Article

Horizontal differences in ecosystem metabolism of a large shallow lake

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Abstract

The causes of horizontal differences in metabolic activities between lake zones are still poorly understood. We carried out a two-year study of lake metabolism in two contrasting parts of a large shallow lake using the open-water technique based on high-frequency measurements of dissolved oxygen concentrations. We expected that the more sheltered and macrophyte-rich southern part of the lake receiving a high hydraulic load from the main inflow will exhibit equal or higher rate of metabolic processes compared to the open pelagic zone, and higher temporal variability, including anomalous metabolic estimates such as negative gross primary production (GPP) or community respiration (CR) due to rapid water exchange. Our results showed that anomalous metabolic estimates occurred at both stations with a similar frequency and were related rather to certain wind directions, which likely contributed to stronger water exchange between the littoral and pelagic zones. Periods of auto- and heterotrophy (daily mean NEP> or <0) had a 50:50 distribution at the Central Station while the proportions were 30:70 at the Southern Station. High areal GPP estimated in our study exceeding nearly twice the long-term average 14C primary production, showed the advantages of the free-water technique in integrating the metabolism of all communities, a large part of which has remained undetected by the traditional bottle or chamber incubation techniques.

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... In addition, there is a profound lack of studies that have dealt with the dynamics of ecosystem metabolism in shallow warm lakes. Particularly in polymictic shallow lakes, which are susceptible to warming and wind induced mixing effects, aquatic metabolism is expected to display highly dynamic behavior with abrupt changes within short periods (e.g., weeks) and to present larger variability in the littoral than the open water due to the presence of benthic littoral communities and their interaction with the land-water interface (e.g., benthic aquatic vegetation) [22]. ...
... This seasonal pattern was evidenced by the high dependency found between daily estimates of GPP, water temperature, and PAR. This agrees with the results for other shallow lakes (where seasonal patterns of GPP were driven by the temporal variations in water temperature and irradiance [14,22,45,46]). Besides, high summer temperatures can trigger high algal biomass that contributes to high rates of GPP [14]. However, in this study GPP showed weak correlation with algal biomass (chl-a) while both GPPd and GPPm models included chl-a as a non-significant predictor. ...
... Previous studies also had shown the significant effect of benthic vegetation in the metabolic processes by comparing vegetated littoral habitats with pelagic open water [18,19,47]. PAR and water temperature also were significant predictors for both Rd and Rm models which agrees with previous studies [14,22,46]. ...
Article
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The metabolic balance between gross primary production (GPP) and ecosystem respiration (R) is known to display large spatial and temporal variations within shallow lakes. Thus, although estimation of aquatic metabolism using free-water measurements of dissolved oxygen concentration has become increasingly common, the explanation of the variance in the metabolic regime remains an extremely difficult task. In this study, rates of GPP, respiration (R) and the metabolic balance (net ecosystem production, NEP) were estimated in four littoral habitats with different macrophyte growth forms (floating-leaved vs submerged) over a 28-month period in lake of Kastoria (Greece), a shallow eutrophic lake. Our results showed that net heterotrophy prevailed over the studied period, suggesting that allochthonous organics fuel respiration processes in the littoral. Temporal variation in the metabolic rates was driven mainly by the seasonal variation in irradiance and water temperature, with the peak of metabolic activity occurring in summer and early autumn. Most importantly, significant spatial variation among the four habitats was observed and associated with the different macrophyte growth forms that occurred in the sites. The results highlight the importance of habitat specific characteristics for the assessment of metabolic balance and underline the potentially high contribution of littoral habitats to the whole lake metabolism.
... In the present study, as shown in Table 1, Rdaytime, Rnight and R24h were estimated using sensor data and certain periods of the instantaneous NCP data and were integrated either from sunrise to sunset or only over the production period (e.g., only when the instantaneous NCP was positive). Means of instantaneous NCP over certain time periods were used as in previous studies, including Staehr et al. (2010), Laas et al. (2012); Idrizag et al. (2016); Richardson et al. (2017); and Chiu et al. (2020) . More precisely, Rdaytime was estimated either with the mean of the instantaneous NCP during a 1-h period centered on the maximum of the instantaneous Negative NCP (hereafter referred to as the "Max" method) or with the mean of the instantaneous NCP during a 1-h period following sunset, as in Mostajir et al. (2013) (hereafter referred to as the "Most" method). ...
... Estimation of R during the day (Rdaytime), at night (Rnight), and daily R (R24h), using sensor data In the present study, as shown in Table 1, Rdaytime, Rnight, and R24h were estimated using sensor data and certain periods of the instantaneous NCP data and were integrated either from sunrise to sunset or only over the production period (e.g., only when the instantaneous NCP was positive). Means of instantaneous NCP over certain time periods were used as in previous studies, including Staehr et al. (2010a,b), Laas et al. (2012), Idrizag et al. (2016), Richardson et al. (2017), andChiu et al. (2020). More precisely, Rdaytime was estimated either with the mean of the instantaneous NCP during a 1-h period centered on the maximum of the instantaneous Negative NCP (hereafter referred to as the "Max" method) or with the mean of the instantaneous NCP during a 1-h period following sunset, as in Mostajir et al. (2013) (hereafter referred to as the "Most" method). ...
Thesis
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Planktonic processes playing a key role in the fate of matter and global biogeochemical cycles, gross primary production (GPP), net community production (NCP) respiration (R), growth (µ) and loss (L) rates of phytoplankton were quantified using high-frequency (HF) data acquired by sensors immersed in in situ mesocosms during experiments simulating warming (Thau Lagoon) and brownification (Hopavågen Bay, Norway). A new method for estimating GPP, R and NCP was established using sensor data measuring dissolved O2 concentration from experiments in Thau lagoon. It was compared with an existing method using HF data and with the classical incubation technique (Winkler). It has the advantage of considering the variability in the coupling between day-night and dissolved O2 cycles and allowed to estimate a daytime respiration on average 41% higher than the nighttime one, in agreement with the positive theoretical effect of light on respiration. Application of this new method in an experiment testing the effects of brownification on planktonic community functioning in Hopavågen Bay revealed a negative effect of brownification of about 30% on GPP and R, associated with significant changes in phytoplankton pigment concentration related to physiological acclimation to low light conditions. The warming scenario for 2100 in the Mediterranean was tested in two in situ mesocosm experiments in spring and autumn 2018 in Thau lagoon. Warming increased phytoplankton’s µ and L, estimated with HF fluorescence data. The estimates of µ and L were compared to the growth and grazing rates obtained via the dilution technique, highlighting a good agreement between the two methods, confirming the robustness of the estimates obtained with the HF data, despite some differences between results due to the fact that L, estimated with the HF data, considers sedimentation, mesozooplankton grazing mortality and viral lysis while the dilutions only allow the estimation of microzooplankton grazing. An experiment simulating a heatwave was also carried out in Thau in spring 2019. The HF data showed a positive effect of the heat wave on GPP, R, µ and L that lasted for several days after the end of the heatwave, except for µ. The heatwave shifted the trophic state of the system towards heterotrophy and favored cyanobacteria at the expense of dinoflagellates, however most of the studied processes showed good resistance and recovery from the heatwave. Warming induced contrasting responses in Thau lagoon planktonic community, shifting the system towards autotrophy or heterotrophy depending on the investigated season. The community from Thau lagoon was more resilient and recovered better from a punctual climate change event than the community from Hopavågen, potentially because the planktonic community from Thau evolved in an environment naturally subject to strong temperature variations and/or because warming is a less drastic disturbance than brownification. The methods established in the thesis represent a novel approach to obtain reliable estimates of planktonic processes highlighting the effects of climate change on the functioning of coastal ecosystems. There are many perspectives to continue this work, using these new methods to study other disturbances in other ecosystems.
