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Summer depth distribution profiles of dissolved CO2 and O2 in shallow temperate lakes reveal trophic state and lake type specific differences
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.
... 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 ). ...
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). ...
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]. ...
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]. ...
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. ...
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. ...
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. ...
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. ...
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]. ...
This study explores the dynamic changes in the partial pressure of CO2 (pCO2) with depth, and the temporal variations of CO2 net atmospheric flux (NAF) in a stratified reservoir. A total of 16 field campaigns were conducted from the summer stratification to fall turnover period in 2017. A random forest (RF) model was developed to estimate the pCO2 using concurrently measured water quality variables. The results showed that the vertical distribution of pCO2 and associated temporal variations of the NAF are closely related to the stratification strength of the reservoir. The reservoir surface pCO2 was supersaturated (1542 µatm) in summer (July 11), but this decreased to undersaturation as algae grew. Meanwhile, dissolved CO2 continuously accumulated below the reservoir mixed-layer due to the thermal stratification barrier and organic-rich floodwater intrusion. Vertical mixing began instantly as the stratification strength began to weaken in mid-October, and the surface pCO2 increased sharply up to 1934 µatm. Consequently, the NAF drastically increased to 3235 mg−CO2 m−2·day−1, which implies that the NAF changes seasonally and large CO2 pulsing occurs during the turnover events. The results provide valuable information about pCO2 variability and physical mixing processes, as well as carbon budget estimation in stratified reservoirs, and offer an improved understanding of these phenomena.
The ratio of primary production to ecosystem respiration rates (P:R ratio) is an ostensibly simple calculation that is used to characterize lake function, including trophic status, the incorporation of terrestrial organic carbon into lacustrian food webs, and the direction of carbon dioxide (CO2) flux between a lake and the atmosphere. However, many predictive links between P:R ratios and lake ecosystem function stem from a historically plankton‐centric perspective and the common use of the diel oxygen curve approach. We review the evolution of the use of P:R ratios and examine common assumptions underlying their application to (1) eutrophication, (2) carbon flux through lake food webs, and (3) the role of lakes in the global carbon budget. Foundational P:R studies have been complicated principally by the following: most P:R ratios were calculated from mid‐lake measurements and failed to incorporate nonplanktonic dynamics; there has been confusion regarding the food web implications when P:R ≠ 1; and CO2 fluxes between lakes and the atmosphere are influenced by nonmetabolic processes. We argue for a re‐assessment, or shoring up, of several fundamental assumptions that continue to guide metabolism research in lakes by accounting for mixing, benthic‐littoral processes, groundwater fluxes, and abiotic controls on gas dynamics to better understand lake food webs and accurately integrate lake ecosystems into landscape‐scale carbon cycling models.
Several findings suggest that CO2 emissions in lakes are not always directly linked to changes in metabolism but can be associated with interactions with the dissolved inorganic carbon equilibrium. Alkalinity has been described as a determining factor in regulating the relative contributions of biological and inorganic processes to carbon dynamics in lakes. Here we analyzed the relationship between metabolic changes in dissolved oxygen (DO) and dissolved inorganic carbon (DIC) at different timescales in eight lakes covering a wide range in alkalinity. We used high-frequency data from automatic monitoring stations to explore the sensitivity of DIC to metabolic changes inferred from oxygen. To overcome the problem of noisy data, commonly found in high-frequency measurements datasets, we used Singular Spectrum Analysis to enhance the diel signal-to-noise ratio. Our results suggest that in most of the studied lakes, a large part of the measured variability in DO and DIC reflects non-metabolic processes. Furthermore, at low alkalinity, DIC dynamics appear to be mostly driven by aquatic metabolism, but this relationship weakens with increasing alkalinity. The observed deviations from the metabolic 1:1 stoichiometry between DO and DIC were strongly correlated with the deviations expected to occur from calcite precipitation, with a stronger correlation when accounting also for the benthic contribution of calcite precipitation. This highlights the role of calcite precipitation as an important driver of CO2 supersaturation in lakes with alkalinity above 1 meq L⁻¹, which represents 57% of the global area of lakes and reservoirs around the world.
