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Role of a productive lake in carbon sequestration within a calcareous catchment
... Researchers have found that atmospheric CO 2 can be converted into organic carbon via a series of physical, chemical, and biological processes and can be buried in sediments [7][8][9][10]. Compared to allochthonous sources [11], the organic matter (OM) component produced in situ by aquatic plants is termed autochthonous [9]; this OM is synthesized from dissolved CO 2 (CO 2aq ), either photosynthetically or chemosynthetically, by submerged macrophytes, algae, and other microbial organisms. ...
... Researchers have found that atmospheric CO2 can be converted into organic carbon via a series of physical, chemical, and biological processes and can be buried in sediments [7][8][9][10]. Compared to allochthonous sources [11], the organic matter (OM) component produced in situ by aquatic plants is termed autochthonous [9]; this OM is synthesized from dissolved CO2 (CO2aq), either photosynthetically or chemosynthetically, by submerged macrophytes, algae, and other microbial organisms. ...
... If the resulting autochthonous OM is deposited and buried in lake sediments, it can remove carbon from the short-term carbon cycle and form long-term stable carbon storage, which is similar to the marine biological carbon pump (BCP) effect [12]. Recent studies have revealed a BCP effect in which aquatic photosynthesis takes up DIC derived from carbonate weathering, and part of the generated autochthonous organic carbon is buried [5,9,10,[13][14][15][16]. Researchers have presented the carbon sink mechanism of the carbonate-weathering-derived carbon sink coupled with aquatic photosynthesis (coupled carbonate weathering (CCW)), which indicates that carbonate weathering can form long-term effective carbon sinks via organic carbon burial [14,17]. ...
Sedimentary organic matter is an important component of the metabolism of a lake’s ecosystem, and it is generally derived from both the watershed and the primary productivity of a lake. Understanding the sources of organic matter in lakes and lake trophic status is important when evaluating the quality of lake ecosystems. We summarize the spatial distribution of total nitrogen (TN), total organic carbon (TOC), TOC/TN (C/N) molar ratios, and organic carbon isotope (δ13Corg) of the surface sediments of Fuxian Lake, Yunnan–Guizhou Plateau, Southwest China, which is the second deepest freshwater oligotrophic lake in China. The results show that the distributions of TN, TOC, C/N, and δ13Corg of the surface sediments are spatially heterogeneous, which is also the case for the trophic conditions of the lake. Compared with the adjacent eutrophic lakes and typical lakes in other areas with strong human activities, the content of organic matter is at a low level. Meanwhile, the autochthonous organic carbon in the surface sediments was characterized by lower δ13Corg (−25.3~−28.5) and C/N (8.7~12.9), suggesting that the biological carbon pump effect plays a significant part in the stability of carbon sinks by coupling with carbonate weathering. Our results emphasize the importance of the carbon sink of coupled carbonate weathering and aquatic photosynthesis in the evolution of the carbon cycle in lakes. Although modern monitoring shows that Fuxian Lake is an oligotrophic lake, there are potential risks of organic nitrogen pollution with respect to surface sediments, especially in northern and southern shallow-water areas. The organic pollution of lakes can be reduced by controlling the discharge of wastewater and reducing the nutrient loading of agricultural runoff.
... Nowadays, inland lake is increasingly recognized as playing an important role in the transport, mineralization, and burial of terrestrial C, especially in the karst watershed (Nõges et al. 2016;Sironić et al. 2017;Lu et al. 2018). It has been estimated that 2.9 Pg C a −1 discharged from global landscapes, in which about 20 percent was detained in the freshwater sediments Tranvik et al. 2009). ...
... Among them, approximately 11-46% of organic C inputs were permanently captured by lakes, which originated from allochthonous or/and autochthonous organic matter (OM) Lu et al. 2018). Human activities can largely affect the origin, transport, and fate of organic C in the lake ecosystem (Li et al. 2016;Nõges et al. 2016). Recent studies have shown that the burial rate of organic C rose dramatically in the subtropical lakes of Southwest China in the past decades in response to increasing agricultural intensification and urban expansion He et al. 2020). ...