... For meaningful PP results over a longer time period (days, months, and years), a large number of direct consecutive contact measurements of photosynthesis rate (e.g., using the 14 C [16], 13 C [17], or dissolved oxygen method [18,19]) are needed. However, these methods are costly, time-consuming, in some cases need the manipulation of radioactive material, and are difficult to apply for large-scale routine monitoring [20]. ...
... There have been large discrepancies in the PP results obtained in different studies using different methods. The long-term study ) by Nõges et al. [68] estimated the average PP from June to October to be 880 mg C m −2 d −1 , based on 14 C methods, while another study [19] estimated the PP from oxygen measurements to be 1632 mg C m −2 d −1 (June to October 2011-2012). Our results would be placed in the middle of those two past studies, with the PP estimated to be 1188 mg C m −2 d −1 from June to October 2018. ...
Article
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Phytoplankton primary production (PP) in lakes play an important role in the global carbon cycle. However, monitoring the PP in lakes with traditional complicated and costly in situ sampling methods are impossible due to the large number of lakes worldwide (estimated to be 117 million lakes). In this study, bio-optical modelling and remote sensing data (Sentinel-3 Ocean and Land Colour Instrument) was combined to investigate the spatial and temporal variation of PP in four Baltic lakes during 2018. The model used has three input parameters: concentration of chlorophyll-a, the diffuse attenuation coefficient, and incident downwelling irradiance. The largest of our studied lakes, Võrtsjärv (270 km2), had the highest total yearly estimated production (61 Gg C y−1) compared to the smaller lakes Lubans (18 Gg C y−1) and Razna (7 Gg C y−1). However, the most productive was the smallest studied, Lake Burtnieks (40.2 km2); although the total yearly production was 13 Gg C y−1, the daily average areal production was 910 mg C m−2 d−1 in 2018. Even if lake size plays a significant role in the total PP of the lake, the abundance of small and medium-sized lakes would sum up to a significant contribution of carbon fixation. Our method is applicable to larger regions to monitor the spatial and temporal variability of lake PP.
... 6,7 While most shallow lakes usually lack stable vertical gradients due to their polymictic character, they can show pronounced horizontal differences in chemistry and sediment composition due to wind action and sheltering effects in the littoral zone. [8][9][10][11] Such spatial differences, combined with pronounced short-term temporal variability, represent a challenge for monitoring impacts of external pressures on hydrochemistry and biology. This is also true for Lake Neusiedl, the largest endorheic lake in Central Europe. ...
Article
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Shallow lakes provide a multitude of ecosystem functions, but they are particularly vulnerable to natural and anthropogenic disturbances. Understanding the driving factors determining the fate and spatial distribution of nutrients and pollutants in such systems is fundamental to assess the impact of ongoing or future external pressures endangering their ecological integrity. This study investigates the fate of trace contaminants transported into the large shallow Lake Neusiedl, including contaminants representative of different patterns of sources and emission pathways and of environmental behavior, namely metals, pharmaceuticals, an artificial sweetener and perfluoroalkyl substances. Further, it examines the horizontal spatial distribution of nutrients, ions and physico-chemical parameters with an unprecedented detailed focus on the internal variability within the large reed belt. As described in the past e.g. for chloride, evaporation was identified as the process leading to a substantial concentration enrichment of the industrial chemical PFOA and the sweetener acesulfame K from the tributary river into the open lake. This is particularly relevant in view of the predicted future increase of evapotranspiration due to climate change. In contrast, the observed loss of diclofenac, but also of PFOS and carbamazepine suggests that the well-mixed, humic-rich and alkaline Lake Neusiedl offers favorable conditions for the photodegradation of otherwise very persistent chemicals. Another important finding, in the context of possible modifications in lake water levels due to climate change, is the fundamental role played by the connectivity between open lake and reed belt but also by the presence and characteristics of inner water areas within the reed belt region in determining the hydrochemistry of the lake system. By revealing systematic spatial patterns and by focusing on the underlying factors and processes, the understanding offered by this study is of high value for the conservation of shallow lakes.
... Lake metabolism provides a framework for understanding the balance of carbon fixation (GPP) and oxidation (R) in lake ecosystems. There is a long history of quantifying metabolism in lakes to understand the spatial and temporal variation within and among lakes, drivers of metabolism at different spatial scales (Alfonso et al., 2018;Hanson et al., 2008;Idrizaj et al., 2016;Staehr et al., 2012;Stefanidis & Elias, 2019). However, quantifying how years with different hydroclimatic conditions influence metabolism in littoral and open water lake habitats has not been addressed to date despite the relatively larger contribution of littoral habitats to whole lake function. ...
Article
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Lakes integrate the signals of changing hydroclimate in their surrounding basin, which substantially influence gross primary production (GPP), respiration (R), and net ecosystem production (NEP). Most research focuses only on the changes to the open water habitat despite the littoral habitat's importance to lakes and its sensitivity to hydroclimatic variability. We analyze how years with different ice‐out dates and snow water equivalent (SWE) triggered different metabolism responses in the open water and littoral habitats of a subalpine lake. The dry (early ice‐out date and low SWE) and wet (late ice‐out date and high SWE) years had lower GPP and R rates in both habitats compared to the average hydroclimatic year. Furthermore, consumer biomass decreased during dry and wet years. GPP and R reduced the most in the littoral habitat. Consequently, the relative contribution to total lake GPP and R of the littoral habitat decreased, and the contribution of the open water habitat increased during the dry and wet years. We demonstrate that hydroclimatic conditions affect productivity and consumer biomass and show that within‐lake habitats do not experience equivalent responses to climate forcing. Our study has implications for how ecosystem scientists and managers quantify the absolute and relative contributions of the littoral habitat to whole lake production in the context of climate variation.
... We believe that aquatic metabolism studies are on a positive trajectory, providing methodological and conceptual guidance for future research. P:R ratios can provide valuable information for lake managers, highlighting the effects of changing hydrological inputs on lake metabolism (Idrizaj et al. 2016;Alfonso et al. 2018). Temporal trends in measured P:R ratios may signal browning events such as those caused by storms and high runoff (Sadro and Melack 2012). ...
Article
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.
... In the present study, as shown in Table 1, Rdaytime, Rnight, and R24h were estimated using sensor data and certain periods of the instantaneous NCP data and were integrated either from sunrise to sunset or only over the production period (e.g., only when the instantaneous NCP was positive). Means of instantaneous NCP over certain time periods were used as in previous studies, including Staehr et al. (2010a,b), Laas et al. (2012), Idrizag et al. (2016), Richardson et al. (2017), and Chiu et al. (2020). More precisely, Rdaytime was estimated either with the mean of the instantaneous NCP during a 1-h period centered on the maximum of the instantaneous Negative NCP (hereafter referred to as the "Max" method) or with the mean of the instantaneous NCP during a 1-h period following sunset, as in Mostajir et al. (2013) (hereafter referred to as the "Most" method). ...