To elucidate the patterns and characteristics of dissolved oxygen (DO) decline near bottom in shallow polymictic eutrophic lakes, we conducted a statistical analysis of monthly measurements for 12 years at 14 stations and hourly observation for 8 years at two stations in Lakes Kasumigaura and Kitaura. Results indicated that DO decline (hereafter, <2 mg/L) was negligible at stations with a depth of <5 m. Thus, we consider this depth to be critical depth for studying DO decline conditions in these lakes. The collected data revealed that DO decline events lasting more than 2 days occurred a few times a year. During these events, wind blew weakly (usually, <2 m/s), and diurnal stratification was formed by solar radiation. The averaged DO declining rate leading up to these events was 4.3 (±2.7) mg L−1 d−1, suggesting that within a few days, DO can fall into a declined state. The occurrence probability of DO declined events in the respective summer increased significantly with the proportion of weak wind hours (<2 m/s). Significant correlations between wind velocity and DO change rate (1 hr difference in DO) supported the importance of wind on DO changes in polymictic lakes. The influence of mean air temperature on DO decline events was insignificant.
Carbon dioxide (CO2) concentrations in lakes vary strongly over time. This variability is rarely captured by environmental monitoring, but is crucial for accurately assessing the magnitude of lake CO2 emissions. However, it is unknown to what extent temporal variability needs to be captured to understand important drivers of lake carbon cycling such as climate and land management. We used environmental monitoring data of Swedish forest lakes collected in autumn (n=439) and throughout the whole open‐water season (n=22) from a wet and a dry year to assess temporal variability in effects of climate and forestry on CO2 concentrations across lakes. Effects differed depending on the season and year sampled. According to cross‐lake comparisons based on autumn data, CO2 concentrations increased with annual mean air temperature (dry year) or catchment forest productivity (wet year) but were not related to colored dissolved organic matter (CDOM) concentrations. In contrast, open‐water season averaged CO2 concentrations were similar across temperature and productivity gradients but increased with CDOM. These contradictions resulted from scale mismatches in input data, lead to weak explanatory power (R2=9‐32%) and were consistent across published data from 79 temperate, boreal and arctic lakes. In a global survey of 144 published studies, we identified a trade‐off between temporal and spatial coverage of CO2 sampling. This trade‐off clearly determines which conclusions are drawn from landscape‐scale CO2 assessments. Accurate evaluations of the effects of climate and land management require spatially and temporally representative data that can be provided by emerging sensor technologies and forms of collaborative sampling.
We assessed the allochthonous organic carbon (OC) budgets for thirteen hemiboreal lakes using a simple equilibrium model coupled with a Bayesian framework for estimating parameter distribution and uncertainty. Model inputs consisted of hydrological, bathymetric and chemical data that are easily measurable at the lake and basin scale. Among the model outputs were mean OC loads (5–123 g m⁻² y⁻¹), exports (1.10⁻³–108 g m⁻² y⁻¹), mineralization (3–12 g m⁻² y⁻¹), and sedimentation (2–6 g m⁻² y⁻¹). “Active” lake-catchment systems received and emitted the largest amounts of allochthonous OC, whereas lakes depending mostly on atmospheric inputs exhibited much more modest OC fluxes. Simulated organic carbon retention varied accordingly from 12% in some drainage lakes to 99% in seepage lakes. Lake allochthonous OC loads and exports were strongly correlated to drainage ratio (catchment area/lake area, R²: 0.89 and 0.92, respectively) and to forest ratio (catchment forested area/lake area, R²: 0.86 and 0.89), but not to wetland ratio. The simplicity of the model makes it easily transposable to a large variety of lakes. For a better insight into carbon processing, we suggest to follow a more integrative approach accounting for interactions between lake hydrology and catchment land cover.
Graphical Abstract
Autotrophic structure refers to the partitioning of whole-ecosystem primary production between benthic and planktonic primary producers. Autotrophic structure remains poorly understood especially because of the paucity of estimates regarding benthic primary production. We used a conceptual model for numerically exploring the autotrophic structure of 13 hemiboreal lakes situated in the Baltic Sea catchment. We also used diel variations in primary production profiles to graphically evaluate levels of light and/or nutrient limitation in lakes. The input morphometric data, light extinction coefficients and dissolved carbon parameters were mostly obtained from in situ measurements. Results revealed that cross- and within-lake autotrophic structure varied greatly: one lake was clearly dominated by benthic production, and three lakes by phytoplankton production. In the rest, phytoplankton production was generally dominant but switch to benthic dominance was possible. The modelled primary production profiles varied according to lake water clarity and bathymetry. Our results clearly indicate that the relative contribution of benthic primary production to whole-lake primary production should be taken into account in studies about hemiboreal and boreal lakes.