... However, the patterns of the lake C enrichment and the depositional processes in the karst watershed differed from those of other geologic settings because of the distinct hydrological processes of runoff, sediment yield, and nutrient transportation (Jiang and Ji 2013;Li et al. 2016). Little information is currently available on the composition and transformation mechanism of OM, especially organic C, as it moves through karst ecosystems (Nõges et al. 2016;Lechleitner et al. 2017). ...
Human activities have greatly altered terrestrial carbon (C) dynamics associated with vegetation cover and land use changes, thereby influencing the C sink in downstream ecosystems. However, the transport and preservation of organic C from soils that experience serious erosion in the karst area are scarce, particularly at catchment scales. In this study, chemical characteristics of organic matter (OM) isolated from the topsoil, overlying water, and lake sediments, as well as subsequent source identification, were inferred from the molecular, spectroscopic, and carbon isotopic (δ13C) signatures in a typical karst catchment, Southwestern China. The results indicated that the elemental compositions of the calcareous soil and paddy soil significantly differed from the yellow soil. High similarities existed in the fluorescence spectra of humic substances (HS) extracted from the front two soil types with those of lake sediments, indicating the homogeneous nature of OM molecular structure. The C/N ratios of six dissolved OM fractions and sedimentary HS along with δ13C values consistently reflected the primary terrestrial source. It was estimated to account for 60% of total organic C in sedimentary OM by end-member mixing modeling in accordance with soil erosion intensity and large recharge coefficient of this catchment. The evolution of soil loss and lake productivity can be well deduced from sediment records of organic C content, C/N ratio, and the specific information of HS. This research highlighted that the composition, source, and fate of OM in the karst lake was mainly dominated by the terrestrial C flux, rather than in-lake production. Furthermore, soil type and erosion intensity have significant effects on the nature of eroded OM and ultimate preservation.
... Although lakes release a large amount of carbon transported from the catchment, they currently contain more organic carbon in their sediments than the entire ocean (Downing et al. 2008). On the other hand, the inorganic carbon accumulation in sediments of lakes with high DIC input cannot be fully considered to be carbon sequestration because part of this carbon originates from weathering of fossil carbonates in the catchment (Nõges et al. 2016b). Moreover, as the majority of photosynthesizing organisms in lakes, such as cyanobacteria, green algae, and submerged macrophytes, can use HCO 3 − as a C source instead of CO 2 (Allen and Spence 1981; Aizawa and Miyachi 1986), part of the organic carbon accumulated in lake sediments originates from fossil carbonates and not from the atmosphere. ...
... In Estonia, major long-term efforts have been directed to multifaceted studies of Lake Võrtsjärv, a large shallow lake where the water level is unregulated and has a natural variability strongly associated to climatic drivers (Nõges 2004). Based on these studies, a carbon budget of a large shallow model lake system was prepared (Fig. 3, Cremona et al. 2014a, b, c;Nõges et al. 2016b). In this assessment, the balance of autotrophic and heterotrophic processes in the lake were estimated and the yearly production and destruction of organic matter and the rate of organic matter sedimentation were quantified using riverine budget calculations, atmospheric gas exchange, biomass, and production measurements of different trophic levels in the food chain, as well as stable isotope (δ 13 C, δ 15 N) analyses of most of the abiotic and biotic components of the ecosystem. ...
... For example, due to flat c d b a Fig. 3 The carbon balance of a waterbody is an integrative indicator of the integrity and functioning of aquatic food webs in a complex catchment-lake system (Binzer et al. 2016). An example of the complexity of carbon flows into and out of aquatic ecosystems: the carbon budget of the North-European shallow lake Võrtsjärv, based on the data from Cremona et al. (2014a, b, c) and Nõges et al. (2016b). a Võrtsjärv receives annually 250-420 g m −2 of organic carbon (OC): ca 70% from autochthonous sources-phytoplankton and macrophyte production-and 30% is allochthonous sources coming from the catchment. ...
