CO2 and CH4 over the summer period were 44·92 ± 7·86 and 0·44 ± 0·25 µg C m−2s−1, respectively. Open water at Mer Bleue is a significant summer hotspot for greenhouse gas emissions within the catchment. Our results suggest that CO2 concentrations during the summer in beaver ponds at Mer Bleue are strongly influenced by biological processes within the water column involving aquatic plants and algae (in situ photosynthesis and respiration). In terms of carbon cycling, soil-stream connectivity at this time of year is therefore relatively weak. Copyright © 2008 John Wiley & Sons, Ltd.
Article
Understanding of the processes that control CO2 concentrations in the aquatic environment has been hampered by the absence of a direct method to make continuous measurements over both short- and long-term time intervals. We describe an in situ method in which a non-dispersive infrared (NDIR) sensor is enclosed in a water impermeable, gas permeable polytetrafluoroethylene (PTFE) membrane and deployed in a freshwater environment. This allows measurements of CO2 concentration to be made directly at a specific depth in the water column without the need for pumps or reagents. We demonstrate the potential of the method using examples from different aquatic environments characterized by a range of CO2 concentrations (0·5–8·0 mg CO2-C l−1, equivalent to ca 40–650 µmol CO2 l−1). These comprise streams and ponds from tropical, temperate and boreal regions. Data derived from the sensor was compared with direct measurements of CO2 concentrations using headspace analysis. Sensor performance following long-term (>6 months) field deployment conformed to manufacturers' specifications, with no drift detected. We conclude that the sensor-based method is a robust, accurate and responsive method, with a wide range of potential applications, particularly when combined with other in situ sensor-based measurements of related variables. Copyright © 2009 John Wiley & Sons, Ltd.
Article
Estimates of net primary production, community respiration (R'), and gross primary production (P g) are developed and presented for the productive layers of eutrophic Onondaga Lake, NY, U.S.A., for time scales ranging from diel to several months, based on 4 months of robotic diel profiles of dissolved oxygen (DO) and temperature. Metabolic rate calculations are made through application of a DO mass balance framework that also accommodates inputs and losses of DO mediated by exchange across the air–water interface and across the lower boundary of the productive layers. It is demonstrated that the dynamics of the flux across the air–water interface are important to the metabolic rate estimates, while vertical mixing-based losses to the underlying layers can be ignored. Study average estimates of R' (1.49 g O2 m–3 d–1) and P g (1.60 g O2 m–3 d–1) obtained by this non-isolated community approach are consistent with levels reported in the literature for similar chlorophyll a concentrations, based on isolated community (bottle experiment) protocols to measure these metabolic rates. The non-isolated community approach is shown to have limited utility for quantifying day-to-day changes in these rates in this lake, apparently because of horizontal exchange with waters of different DO concentrations. However, this approach may support reliable estimates of metabolic rates at intermediate time scales; e.g., several days to a week. The DO mass balance framework is demonstrated to be valuable in resolving the relative roles of various physical and biological processes in regulating the DO pool of the productive layers.
Article
The fluxes of CH4 and CO2 to the atmosphere, and the relative contributions of ebullition and molecular diffusion, were determined for a small hypertrophic freshwater lake (Priest Pot, UK) over the period May to October 1997. The average total flux of CH4 and CO2 (estimated from 7 sites on the lake) was approximately 52 mmol m–2 d–1 and was apportioned 12 and 40 mmol m–2 d–1 toCH4 and CO2 respectively. Diffusion across the air-water interface accounted for the loss of 0.4and 40 mmol m–2 d–1 of CH4 and CO2 respectively whilst the corresponding figures for ebullition losses were 12.0 (CH4) and 0.23 (CO2) mmol m–2 d–1. Most CH4 (96%) was lost by ebullition, and most CO2 (99%) by diffusive processes. The ebullition of gas, measured at weekly intervals along a transect of the lake, showed high spatial and temporal variation. The CH4 content of the trapped gas varied between 44 and 88% (by volume) and was highest at the deepest points. Pulses of gas ebullition were detected during periods of rapidly falling barometric pressure. Therelevance of the measurements to global estimates ofcarbon emission from freshwaters are discussed.
Article
Cultural eutrophication has become the primary water quality issue for most of the freshwater and coastal marine ecosystems in the world. However, despite extensive research during the past four to five decades, many key questions in eutrophication science remain unanswered. Much is yet to be understood concerning the interactions that can occur between nutrients and ecosystem stability: whether they are stable or not, alternate states pose important complexities for the management of aquatic resources. Evidence is also mounting rapidly that nutrients strongly influence the fate and effects of other non-nutrient contaminants, including pathogens. In addition, it will be important to resolve ongoing debates about the optimal design of nutrient loading controls as a water quality management strategy for estuarine and coastal marine ecosystems.
Article
A new design of equilibrator for carbon dioxide monitoring in natural waters is described. It consists in a vertical tube filled with marbles through which water is flowing while equilibrating with a closed air circuit. It offers several advantages compared with classical equilibrators, among which is a fast response time (half-life constant approximately 30 s) and the potential to work in very turbid water. The proposed equilibrator is of particular interest to monitor carbon dioxide in coastal ecosystems, such as estuaries, which are known to be turbid and highly dynamic. Two performance tests and some field results are presented to illustrate the efficiency of the proposed system.
Article
Genome sequencing projects are revealing new information about the distribution and evolution of photosynthesis and phototrophy. Although coverage of the five phyla containing photosynthetic prokaryotes (Chlorobi, Chloroflexi, Cyanobacteria, Proteobacteria and Firmicutes) is limited and uneven, genome sequences are (or soon will be) available for >100 strains from these phyla. Present knowledge of photosynthesis is almost exclusively based on data derived from cultivated species but metagenomic studies can reveal new organisms with novel combinations of photosynthetic and phototrophic components that have not yet been described. Metagenomics has already shown how the relatively simple phototrophy based upon rhodopsins has spread laterally throughout Archaea, Bacteria and eukaryotes. In this review, we present examples that reflect recent advances in phototroph biology as a result of insights from genome and metagenome sequencing.