Net ecosystem production (NEP) is the difference between gross primary production (GPP) and community respiration (R). We estimated in situ NEP using three independent approaches (net CO2 gas flux, net O-2 gas flux, and continuous diel O-2 measurements) over a 4-7 yr period in a series of small lakes in which food webs were manipulated and nutrient loadings were experimentally varied. In the absence of manipulation, these lakes were net heterotrophic according to all three approaches. NEP (NEP = GPP-R) was consistently negative and averaged -35.5 +/- 3.7 (standard error) mmol C m(-2) d(-1). Nutrient enrichment, in the absence of strong planktivory, tended to cause increases in estimates of both GPP and R (estimated from the continuous O-2 data) but resulted in little change in the GPP/R ratio, which remained <1, or NEP, which remained negative. When planktivorous fish dominated the food web, large zooplankton were rare and nutrient enrichment produced positive values of NEP by all three methods. Among lakes and years, daily values of NEP ranged from -241 to +175 mmol m(-2) d(-1); mean seasonal NEP was positive only under a combination of high nutrient loading and a planktivore-dominated food web. Community R is significantly subsidized by allochthonous sources of organic matter in these lakes. Combining all lakes and years, we estimate that 26 mmol C m(-2) d(-1) of allochthonous origin is respired on average. This respiration of allochthonous organic matter represents 13 to 43% of total R, and this fraction declines with increasing GPP.
Boreal and northern temperate lakes (hereinafter referred to as northern lakes) are sites of intense processing of dissolved organic carbon (DOC), which is reflected in part in the persistent CO2 supersaturation of their surface waters. These ecosystems are subjected to strong seasonal fluctuations in both irradiance and DOC amount and quality, which in turn should result in temporal shifts in the magnitude of DOC photo-degradation. Here we explore the temporal patterns in the magnitude of water column DOC photo-mineralization, and its potential contribution to pelagic CO2 production in three northern lakes of different DOC content. We performed laboratory DOC photo-degradation incubations, and combined the resulting rates with field measurements and modeling to reconstruct the annual cycle in depth-integrated DOC photo-mineralization. We found that areal rates of DOC photo-mineralization were driven by both irradiance and intrinsic DOC photo-reactivity, both of which showed seasonality. Over an annual cycle, depth-integrated DOC photo-mineralization rates were remarkably similar across lakes, averaging 4.4 (SD = 0.7) g C m-2 yr-1, and daily rates followed an apparent seasonal pattern. The contribution of DOC photo-mineralization to total pelagic CO2 production (as the sum of respiration and DOC photo-mineralization) peaked after ice melt (up to 49%), averaging 14% for the entire open water season. Our study identifies potential hot periods of photochemical activity that result from the interplay between DOC properties and environmental conditions, which should be incorporated into models of lake functioning.
We explored the whole-lake accumulation of CO2 during winter ice cover and in the hypolimnion during summer stratification, in 15 temperate and boreal lakes, and how these processes vary with lake trophic state and morphometry. We further estimated an annual CO2 budget for each lake that incorporates the fluxes resulting from winter ice-cover and summer hypolimnetic CO2 accumulation to assess their relative importance. The volumetric rates of CO2 accumulation during the winter ice cover ranged from 4.1 to 42.1 mg C m−3 d−1, and from 9.3 to 54.5 mg C m−3 d−1 in the summer hypolimnion, and both varied mainly as a function of water temperature. The total CO2 accumulation at the end of winter/summer was most strongly related to lake mean depth and was significantly greater under ice than in the hypolimnion. This greater CO2 accumulation resulted in relatively high and unvarying spring outflux, averaging 404 mg C m−2 d−1 (C.V. 27%). Significant hypolimnetic CO2 accumulation also resulted in relatively high fluxes (average 228 mg C m−2 d−1) during autumnal mixing, which were more variable (C.V. 43%) than spring fluxes. Average fluxes were lowest and most variable in summer (198 mg C m−2 d−1, C.V. 46%). The net annual CO2 flux ranged from 14 to 68 g C m−2 y−1, and was positively related to DOC concentration. The winter ice-cover CO2 accumulation accounted from 3 to 80% (average 17%) of the annual CO2 flux, whereas summer hypolimnetic accumulation accounted for a smaller but still significant proportion (from 1.4 to 46%). These winter and summer hypolimnetic CO2 accumulations are released to the atmosphere through short but intense emission bursts that are rarely captured in regular lake sampling schemes, yet they have the potential to profoundly influence the annual lake CO2 budgets in northern landscapes.