Article
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Understanding how aquatic ecosystems respond to perturbations has emerged as a crucial way to predict the future of these ecosystems and to assess their capacity to produce oxygen and store atmospheric carbon. In this context, in situ mesocosm experiments are a useful approach for simulating disturbances and observing changes in planktonic communities over time and under controlled conditions. Within mesocosm experiments, the estimation of fundamental parameters such as gross primary production (GPP), net community production (NCP), and respiration (R) allows the evaluation of planktonic metabolic responses to a perturbation. The continuous estimation of these metabolic parameters in real time and at high frequency is made possible by employing noninvasive automated sensors in the water column. However, some uncertainties and methodological questions about the estimation of daytime respiration remain to be addressed for this method, and notably to address the fact that respiration could be significantly higher during the day than during the night. In this study, data from two in situ mesocosm experiments performed in fall and spring in a coastal Mediterranean area were used to develop a new method of estimating daytime respiration, and in turn daily GPP, R, and NCP, by considering the maximum instantaneous R, and that takes into account the variability of the coupling between day-night and dissolved oxygen cycles. This new method was compared with the Winkler incubation technique and with another existing method. Results showed that using this existing method, daytime R was significantly underestimated relative to estimates obtained with the newly proposed method.
... Since late 1970s when the two Limnothrix species outcompeted P. limnetica, which still occurs in moderate numbers, no major change in the phytoplankton community composition occurred, while the total biomass has slowly increased over the years. The Limnothrix species representing slow growing K-strategists are sensitive to hydraulic flushing and cannot colonize areas with short water retention time (Idrizaj et al. 2016). Although enhanced gas-vacuolation reaching up to 50% of cell volume has been described in some strains of L. redekei in winter (Gkelis et al. 2005), the gas-vacuolation in Võrtsjärv remains low and no gradients in the vertical distribution of the Limnothrix species have been observed. ...
Article
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As part of a global phenomenon, a 30% decrease in average wind speed since 1996 in southern Estonia together with more frequent easterly winds resulted in 47% decrease in bottom shear stress in the large (270 km 2), shallow (mean depth 2.8 m), and eutrophic Lake Võrtsjärv. Following a peak in eutrophication pressure in the 1970s-80s, the concentrations of total nutrients were declining. Nonmetric Multidimensional Scaling (NMDS) ordination of a 54-year phytoplankton community composition time-series (1964-2017) revealed three distinct periods with breaking points coinciding with changes in wind and/or water level. Contrary to expectations, we detected no decrease in optically active substances that could be related to wind stilling, whereas phytoplankton biomass showed an increasing trend despite reduced nutrient levels. Here we show how opening of the "light niche," caused by declining amount of suspended sediments, was capitalized and filled by the light-limited phytoplankton community. We suggest that wind stilling is another global factor, complementary to climate warming that counteracts eutrophication mitigation in lakes and may provide a challenge to assessment of the lake ecological status.
... Benthic primary production might also contribute to this upward trend in GPP (see below). Several studies have demonstrated a significant contribution of benthic/littoral metabolism in both seasonally stratified and shallow lakes, even though availability of benthic habitats was limited in some of these lakes (Lauster et al. 2006;Van de Bogert et al. 2007;Sadro et al. 2011;Idrizaj et al. 2016). A rich and abundant algal flora inhabits the sediments of Lake Balaton (Uherkovich and Lantos 1987). ...
Article
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High frequency time series of dissolved oxygen (DO), delayed chlorophyll fluorescence (Chl) and relevant background variables were recorded on nearly 900 d during 7 yr in large, shallow, meso-eutrophic Lake Balaton (Hungary). Novel models were developed for coupled simulation of diel dynamics of DO and Chl using sequential learning and uncertainty assessment in a Bayesian framework. Despite the generally good model fit for both variables, the uncertainty of the metabolic estimates was high, due primarily to the identification problem of individual metabolic processes. Deviations between observed and simulated DO concentrations suggested that neglect of transient stratification might be responsible for the bulk of the systematic model errors. Net ecosystem production (NEP) was uncertain. Unless air-water oxygen exchange can be estimated from direct measurements, the free-water DO method cannot reliably estimate NEP. Gross primary production (GPP) could satisfactorily be hindcasted assuming non-linear multiplicative dependence on Chl, water temperature and light. Hindcast of community respiration (CR) was less successful, possibly due to the impact of local benthic respiration. Results suggested a major shift in lake metabolism at about 16°C. Below and above this temperature, 70% and 90% of net primary production could be utilized by heterotrophs within a day, respectively. Indirect evidence suggested that biomass-specific net primary production was determined by phosphorus. The large difference between reproductive rates and net growth rates estimated from GPP and Chl and from daily change in Chl, respectively indicated that loss rates of phytoplankton were as important determinants of algal dynamics as reproductive rates. © 2017 Association for the Sciences of Limnology and Oceanography.
... Considering the potentially large variations in metabolic rates between the water column, benthos, and littoral zones of a given lake, the chosen measurement location may influence the resulting data (Caraco and Cole 2002;Lauster et al. 2006; Van de Bogert et al. 2007;Sadro et al. 2011;Staehr et al. 2012). Littoral macrophyte beds can boost net ecosystem production (NEP), producing high dissolved O 2 concentrations in the littoral surface waters compared to offshore surface waters (Unmuth et al. 2000;Lauster et al. 2006;They et al. 2013;Idrizaj et al. 2016; Fig. 1). With rapid and/or random mixing of lake waters, elevated littoral primary productivity rates might influence long-term offshore O 2 curves. ...
Article
The diel (24-h) oxygen (O2) curves approach has become a popular method for analyzing gross primary production (GPP) and ecosystem respiration (ER) rates in aquatic systems. Despite the simplicity of this approach, there remain aspects of the calculation and interpretation of diel O2 curves which may skew results, with potentially large implications for estimates of metabolic rates. One common problem in lakes is the occurrence of unexpected changes in O2 concentrations (for instance, increasing overnight O2 concentrations). Such changes have typically been ascribed to the random mixing of pockets of O2. It has thus been suggested that negative GPP or positive ER values should be included in calculations, on the assumption that under- and overestimates should occur with equal frequency, and thus cancel each other out. Our data from a shallow, eutrophic lake provided a high share of negative GPP values. We argue that these may have been the result of elevated littoral productivity coupled with convective currents produced by consistent differences in the heating or cooling of littoral and offshore waters. Such phenomena might be common in small, sheltered lakes where the role of mixing by wind is diminished. We conclude that a failure to account for consistent metabolic gradients and periodic convective mixing may lead to a chronic underestimation of metabolic rates in lakes when using the diel O2 curves method.