Globally increasing temperature and modifications in precipitation patterns induce major environmental alterations in aquatic ecosystems. Particularly profound changes are predicted for arctic to temperate shallow lakes where modifications in temperature affect the distribution of ice and ice-free periods, thereby altering the timing of peak productivity, while changes in precipitation strongly alter water table depth with concomitant modifications in light distribution, temperature, and water chemistry, collectively altering the balance between primary production, organic matter consumption, and decomposition. Due to direct effects of temperature on primary productivity and microbial decomposition, raising temperatures alter the capacity of aquatic ecosystems for carbon sequestration and greenhouse gas release, and this affects atmospheric greenhouse gas concentrations and temperature, implying a feedback loop between environmental effects on ecosystems and climate change. Moreover, elevated temperature can modify the bioavailability of pollutants deposited in the past, and increase the probability for their uptake by aquatic organisms. The latter processes in turn reduce primary productivity and alter microbial decomposition, creating thus another key feedback loop between productivity, climate change, and environmental pollutants. However, warming can also enhance eutrophication and deposition of pollutants in organic sediments, further speeding up productivity and eutrophication, with the overall net effects depending on the quantitative significance of different processes. Therefore, the feedbacks arising from pollution stress must be incorporated in models intending to predict the carbon balance of aquatic ecosystems under globally changing environmental conditions. Further work on carbon balance and greenhouse gas release of aquatic ecosystems should focus on quantitative characterization of the feedback loops operative, and on how global change affects these feedback loops.
... In fluvial networks, there are two major processes that determine the concentration gradient of CO2 (by adding CO2 to or removing CO2 from the water) between lentic (solid arrows in Figures 1.2a) or lotic (solid arrows in Figures 1.3a) waterbodies and the atmosphere: i) internal aquatic mineralization of organic matter (OM) [Cole et al., 2000;Duarte and Prairie, 2005]), and ii) external surface and subsurface hydrological inputs of water with high dissolved inorganic carbon (DIC) content derive from either soil respiration [Humborg et al., 2010a;Maberly et al., 2012] or mineral weathering within the catchment Nõges et al., 2016]. Likewise, the role of geochemical reactions such as precipitation and dissolution of carbonate minerals [Otsuki and Wetzel, 1974;Stets et al., 2009;Nõges et al., 2016] or photochemical mineralization of organic solutes [Amon and Benner, 1996;Cory et al., 2014;Vachon et al., 2016] on the concentration gradient of CO2 remains largely undefined both in lentic (dashed arrows in Figure 1.2a) and lotic (dashed arrows in Figure 1.3a) freshwater ecosystems. ...
... In fluvial networks, there are two major processes that determine the concentration gradient of CO2 (by adding CO2 to or removing CO2 from the water) between lentic (solid arrows in Figures 1.2a) or lotic (solid arrows in Figures 1.3a) waterbodies and the atmosphere: i) internal aquatic mineralization of organic matter (OM) [Cole et al., 2000;Duarte and Prairie, 2005]), and ii) external surface and subsurface hydrological inputs of water with high dissolved inorganic carbon (DIC) content derive from either soil respiration [Humborg et al., 2010a;Maberly et al., 2012] or mineral weathering within the catchment Nõges et al., 2016]. Likewise, the role of geochemical reactions such as precipitation and dissolution of carbonate minerals [Otsuki and Wetzel, 1974;Stets et al., 2009;Nõges et al., 2016] or photochemical mineralization of organic solutes [Amon and Benner, 1996;Cory et al., 2014;Vachon et al., 2016] on the concentration gradient of CO2 remains largely undefined both in lentic (dashed arrows in Figure 1.2a) and lotic (dashed arrows in Figure 1.3a) freshwater ecosystems. ...