Most lakes and reservoirs have surface CO2 concentrations that are supersaturated relative to the atmosphere1. The resulting CO2 emissions from lakes represent a substantial contribution to the continental carbon balance2, 3, 4. Thus, the drivers of CO2 supersaturation in lakes need to be understood to constrain the sensitivity of the land carbon cycle to external perturbations4, 5, 6. Carbon dioxide supersaturation has generally been attributed to the accumulation of inorganic carbon in lakes where respiration exceeds photosynthesis7, 8, but this interpretation has faced challenges9, 10, 11. Here we report analyses of water chemistry data from a survey of Spanish reservoirs that represent a range of lithologies, using simple metabolic models. We find that, above an alkalinity threshold of 1 mequiv. l−1, CO2 supersaturation in lakes is directly related to carbonate weathering in the watershed. We then evaluate the global distribution of alkalinity in lakes and find that 57% of the surface area occupied by lakes and reservoirs—particularly in tropical and temperate latitudes—has alkalinity exceeding 1 mequiv. l−1. We conclude that lake inputs of dissolved inorganic carbon from carbonate weathering should be considered for the CO2 supersaturation of lakes at both regional and global scales.
In lentic systems, hydrology can be dramatically altered after storm events, potentially modifying the carbon budget. In particular, rapid increases in the surface water carbon dioxide partial pressure (pCO 2) have been observed following such events. Several processes may explain these shifts in lake CO 2 dynamics, including vertical mixing, increases in metabolism, and increases in external loading. To evaluate the relative importance of these vari-ous processes, we reconstructed the whole-lake daily CO 2 budget using concurrent estimates of lake metabolism and daily CO 2 mass balance bud-gets in two lakes with distinct morphometries lo-cated in Qué bec, Canada. We found that storm events caused variable, but significant, changes in whole-lake CO 2 mass. Such events influenced CO 2 dynamics indirectly by inducing shifts in lake metabolism, and directly by importing CO 2 by the inflowing storm waters. Storm intensity (in terms of total amount of precipitation) influences the balance between these two processes, but the final outcome depends on lake morphometry. Our re-sults suggest that when storms are intense enough to drive lake water renewal rate beyond 1% day -1 , external CO 2 loadings became the dominant pro-cess, overwhelming internal CO 2 production. Lakes with slower hydrological turnover, however, are more susceptible to internal regulation and may simply re-allocate CO 2 from the hypolimnion to the epilimnion following a storm event. Our results thus suggest that this tightening of the watershed-lake-atmosphere linkage by climatic events is strongly modulated by lake morphometry. These features should be considered when predicting the impact of future climate change on regional C budgets and emissions.
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 undersaturated 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.
Dissolved CO2 dynamics in stormflow and event water versus
preevent water contributions to storm hydrographs were assessed in a
forested headwater catchment of the Brazilian Amazon using
high-frequency data. We applied the transfer function hydrograph
separation model (TRANSEP) using specific conductance as a conservative
tracer, finding preevent water to average 0.79 ± 0.03 of storm
discharge (mean ± 1 SE for n = 14 storms). In situ, direct
measurements of dissolved CO2 were able to capture new
hydrobiogeochemical processes in real time, including CO2
pulses observed on the falling limb of storm hydrographs, the magnitudes
of which were inversely related to preevent water fractions (r = -0.97,
p < 0.0001).