Article
Dissolved organic matter (DOM) plays an important role in biogeochemistry of lake ecosystems. Studies measuring DOM at short intervals in lakes are still rare, thus its short-term dynamics are largely unknown. We investigated DOM temporal variation in large and shallow eutrophic Lake Võrtsjärv (Estonia) during one growing season (May–September 2016) using field-deployable spectrometer In Situ Spectral Analyzer (GO Systemelektronik GmbH) to measure absorbance spectra (wavelength range 200–708 nm) at a 2-hour interval coupled with monthly discrete water sampling. Collected spectra were analyzed together with some in-lake variables, lake metabolic rates, and meteorological and hydrological data using Boosted Regression Tree (BRT) and Random Forest (RF) models. Different spectral parameters were used to assess total and allochthonous DOM quantity, and relative share of autochthonous DOM. All parameters, i.e., DOM quantity and quality, varied on a large scale. For example, dissolved organic carbon (DOC) concentrations ranged from 12.0 to 17.3 mg L-1. High levels of DOM were mainly of allochthonous origin; strong relationship with inflow indicated the same. Relative share of autochthonous DOM increased with rising air temperature as primary production rose in warm water; however, we did not find any direct relationships with gross primary production. RF and BRT models explained up to 38% and 63% of DOM temporal variability, respectively. Our results showed that monthly water samples did not capture large variation in DOM. Therefore, high-frequency measurements using in situ spectrometer improve temporal representativeness of DOM monitoring in lakes compared to traditional sampling methods.
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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.
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Lake Võrtsjärv is a large (270 km2) and shallow (mean depth 2.8 m) lake in Central Estonia. About 2/3 of the lake bottom is covered with mud lying on marl having a total thickness up to 7.6 m. Eighteen rivers and streams collect the water from mainly cultivated catchment area (3374 km2). The outflow into Lake Peipsi occurs via the Suur Emajõgi River. The mean water renewal time is one year. The ice cover lasts from November to April, on average 135 days. The lake is eutrophic (mean total N 1.6 g m-3 and total P 54 mg m-3). The water is slightly alkaline (pH ∼8) with a salinity of about 300 mg 1-1 and a high seston content. During the ice-free period, the Secchi depth is usually less than 1 m. Monthly means of the hydrobiological characteristics range as follows: Chla 8-43 mg m-3, phytoplankton biomass 1-30 g m-3, phytoplankton particulate production 27-1163 mg C m-2 d-1, decomposition 182-1660 mg C m-2 d-1, sedimentation 602-4786 mg C m-2 d-1, metazooplankton biomass 0.1-2.1 g m-3, production of herbivorous metazooplankton 0.3-14.1 mg C m3 d-1, production of predatory metazooplankton 0.00-1.61 mg C m3 d-1, total number of bacteria 3.3-4.7 · 106 cells ml-1, macrozoobenthos biomass 3.0-8.1 g m-2 in the profundal, 3.0-8.9 g m-2 in the littoral and corresponding abundance 0.4-0.9 · 103 and 1.7-3.5 · 103 ind. m-2. 81 species of macrophytes have been found in the lake which covered 15% of the lake area in 1965. Since then, the macrophyte area has extended remarkably. L. Võrtsjärv was previously referred to as a ruffe lake. Thanks to the rearrangement of the fishing strategy the rapid replacement of rough fishes by commercial fishes took place: the total catch of fish has risen about 1.5 times, the cost of the catch as much as 6-8 times, mainly on account of the eel.
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To characterize the spatial variability of metabolism estimates (gross primary production [GPP], respiration [R], and net ecosystem production [NEP]) in two Northern Wisconsin lakes, we collected data from 27 and 35 dissolved oxygen sensors placed in a two-dimensional array throughout the upper mixed layers over a period of 10 d per lake in midsummer. Averaged over the deployment, aerial metabolism estimates among sensor locations varied 1-2 orders of magnitude and were largely unrelated to physical habitat within the lake. For all sites and days, 76-90% of the explainable variance in GPP and R was attributable to location in the lake rather than day of the deployment. NEP, on the other hand, was less affected by location, with 79-93% of the explained variance attributable to the day of the deployment. Single-location estimates can yield errors of more than an order of magnitude in estimates of daily GPP and R and can mischaracterize the trophic status of the lake. Using a rarefaction approach, we found that using four randomly placed sensors increased the precision of the resulting daily metabolism estimates fourfold over single-location measures in both lakes.
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Automated in situ sensors for measuring changes in dissolved oxygen (DO) at high frequency have facilitated estimates of gross primary production (GPP) and respiration (R) in aquatic systems. Lake researchers usually rely on a single sensor for these estimates, but such point measurements may miss important spatial heterogeneity in within-lake processes and may not accurately represent systemwide values of metabolism. Here we combine simultaneous measurements of metabolism using DO sensors along transects from the shore to the center of a lake with a spatial model to better understand the underlying heterogeneity in processes contributing to whole-lake epilimnetic metabolism. We use this model to achieve better estimates of epilimnetic GPP and R and to determine the relative contributions of benthic-littoral vs. pelagic processes to these estimates. We compared the spatially explicit process-based model to estimates of metabolism from both a single sensor at the lake's center and a spatially explicit averaging of multiple sensor sites. Estimates of both GPP and R varied on average 2.5- to 3.2-fold from site to site within the same lake, whereas variations were sometimes as high as 6-to 7-fold. Estimates of GPP and R near the perimeter of lakes were on average greater than measurements in the middle of the lake. Our model estimates that benthic-littoral processes accounted for ∼40% of epilimnetic GPP and R. A single, centrally located sensor often misses a significant component of this benthic metabolism and accounts for only ∼81% of lakewide GPP and R. © 2007, by the American Society of Limnology and Oceanography, Inc.
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Carbon and oxygen metabolism in a densely vegetated lagoon: implications of spatial heterogeneity Ecosystem metabolism is an integrated descriptor of lake functioning. In systems dominated by submerged vegetation, as in many coastal environments, estimates of whole-system metabolism that are calculated through free-water diel techniques can be compromised by the high spatial heterogeneity of the metabolic signal. We investigated the spatial variability in the dissolved inorganic carbon (DIC) and dissolved oxygen (DO) concentrations and in the derived ecosystem metabolism in a coastal lagoon dominated by dense meadows of canopy-forming macrophytes. We analysed the gross primary production (GPP), respiration (R) and net ecosystem production (NEP) from diel variations of both DIC and DO at different sites during the period of maximum activity of the meadows. Our results showed high spatial variability in the DIC and DO concentrations in the vertical and horizontal dimensions as a result of the intense metabolic activity of macrophytes in the littoral surface waters. High heterogeneity in metabolism was also observed in the lagoon, with a mean coefficient of variation of up to 48 % for GPP and R rates. Most of this variability occurred within the littoral areas with macrophytes. The DIC-derived metabolic rates were systematically higher than the DO-derived rates (slope 2.074), indicating the existence of strong inorganic carbon fluxes. Our results stress the need for high sampling efforts based on multiple sampling sites and coupling of DIC and DO estimates to allow accurate quantification of ecosystem metabolism in shallow lakes and lagoons that are dominated by submerged vegetation.
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The patterns of photosynthetic 14C uptake predicted by four models, each of which incorporates different assumptions about the fate of carbon within a few hours of its fixation by phytoplankton, have been compared with the patterns of net 12C uptake predicted by the same models. According to all models tested, 14C will accumulate in the cells more rapidly (relative to its abundance) than 12C, until an equilibrium is established between the 14C:12C ratio inside the cells and that in the external medium. Since several hours are required for cells to approach this equilibrium, all models predict that measurements of photosynthesis based on 14C uptake overestimate net photosynthesis in incubations of up to 6-12 h, and that this overestimation will be especially severe close to the compensation point, or at depth in a water column. Experiments showed approximately linear uptake of 14C for up to 4-5 h. The only model predicting this pattern indicated that 14C uptake was proportional to gross photosynthesis, and could not be used to estimate either respiration or net photosynthesis. Other physiological and ecological evidence suggests that the conventional assumption that the radiocarbon technique measures net photosynthesis should be re-examined.