Net ecosystem production (NEP) is the difference between gross primary production (GPP) and community respiration (R). We estimated in situ NEP using three independent approaches (net CO2 gas flux, net O 2 gas flux, and continuous diel O2 measurements) over a 4-7 yr period in a series of small lakes in which food webs were manipulated and nutrient loadings were experimentally varied. In the absence of manipulation, these lakes were net heterotrophic according to all three approaches. NEP (NEP 5 GPP-R) was consistently negative and averaged 235.5 6 3.7 (standard error) mmol C m 22 d 21 . Nutrient enrichment, in the absence of strong planktivory, tended to cause increases in estimates of both GPP and R (estimated from the continuous O2 data) but resulted in little change in the GPP/R ratio, which remained ,1, or NEP, which remained negative. When planktivorous fish dom- inated the food web, large zooplankton were rare and nutrient enrichment produced positive values of NEP by all three methods. Among lakes and years, daily values of NEP ranged from 2241 to 1175 mmol m 22 d 21 ; mean seasonal NEP was positive only under a combination of high nutrient loading and a planktivore-dominated food web. Community R is significantly subsidized by allochthonous sources of organic matter in these lakes. Combining all lakes and years, we estimate that ;26 mmol C m 22 d 21 of allochthonous origin is respired on average. This respiration of allochthonous organic matter represents 13 to 43% of total R, and this fraction declines with increasing GPP.
Euphotic holding capacity is defined as the amount of pure water or a substance that could be contained in the euphotic layer if it would be the single optically active component in the system. Com- paring the actual amount of optically active substances in the euphotic layer of a lake to the holding ca- pacities allows to estimate the contribution of different optically active components to the overall at- tenuation of PAR. Investigations made on 8 optically contrasting lakes in Estonia revealed a high role of yellow substances in most of the lakes studied. Chlorophyll a and particulate matter affected light pene- tration in almost equal proportions whereas the role of pure water was remarkable only in some excep- tional cases.
Autonomous, in situ sensors for the partial pressure of CO2(pCO2) and dissolved oxygen (DO) were deployedin a freshwater lake during the winters of 1997 and 1998 to evaluate magnitudeand sources of variability during ice-covered periods. Gas variability ondiel or shorter time scales was small or undetectable during most of thedeployment periods, only becoming significant prior to ice-out whenrunoff and light penetration increased, promoting convective currents andbiological production. A surprising 7.6 d period oscillation,apparently driven by a baroclinic seiche, dominated the short-termvariability during the first year. The gas trends associated with the seicheoscillations and periodic profile measurements revealed that ice formation ledto gas gradients directly under the ice. Long-term variability wascharacterized by increasing CO2 and decreasing DO as a consequenceofbiological oxidation of organic matter. The results suggest that both spatialand temporal variability can be significant over intervals which would not beresolved by traditional sampling-based studies.
Monitoring purposes determine the selection of variables, location of sampling sites, and
sampling frequency. The selection should provide the best signal to noise ratio for the parameters of
interest. For trend and surveillance monitoring, the deepest point of a lake, where different inputs to
the lake are integrated, is frequently selected. However, the representativeness of a single site is
often questioned, especially for large lakes. Based on data collected from 10 sampling points during
11 survey expeditions in August 20012011 to the large shallow Lake Võrtsjärv, Estonia, we studied
the spatial and annual variability of environmental and phytoplankton variables and analysed
the representativeness of a permanent sampling station for the whole lake conditions. The two
southernmost stations under the influence of the main tributary deviated clearly from the homogeneous
group of the other eight stations, which we termed Võrtsjärv Proper. Among the stations of Võrtsjärv
Proper, the year-to-year variability dominated strongly over the spatial variability, the latter being
almost negligible for most of the variables. Surface water temperature and water level explained
approximately half of the total variability in parameters commonly used in ecological status assessment
of lakes. This has serious implications for using these variables to detect human impacts in Võrtsjärv.
Our study showed that the deep sampling site, which was characterized by the lowest average
variability of the parameters measured and was representative of more than 90% of the lake
aquatory, possesses all necessary qualities required of a permanent surveillance monitoring station.
Partial pressure (pCO 2) and flux to the atmosphere of carbon dioxide (CO 2) were studied in northern alpine and forest lakes along a gradient of dissolved organic carbon (DOC) content (0.4 –9.9 mg L 1). Sixteen lakes were each sampled three times over the course of the ice-free season, and an additional 35 lakes were sampled once at midsummer. pCO 2 data were acquired in the field by a headspace equilibration technique. Most lakes were supersat-urated with CO 2 along the entire DOC gradient, with relatively small seasonal differences. pCO 2 was positively correlated to DOC content, reflecting a close dependence between allochthonous DOC input and heterotrophic respiration in the lakes. Fluxes of CO 2 to the atmosphere were estimated from the pCO 2 measurements. Benthic respiration was indicated to be important for CO 2 emission in lakes with high DOC concentrations. In lakes with low DOC concentrations, pelagic mineralization alone was sufficient to account for a large part of the estimated fluxes.