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We characterized spatial and temporal variability in net ecosystem production (NEP), community respiration (CR), and gross primary production (GPP) over an ice-free season in an oligotrophic high-elevation lake using high-frequency measurements of dissolved oxygen. We combined the use of free-water and incubation chamber measurements to compare pelagic and benthic habitats and estimate their relative contributions to whole-lake metabolism. Despite a brief period of predominant heterotrophy after snowmelt, both free-water and incubation chamber measurements confirmed autotrophy of the epilimnion in all habitats throughout the ice-free season. In contrast, benthic incubation chambers showed the benthos to be consistently heterotrophic. Although temperature was the strongest seasonal driver of benthic metabolism, bacterioplankton density and indexes of organic matter quality explained the most variability in pelagic metabolism. Driven largely by benthic metabolism, free-water measurements of GPP and CR were twice as high in littoral than pelagic habitats. However, rates of water column primary production overlying the littoral benthos were high enough to overcome net benthic heterotrophy, and seasonal mean NEP in littoral habitats remained positive and not significantly different from pelagic habitats. Benthic rates averaged about 25% of whole-lake metabolism. Pelagic metabolism measurements were affected by littoral rates about half the time, with the degree of isolation between the two a function of advection and water column stability. These results emphasize the importance of characterizing spatial and temporal variability in metabolism within the context of physical dynamics and challenge the notion that benthic metabolism will necessarily be larger than pelagic metabolism in oligotrophic lakes.
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We studied temporal dynamics and regulation of oxygen metabolism in the upper mixed layer of a nutrientrich shallow Danish lake by continuous measurements of oxygen, irradiance, wind, and temperature and frequent measurements of algal chlorophyll, organic pools, and inorganic nutrients. Chlorophyll, algal growth rate, and mean irradiance (Emean) \rm (E_{mean}) in the mixed surface layer were calculated daily from continuous measurements of irradiance and temperature with depth. There were three to four distinct maxima in gross primary production (GPP) and community respiration (R) during the summer season and minima from fall to spring after broad-scale changes in irradiance, temperature, mixing depth, and biomass and growth rate of the algal community and concentrations of inorganic nutrients. Lake metabolism was annually balanced (mean GPP:R 1.04 in 2003 and 1.01 in 2004), with net autotrophy occurring mainly from mid-May to mid-September (mean GPP : R 1.14 in 2003 and 1.10 in 2004), and net heterotrophy outside this period (mean GPP:R 0.60 in 2003 and 0.81 in 2004). However, GPP : R varied two-to threefold from day to day because lower surface irradiance, higher mixing depth, and thus lower (Emean) \rm (E_{mean}) significantly reduced GPP. Normalizing GPP to chlorophyll provided an index of algal growth potential (GPPB)\rm (GPP^B) which followed a hyperbolic relationship to (Emean) \rm (E_{mean}) and both parameters were related to blooms and collapses of algal biomass. Metabolic rates were much more variable from day to day than algal biomass, which integrates growth and loss processes over longer periods. The continuous approach to lake metabolism provides better data and can provide a more accurate picture than averages of a few discrete measurements. Weekly averages reflected the characteristic seasonal peaks and troughs also observed for algal biomass, whereas monthly averages did not. Daily measurements of lake metabolism, therefore, can provide the optimal background for evaluating temporal changes and regulation of algal biomass and organic pools in nutrient-rich shallow lakes.
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The Kriging technique is used as the interpolation component of a surface water level estimation system applicable to the estimation of the water surface profile and the associated average water level for a large lake or reservoir. The estimation system is applied to the problem of determining the average surface water elevation of Lake Winnipeg, a large multipurpose lake located in central Manitoba, Canada. The Kriging system is demonstrated to produce daily average water level estimates that are not only comparable to the results from two existing techniques but are also available earlier, in a real-time context. Additional information obtained from the basic estimation system, which can assist in identifying inconsistent input data sources, are also discussed. Key words: lake level estimation, Kriging, spatial interpolation.
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Seasonal changes in rates of gross primary production (GPP), net ecosystem production (NEP), and respiration (R) were determined from frequent automated profiles of dissolved oxygen (DO) and temperature in a clear-water polymictic lake. Metabolic rate calculations were made using a method that integrates rates across the entire depth profile and includes DO exchange between depth layers driven by mixed-layer deepening and eddy diffusivity. During full mixing, NEP was close to zero throughout the water column, and GPP and R were reduced 2-10 times compared to stratified periods. When present, the metalimnion contributed 21% and 27% to whole-lake areal rates of GPP and R, respectively. Net autotrophy prevailed in the epilimnion (NEP = 11 +/- 14 mmol O-2 m(-3) d(-1); mean 6 +/- SD) compared to balanced production in the metalimnion (NEP = 2 +/- 19 mmol O-2 m(-3) d(-1)) and net heterotrophic conditions in hypolimnic waters (NEP = -15 +/- 24 mmol O-2 m(-3) d(-1)). Positive NEP occurred in the metalimnion during periods when the photic depth extended below the mixed-layer depth. Although the single-sonde method estimated higher areal GPP (19%) and R (14%) compared to the two depth-integrated approaches, differences were not significant. During stratification, daily variability in epilimnetic DO was dominated by metabolism (46%) and air-water gas exchange (44%). Fluxes related to mixed-layer deepening dominated in meta- and hypolimnic waters (49% and 64%), while eddy diffusion (1% and 14%) was less important. Although air-water gas exchange rates differed among the three formulations of gas-transfer velocity, this had no significant effect on metabolic rates.
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We estimated organic carbon (OC) burial over the past century in 40 impoundments in one of the most intensively agricultural regions of the world. The volume of sediment deposited per unit time varied as a function of lake and watershed size, but smaller impoundments had greater deposition and accumulation rates per unit area. Annual water storage losses varied from 0.1-20% and were negatively correlated with impoundment size. Estimated sediment OC content was greatest in lakes with low ratios of watershed to impoundment area. Sediment OC burial rates were higher than those assumed for fertile impoundments by previous studies and were much higher than those measured in natural lakes. OC burial ranged from a high of 17,000 g C m-2 a-1 to a low of 148 g C m-2 a-1 and was significantly greater in small impoundments than large ones. The OC buried in these lakes originates in both autochthonous and allochthonous production. These analyses suggest that OC sequestration in moderate to large impoundments may be double the rate assumed in previous analyses. Extrapolation suggests that they may bury 4 times as much carbon (C) as the world's oceans. The world's farm ponds alone may bury more OC than the oceans and 33% as much as the world's rivers deliver to the sea.
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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 2001–2011 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.
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Significant improvements have been made in estimating gross primary production (GPP), ecosystem respiration (R), and net ecosystem production (NEP) from diel, "free-water" changes in dissolved oxygen (DO). Here we evaluate some of the assumptions and uncertainties that are still embedded in the technique and provide guidelines on how to estimate reliable metabolic rates from high-frequency sonde data. True whole-system estimates are often not obtained because measurements reflect an unknown zone of influence which varies over space and time. A minimum logging frequency of 30 min was sufficient to capture metabolism at the daily time scale. Higher sampling frequencies capture additional pattern in the DO data, primarily related to physical mixing. Causes behind the often large daily variability are discussed and evaluated for an oligotrophic and a eutrophic lake. Despite a 3-fold higher day-to-day variability in absolute GPP rates in the eutrophic lake, both lakes required at least 3 sonde days per week for GPP estimates to be within 20% of the weekly average. A sensitivity analysis evaluated uncertainties associated with DO measurements, piston velocity (k), and the assumption that daytime R equals nighttime R. In low productivity lakes, uncertainty in DO measurements and piston velocity strongly impacts R but has no effect on GPP or NEP. Lack of accounting for higher R during the day underestimates R and GPP but has no effect on NEP. We finally provide suggestions for future research to improve the technique.
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High-frequency dissolved oxygen (DO) measurements have been used for estimating gross primary production (GPP) and respiration (R) in lake ecosystems. Most researchers have determined GPP and R only in surface waters, a practice that may underestimate R in general and GPP in clear-water lakes in particular. We deployed oxygen sondes at multiple sites and depths in a clear-water lake. Rates of GPP or R were similar horizontally over the surface waters of the lake. Diel DO signals weakened with depth; however, removing noise from the data, by either wavelet transforms or moving averages, enhanced our ability to resolve diel metabolic signals. While GPP declined sharply with depth, R was unrelated to depth. The majority of GPP and R occurred in the upper mixed layer, but deeper water accounted for 14%-28% of GPP and 20%-43% of R, depending on the statistical filtering technique used. GPP and R were nearly in balance in the surface waters, but for the entire lake R exceeded GPP, and net ecosystem production was negative. Deployment of oxygen sondes in various habitats and at multiple depths allows for a more complete estimate of whole-lake metabolism and a better understanding of the spatial and temporal complexity of lakes.
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Net ecosystem production (NEP) trends among lakes have been ascribed to differences in nutrient and allochthonous carbon inputs, but little is known on how different habitats within lakes contribute to these trends. We sampled pelagic and littoral surface waters using sonde (i.e., free-water) and bottle methods concurrently in lakes spanning a range of trophic conditions. We considered whether the typically higher metabolism estimates found with sonde methods are due to contributions from littoral habitats not reflected by bottle estimates. We sought the source of littoral contributions by selecting sites with maximum differences in macrophyte abundance. Sonde estimates for pelagic primary production and respiration were two-three times greater than bottle estimates. Sonde/bottle ratios were higher in productive lakes and lakes with more littoral area. Bottle estimates were similar among all sites, and sonde estimates in macrophyte-poor sites were similar to pelagic sondes. However, sonde estimates in macrophyte-rich areas were four-nine times greater than bottle estimates. Results suggest littoral zones increase whole-lake NEP in eutrophic systems, whereas the Sphagnum mat surrounding dystrophic lakes decreases NEP. Non-planktonic organisms associated with macrophytes provide important littoral contributions to whole-lake metabolism and to understanding NEP trends among lakes.
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The metabolic status, the difference between organic matter production and consumption of an estuary (Randers Fjord, Denmark) has been assessed based on 2 field cruises in April and August 2001 and a number of approaches: (1) the oxygen (02) incubation method, (2) dissolved inorganic carbon (DIC) budgets, (3) the response surface difference (RSD) method based on diel O-2 changes and (4) land-ocean interaction in the coastal zone (LOICZ) budgets based on dissolved inorganic phosphorus (DIP). Although each method has its own associated limitations and uncertainties, the above approaches converged most of the time in consistent metabolic estimates, both in sign and magnitude, and revealed that this system was near metabolic balance in spring (net ecosystem production: NEP similar to 0) and net heterotrophic in summer (NEP similar to -50 mmol C m(-2) d(-1)). In this shallow estuary (mean depth = 1.6 m), the benthic compartment was very active and represented 70 and 30% of the total gross primary production in April and August, respectively. NEP rates measured during this study are in the range of previously reported rates in estuaries.
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Because freshwater covers such a small fraction of the Earth’s surface area, inland freshwater ecosystems (particularly lakes, rivers, and reservoirs) have rarely been considered as potentially important quantitative components of the carbon cycle at either global or regional scales. By taking published estimates of gas exchange, sediment accumulation, and carbon transport for a variety of aquatic systems, we have constructed a budget for the role of inland water ecosystems in the global carbon cycle. Our analysis conservatively estimates that inland waters annually receive, from a combination of background and anthropogenically altered sources, on the order of 1.9PgCy−1 from the terrestrial landscape, of which about 0.2 is buried in aquatic sediments, at least 0.8 (possibly much more) is returned to the atmosphere as gas exchange while the remaining 0.9Pgy−1 is delivered to the oceans, roughly equally as inorganic and organic carbon. Thus, roughly twice as much C enters inland aquatic systems from land as is exported from land to the sea. Over prolonged time net carbon fluxes in aquatic systems tend to be greater per unit area than in much of the surrounding land. Although their area is small, these freshwater aquatic systems can affect regional C balances. Further, the inclusion of inland, freshwater ecosystems provides useful insight about the storage, oxidation and transport of terrestrial C, and may warrant a revision of how the modern net C sink on land is described.
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Measurements of the production and consumption of organic material have been a focus of aquatic science for more than 80years. Over the last century, a variety of approaches have been developed and employed for measuring rates of gross primary production (Pg), respiration (R), and net ecosystem production (Pn=Pg−R) within aquatic ecosystems. Here, we reconsider the range of approaches and applications for ecosystem metabolism measurements, and suggest ways by which such studies can continue to contribute to aquatic ecology. This paper reviews past and contemporary studies of aquatic ecosystem-level metabolism to identify their role in understanding and managing aquatic systems. We identify four broad research objectives that have motivated ecosystem metabolism studies: (1) quantifying magnitude and variability of metabolic rates for cross-system comparison, (2) estimating organic matter transfer between adjacent systems or subsystems, (3) measuring ecosystem-scale responses to perturbation, both natural and anthropogenic, and (4) quantifying and calibrating models of biogeochemical processes and trophic networks. The magnitudes of whole-system gross primary production, respiration and net ecosystem production rates vary among aquatic environments and are partly constrained by the chosen methodology. We argue that measurements of ecosystem metabolism should be a vital component of routine monitoring at larger scales in the aquatic environment using existing flexible, precise, and durable sensor technologies. Current and future aquatic ecosystem studies will benefit from application of new methods for metabolism measurements, which facilitate integration of process measurements and calibration of models for addressing fundamental questions involving ecosystem-scale processes. KeywordsAquatic ecosystems–Metabolism–Methods–History–Applications–Future
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We studied changes in air temperature (AT) in Tartu, Estonia, since 1866; ice phenology in two Estonian large lakes since the 1920s; and daily surface water temperatures (SWT) in these lakes since the 1940s. The Mann–Kendall test showed increasing AT trends in all seasons with biggest changes in spring. The strongest increase in SWT occurred in April and August. The temperature increase has accelerated since 1961. Despite significant trends in the seasonal AT and SWT of Estonian large lakes, trends in ice phenology were weak or absent, implying that the processes governing ice phenology are more complex than those governing lake SWT. Greater snowfall was associated with later ice breakup, longer duration of ice cover, and greater ice thickness, while the relationship between winter rainfall and these ice parameters was the opposite. In the deeper Lake Peipsi, ice-on occurred later and ice-off earlier than in the shallower Võrtsjärv. The dates of both ice-on and ice-off responded more sensitively to AT in the case of Peipsi than in the case of Võrtsjärv. An increase of the average November–March AT by 2°C would presumably halve the ice cover duration in Peipsi but shorten it only by about 20% in Võrtsjärv.