In many freshwater ecosystems, the contents of NO3- and SO4(2-) have increased, whereas O2 has been depleted due to the increased acid and nutrient loads. These changes may affect carbon turnover and the dynamics of the major greenhouse gases CO2, CH4, and N2O. We studied the effects of O2, NO3-, and SO4(2-) availability on carbon mineralization, and fluxes of CO2, CH4, and N2O in the sediments of hyper-eutrophic Lake Kevätön, Finland. Undisturbed sediment cores from the deep (9 m) and shallow (4 m) profundal were incubated in a laboratory microcosm with oxic and anoxic water flows with NO3- or SO4(2-) concentrations of 0, 30, 100, 300, and 2000 microM. The carbon mineralization rate (i.e., the sum of released CO2-C and CH4-C) was not affected by the oxidants. However, the oxidants did change the pathways of carbon degradation and the release of CH4. All of the oxidants depressed CH4 fluxes in the shallow profundal sediments, which had low organic matter content. In the deep profundal sediments rich in organic matter, the CH4 release was reduced by O2 but was not affected by SO4(2-) (the effect of NO3- was not studied). There was an increase in N2O release as the overlying water NO3- concentration increased. Anoxia and highly elevated NO3- concentrations, associated with eutrophication, increased drastically the global warming potential (GWP) of the sedimentary gases in contrast to the SO4(2-) load, which had only minor effects on the GWP.
Arctic tundra has large amounts of stored carbon and is thought to be a sink for atmospheric carbon dioxide (CO2) (0.1 to 0.3 petagram of carbon per year) (1 petagram = 1015 grams). But this estimate of carbon balance is only for terrestrial ecosystems. Measurements of the partial pressure of CO2 in 29 aquatic ecosystems across arctic Alaska showed that in most cases (27 of 29) CO2 was released to the atmosphere. This CO2 probably originates in terrestrial environments; erosion of particulate carbon plus ground-water transport of dissolved carbon
from tundra contribute to the CO2 flux from surface waters to the atmosphere. If this mechanism is typical of that of other tundra areas, then current estimates
of the arctic terrestrial sink for atmospheric CO2 may be 20 percent too high.
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 mg O2l-1. 1% 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. -from Author
Sensors for the partial pressure of CO2 (pCO(2)) and dissolved O-2 (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 pCO(2) 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 pCO(2) and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO(2) 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 muatm daily changes in pCO(2) but, because of their counterbalancing effects, surface pCO(2) remained > 1,000 muatm for nearly I month after ice-out. Upon stable stratification, net production and air-water exchange overcame pCO(2) gains from mixing and heating and reduced pCO(2) to near atmospheric levels within 20 d. Net production and gas exchange accounted for similar to75% and 25%, respectively, of the decrease in surface pCO(2) observed after ice-out. Diel convection was the dominant mixing process both under ice and after ice-out and may be an important underrepresented mechanism for CO2 and DO exchange between surface and bottom water.
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.
Annual CO2 emissions from lakes and other inland waters into the atmosphere are estimated to almost entirely compensate the total annual carbon uptake by oceans. CO2 supersaturation in lakes, which results in CO2 emissions, is frequently attributed to CO2 produced within the lake. However, lateral inorganic carbon flux through watersheds can also be sizeable. Here we calculated lake surface water CO2 concentrations and emissions using lake pH, alkalinity and temperature from a compilation of data from 5,118 boreal lakes. Autumn surface water CO2 concentrations and CO2 emissions from the 5,118 lakes co-varied with lake internal autumn CO2 production. However, using a mass balance approach we found that CO2 emission in the majority of lakes was sustained by inorganic carbon loading from the catchment rather than by internal CO2 production. Small lakes with high dissolved organic carbon and phosphorus concentrations, shorter retention times and longer ice-free seasons had the highest CO2 concentrations. CO2 emissions from these small lakes was twice that of comparable lakes in colder regions, and similar to emissions from subtropical and tropical lakes. We conclude that changes in land use and climate that increase dissolved inorganic carbon may cause emission levels from boreal lakes to approach those of lakes in warmer regions.
Recent progress of including lake subroutines in numerical weather prediction (NWP) models has led to more accurate forecasts. In lake models, one essential parameter is water clarity, parameterized via the light extinction coefficient, K d , for which a global constant value is usually used. We used direct eddy covariance fluxes and basic meteorological measurements coupled with lake water temperature and clarity measurements from a boreal lake to estimate the performance of two lake models, LAKE and FLake. These models represent two 1-D modeling frameworks broadly used in NWP. The results show that the lake models are very sensitive to changes in K d when it is lower than 0.5 m À1. The progress of thermal stratification depended strongly on K d. In dark-water simulations the mixed layer was shallower, longwave and turbulent heat losses higher, and therefore the average water column temperatures lower than in clear-water simulations. Thus, changes in water clarity can also affect the onset of ice cover. The more complex LAKE modeled the seasonal thermocline deepening, whereas it remained virtually constant during summer in the FLake model. Both models overestimated the surface water temperatures by about 1°C and latent heat flux by >30%, but the variations in heat storage and sensible heat flux were adequately simulated. Our results suggest that, at least for humic lakes, a lake-specific, but not time-depending, constant value for K d can be used and that a global mapping of K d would be most beneficial in regions with relatively clear lakes, e.g., in lakes at high altitudes.
The water layer where photosynthesis takes place (euphotic zone) was studied, and criteria for determining its thickness were compared. Published works give alternative definitions of the euphotic depth: 1) the depth at which radiation falls to 1% of the subsurface irradiance in the photosynthetically active radiation (PAR) region of the spectrum; 2) the depth of some small constant value of downwelling irradiance; 3) the depth of the photocompensation point. We compared values of the eupholic depth obtained by these criteria with each other and with the depth where primary production approaches zero. The data describing the vertical distribution of irradiance in the PAR region (90 profiles) and primary production (41 profiles) in 13 Estonian and Finnish lakes collected in 1995-97 were used. Additionally, criteria 1 and 2 were investigated by model calculations. The regression formulae describing the relationship between criteria for of the euphotic zone and the level of zero primary production were obtained. The respective correlation coefficients were different for each criteria and depended on the conditions of the data collection. The relationship between the 1%-level and constant irradiance level was strong when the incoming irradiance varies within narrow limits. The 1%-depth, widely used in practice, corresponded well to the level where primary production approached zero. By our results the 1%-depth and the depth of some constant irradiance describe the zone of positive gross primary production rather than the zone of positive net production.
Streams are typically supersaturated and emit methane to the atmosphere. Much of this gas appears to be imported from groundwater and riparian zones, where anoxia and methane production are common. We investigated how methane evasion and the concentration in groundwaterd discharge varied between five stream sections with differing rates of groundwater discharge along the east fork of Walker Branch in eastern Tennessee. Evasion and groundwater concentration were determined from measurements of methane in surface water in conjunction with coinjection of conservative solute and volatile gas tracers. Methane in surface water was supersaturated, varying from 0.67 to 1.56 μg CH4 liter-1, which translates to concentrations 17.6-41.4 times greater than those at atmospheric equilibrium. Methane evasion rates ranged from 0.4 to 13.2 mg CH4 m-2 d-1. Differences in methane concentration and evasion were related to variation in subsurface discharge and concentration in groundwater. All study sections gained flow, although the rate of subsurface discharge into the second study section (section 2) was particularly low. Furthermore, the specific conductance of groundwater flowing into section 2 averaged only 82.3 μS cm-1, compared with 110-125 μS cm-1 in the other study sections, indicating that groundwater discharge was derived from riparian soils as opposed to deeper flow from fractured bedrock. The mean concentration of 549.2 μg CH4 liter-1 in subsurface water flowing into section 2 was notably greater than in the other sections, where average groundwater concentration ranged from 158.9 to 376.2 μg CH4 liter-1. Our results suggest that subsurface flow from riparian soils appears to be the major source of methane to streams, although deeper bedrock flow also supplies methane at a lower concentration to surface waters.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Baltic Marine Biologists. Publication ; 5 Pages: 38 p.
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.
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.
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.