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We report rates of gross and net O2 production measured in vitro during JGOFS cruises in the equatorial Pacific in spring and fall, 1992. We scale O2 productivities to net and gross C production. We then compare the calculated rates with 14C production and with new/export production measured by various techniques. 14C productivities in samples incubated for 24h are about 45% of gross carbon production rates calculated from gross O2 production. The difference is compatible with expected rates of the Mehler reaction, photorespiration, excretion, and community mitochondrial respiration. 14C production rates are similar to net carbon production rates in the upper half of the euphotic zone. At lower irradiances, where net C production can be zero or less, 14C productivities lie between net community production and gross primary production. Net carbon production rates in vitro are a factor of =4–20 times greater than estimates from drifting sediment trap and tracer transport studies. This difference probably reflects anomalous accumulation of POC in bottles because of the exclusion of grazers.
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Respiration, photosynthetic production, and diffusion interact to produce the daily curve of oxygen change in a segment of flowing water. Conversely, the observed curves of oxygen in streams can be used to calculate the component rates of production, respiration, and diffusion. New production values obtained with these analyses of oxygen curves from various sources, as well as a few previously existing estimates of primary production, indicate a generally higher rate of production in flowing waters than in other types of aquatic environments. The ratio of total primary production to total community respiration is used to classify communities quantitatively according to their predominantly heterotrophic or autotrophic characteristics. Longitudinal succession within a stream tends to modify the ratio towards unity from higher values for autotrophic and from lower values for heterotrophic communities. The behavior of this ratio is described for the annual cycle in a stream, for the sequence of pollution recovery, and for diverse types of communities.
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Knowledge of the distribution of biodiversity remains poor. This situation might more readily be resolved If the species richness of certain groups of organisms indicated the richness of other, less well known groups. A spatially explicit exploration of the pattern in the predictive power that one taxon (a potential‘indicator group”) might have for the diversity of another has been performed previously. In this paper we respond to three important points that have been raised. First, we describe an additional graphical technique for visualising spatial aspects of indicator relationships. Second, we examine some of the consequences of smoothing species richness data on observed indicator relationships. Third, we consider some of the factors that may contribute to strong indicator relationships.
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We investigated the dependence of macrophyte parameters (distribution depth, width of the reed belt, density and length of shoots) on a number of abiotic factors (wind patterns, nutrient loading, slope of the littoral, granulometric sediment composition) in a large, shallow, and eutrophic Lake Võrtsjärv (270km2, average depth 2.8m) in Estonia in 1997. The macrophytes colonized 19% of the lake area, whilst 95% was potentially suitable by depth. The most affected were the floating-leaved plants that colonized the smallest percentage (6%) of areas suitable for them with regard to depth. Factor analysis revealed a strong polarisation of the lake's vegetation both in west–east and north–south direction caused by (1) dominating westerly and south-westerly winds, (2) the shape of the lake narrowing down from north to south, and (3) concentration of bigger inflows at the west and south shores of the lake. The eutrophication process caused the disappearance of several species, on one hand, and the extension of the distribution areas of other species, on the other hand. Myriophyllum spicatum L. has become the dominating species among the submerged plants, replacing the earlier dominant, Potamogeton perfoliatus L.
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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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Abstract Carbon dioxide gas flux across the air-water interface is most often treated as a ‘simple’ physical process, primarily responding to wind speed and water temperature. Available experimental data yield an exponential regression equation relating wind speed to the thickness of a stagnant boundary film through which gas diffuses to or from the water. Flux of CO2 is influenced by CO2 hydration reactions in the stagnant boundary layer. High pH and a thick stagnant boundary layer favour chemical enhancement of the CO2 gas flux. The rate of CO2 flux reflects the sum of net organic metabolism plus CaCO3 reactions. Some interesting gas-flux constraints on the rate of net organic carbon production and on global geochemical cycling of CaCO3 emerge. At high pH (circa 10), the maximum net organic carbon production which can be supported by CO2 flux across the air-water interface is about 0.06 mol C m&2 d&1. On a global scale, organic C, not atmospheric C, appears to account for the ‘CO2’ term in the classical CaCO3 dissolution-precipitation reaction.
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Free-water measurements of dissolved oxygen (DO) in lakes are becoming common and provide opportunities for estimating ecosystem processes, such as gross primary production (GPP) and ecosystem respiration (R). The models used to estimate metabolism often subsume biological processes into one parameter each for GPP and R. However, high-frequency DO observations made over days show diverse patterns at multiple time scales, suggesting a complex suite of processes controls DO dynamics. Can we improve metabolism estimates and predictive ability for DO at diel scales by adding complexity to the models? In this study, we use data from two north temperate lakes to test a variety of metabolism models representing a suite of non-linear metabolic processes. To test whether alternative models can be discriminated, we simulated DO with assumed parameter values and auto-regressive noise, and fit the models to the simulated DO. The most complex model could be discriminated from simpler models and provided the most accurate and precise predictions. However, when models were fit to observed DO data from the sensor network, the simplest model predicted DO as well as the most complex one. The added complexity did not improve model performance. An analysis of the model residuals indicates that physics may explain some of the DO pattern not predicted, especially high-frequency oscillations and anomalies that appear to coincide with weather patterns. Under reasonably stable weather conditions and at scales of a few days, simple metabolism models explain the bulk of diel DO variability. © 2008, by the American Society of Limnology and Oceanography, Inc.
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Aiming at building the carbon budget for further climate change impact research in the large and shallow northern temperate Lake Võrtsjärv, the present paper focuses on reconstructing the full phytoplankton primary production (PP) data series for the lake for the period of 1982–2009 covered by disconnected measurements, and testing the uncertainties involved both in the PP measurements and bio-optical modelling. During this 28-year period, in situ PP was measured in Võrtsjärv in 18 years with 14C-assimilation technique. We reconstructed the full time series using a semi-empirical PP simulation model based on continuously measured PAR irradiance and interpolated values of monthly measured chlorophyll a (C chl). The modelling results, which proved highly reliable during the calibration phase, correlated rather weakly with the annual PP estimates for the 18 years, which were based on 2-h incubations at midday, 1–2 times per month. Being based on continuous irradiance data, the modelled PP can be considered more reliable than the sparse measurements, especially for short to medium term studies. We demonstrate that in the long-term, the bio-optical method can be biased if changes in water colour or water level alter the light climate causing adaptive responses in the cellular chlorophyll content of light-limited phytoplankton.
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In shallow lakes with large littoral zones, epiphytes and submerged macrophytes can make an important contribution to the total annual primary production. We investigated the primary production (PP) of phytoplankton, submerged macrophytes, and their epiphytes, from June to August 2005, in two large shallow lakes. The production of pelagic and littoral phytoplankton and of the dominant submerged macrophytes in the littoral zone (Potamogeton perfoliatus in Lake Peipsi and P. perfoliatus and Myriopyllum spicatum in Lake Võrtsjärv) and of their epiphytes was measured using a modified 14C method. The total PP of the submerged macrophyte area was similar in both lakes: 12.4gCm−2day−1 in Peipsi and 12.0gCm−2day−1 in Võrtsjärv. In Peipsi, 84.2% of this production was accounted for by macrophytes, while the shares of phytoplankton and epiphytes were low (15.6 and 0.16%, respectively). In Võrtsjärv, macrophytes contributed 58%, phytoplankton 41.9% and epiphytes 0.1% of the PP in the submerged macrophyte area. Epiphyte production in both lakes was very low in comparison with that of phytoplankton and macrophytes: 0.01, 5.04, and 6.97gCm−2day−1, respectively, in Võrtsjärv, and 0.02, 1.93, and 10.5gCm−2day−1, respectively, in Peipsi. The PP of the littoral area contributed 10% of the total summer PP of Lake Peipsi sensu stricto and 35.5% of the total summer PP of Lake Võrtsjärv. KeywordsPrimary production-Epiphytes-Submerged macrophytes-Large shallow lake
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We calculated the carbon loss (mineralization plus sedimentation) and net CO2 escape to the atmosphere for 79 536 lakes and total running water in 21 major Scandinavian catchments (size range 437-48 263 km(2)). Between 30% and 80% of the total organic carbon that entered the freshwater ecosystems was lost in lakes. Mineralization in lakes and subsequent CO2 emission to the atmosphere was by far the most important carbon loss process. The withdrawal capacity of lakes on the catchment scale was closely correlated to the mean residence time of surface water in the catchment, and to some extent to the annual mean temperature represented by latitude. This result implies that variation of the hydrology can be a more important determinant of CO2 emission from lakes than temperature fluctuations. Mineralization of terrestrially derived organic carbon in lakes is an important regulator of organic carbon export to the sea and may affect the net exchange of CO2 between the atmosphere and the boreal landscape.
Article
Climatological measurements, including carbon dioxide flux density, were made from April to September in 1994 and from April to November in 1996 at a fen wetland near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study (BOREAS). For both years, the study period was warmer and drier than the 24-year climate normals. The period of CO2 uptake was similar for both years, reaching maximum measured assimilation rates of -0.55 mg m-2 s-1 in midsummer. However, warmer air temperatures and an earlier snowmelt in the spring of 1994, which led to an earlier thaw for the fen surface, and warmer and drier conditions in the fall of 1994 promoted CO2 production at times when the vascular vegetation was not photosynthesizing. As a result, in 1994 over the study period of 124 days the fen was a net source of CO2-carbon to the atmosphere, losing 30.8 g C m-2; for the same period in 1996 the fen was a net sink of CO2-carbon, assimilating -91.6 g C m-2. Given the immense store of carbon in boreal peatlands and given a growing understanding of the relative importance of the soil carbon pool to net ecosystem exchange and of the sensitivity of this carbon storage to temperature and wetness, this boreal fen's response to earlier spring warming and drier conditions extends our understanding of the impact of climate change on the carbon balance for northern ecosystems.
Article
A discussion is presented of the measurement of primary production by short‐term experimental methods and by measurement of in situ environmental changes occurring over longer periods of time. The methods described include the measurement of photosynthesis by oxygen production, carbon dioxide assimilation and the associated pH change, and uptake of radioactive carbon (C ¹⁴ ). In situ changes in the standing crop and the concentration of inorganic plant nutrients are also discussed, and the possibility explored of relating the rate of uptake of radioactive inorganic tracers to primary production. The relationship between chlorophyll and photosynthesis is reviewed and some new experimental evidence is presented which indicates the possibility of using chlorophyll concentration as an index of primary production. In the discussions of each method emphasis is placed on sensitivity, reliability, and the interpretation of the data.
Article
We have examined how some major catchment disturbances may affect the aquatic greenhouse gas fluxes in the boreal zone, using gas flux data from studies made in 1994-1999 in the pelagic regions of seven lakes and two reservoirs in Finland. The highest pelagic seasonal average methane (CH(4)) emissions were up to 12 mmol x m(-2) x d(-1) from eutrophied lakes with agricultural catchments. Nutrient loading increases autochthonous primary production in lakes, promoting oxygen consumption and anaerobic decomposition in the sediments and this can lead to increased CH(4) release from lakes to the atmosphere. The carbon dioxide (CO(2)) fluxes were higher from reservoirs and lakes whose catchment areas were rich in peatlands or managed forests, and from eutrophied lakes in comparison to oligotrophic and mesotrophic sites. However, all these sites were net sources of CO(2) to the atmosphere. The pelagic CH(4) emissions were generally lower than those from the littoral zone. The fluxes of nitrous oxide (N(2)O) were negligible in the pelagic regions, apparently due to low nitrate inputs and/or low nitrification activity. However, the littoral zone, acting as a buffer for leached nitrogen, did release N(2)O. Anthropogenic disturbances of boreal lakes, such as increasing eutrophication, can change the aquatic greenhouse gas balance, but also the gas exchange in the littoral zone should be included in any assessment of the overall effect. It seems that autochthonous and allochthonous carbon sources, which contribute to the CH(4) and CO(2) production in lakes, also have importance in the greenhouse gas emissions from reservoirs.
Article
Lakes play an important role in the cycling of organic matter in the boreal landscape, due to the frequently high extent of bacterial respiration and the efficient burial of organic carbon in sediments. Based on a mass balance approach, we calculated a carbon budget for a small humic Swedish lake in the vicinity of a potential final repository for radioactive waste in Sweden, in order to assess its potential impact on the environmental fate of radionuclides associated with organic matter. We found that the lake is a net heterotrophic ecosystem, subsidized by organic carbon inputs from the catchment and from emergent macrophyte production. The largest sink of organic carbon is respiration by aquatic bacteria and subsequent emission of carbon.dioxide to the atmosphere. Although the annual burial of organic carbon in the sediment is a comparatively small sink, it results in the build-up of the largest carbon pool in the lake. Hence, lakes may simultaneously disperse and accumulate organic-associated radionuclides leaking from a final repository.
Ogsn bpyxeybz aonocbynepa b lsxaybz d dolyoq vacce opepa. R dogpocy o ,akayce opuaybxecrouo deoecnda (Experience gained by investigations of photosynthesis and respiration in lake's water mass. Towards the question on the balance of organic matter)
  • G G Winberg
  • Bby
  • U U Epu
Winberg, G.G. (Bby,epu, U.U.), 1934. Ogsn bpyxeybz aonocbynepa b lsxaybz d dolyoq vacce opepa. R dogpocy o,akayce opuaybxecrouo deoecnda (Experience gained by investigations of photosynthesis and respiration in lake's water mass. Towards the question on the balance of organic matter). Tpyls Kbvyokoubxecroq cnaywbb d Rocbye (Proceedings of Kosino Limnological Station) 18, 5-24 (in Russian with English summary).
Present State and Future Fate of Lake Võrtsjärv. The Finnish Environment
  • R Kivimaa
  • T Huttula
  • V Podsetchine
Kivimaa, R., Huttula, T., Podsetchine, V., 1998. Hydrodynamical studies. In: Huttula, T., Nõges, T. (Eds.), Present State and Future Fate of Lake Võrtsjärv. The Finnish Environment, pp. 60-78, 209.
Manual on low-flow estimation and prediction
WMO, 2009. Manual on low-flow estimation and prediction. Operational Hydrology Report, 50, World Meteorological Organization, WMO-1029, Koblenz, 138 pp.