... However, there is still little information about the relative contribution of these major sources to the CO2 emissions from lotic and lentic waterbodies located within fluvial networks. Likewise, the role of other less known processes such as internal geochemical reactions of calcite precipitation usually occurring in alkaline waterbodies [Otsuki and Wetzel, 1974;Stets et al., 2009;Nõges et al., 2016] or photochemical mineralization of organic solutes [Amon and Benner, 1996;Cory et al., 2014;Vachon et al., 2016] on sustaining CO2 supersaturation and emission in aquatic ecosystems is still largely undefined. ...
Inland waters are active components of the global carbon (C) cycle that transform, store and outgas more than half of the C they receive from adjacent terrestrial ecosystems. In particular, C emissions from fluvial networks to the atmosphere represent a substantial flux in the global C cycle. However, fundamental uncertainties regarding the spatiotemporal patterns, controls and sources of C gas fluxes in fluvial networks still exist. For instance, current biogeochemical models addressing C transport and processing in fluvial networks from a continuous perspective, do not integrate the effects of local discontinuities such as river impoundment or stream flow intermittency on the dynamics of C gas fluxes.
The present dissertation aims to examine how flow discontinuities (i.e., river impoundment, flow fragmentation and drying) shape the spatiotemporal patterns, the controls and the sources of C gas fluxes in a Mediterranean fluvial network. The study was performed from December 2012 to March 2015 in the Fluvià River (NE Iberian Peninsula). This river is characterized by a high density of impounded waters associated to small water retention structures (SWRS; i.e., weirs and small to very small impoundments with a surface area < 0.1 km2 and a volume < 0.2 hm3) as well as fragmented river sections dominated by isolated water pools and dry riverbeds coinciding with dry periods.
Results of this dissertation show that river discontinuities associated to SWRS and flow intermittency modulate the spatiotemporal patterns, controls and sources of C gas fluxes in the studied fluvial network. However, the magnitude of these effects varied depending on the nature of the discontinuity (i.e., river impoundment or flow intermittency), the type of C gas (i.e., carbon dioxide (CO2) or methane (CH4)) and the hydrological condition (i.e., high or low flow).
The presence of SWRS, despite their relatively small water capacity, attenuated the turbulent conditions occurring in free-flowing river sections. As a consequence, the diffusive CO2 emissions from impounded waters were significantly lower than from free-flowing river sections. Contrarily, no reduction in CH4 emissions from impounded river sections associated to the presence of SWRS was detected. This result suggests that the higher internal CH4 production at the impounded river sections, which remained very stable over time, compensated the attenuated physical effect on CH4 emissions. Despite potential inaccuracies in capturing the temporal and spatial heterogeneity, ebullition was the predominant pathway of CH4 emissions in impounded river sections. Moreover, sources other than internal metabolism (i.e., external inputs, internal geochemical reactions or photochemical mineralization) sustained most of the fluvial network CO2 emissions. Specifically, the magnitude and sources of CO2 emissions depended on flow conditions in the free-flowing sections, whereas they remained relatively stable and independent of hydrological variation in the impounded river sections.
The channels of temporary rivers remain as active biogeochemical habitats, degassing significant amounts of CO2 to the atmosphere after flow cessation. In contrast, the CH4 efflux from dry beds was undetectable in almost all cases, most likely due to the high aeration limiting the redox requirements for microbial CH4 production. Our results also suggest that the source of CO2 emitted from dry riverbeds remains unclear, although CO2 produced from biological mineralization of fresh and labile organic matter fractions could be an important source.
Future hydrological scenarios considering the combined effects of climate change and human pressures on water resources in the Mediterranean region show the rather low sensitivity of the annual CO2, CH4 and total C emissions to shifts in river discharge. In contrast, they stress the high sensitivity of annual CH4 and total C emissions to shifts in the surface area of lentic waterbodies associated to SWRS.
Overall, the main findings of this dissertation point to the need for a shift away from a continuous and system-centric view to a more inclusive approach that incorporates spatiotemporal discontinuities (i.e., SWRS and flow fragmentation and drying) as a suitable framework to understand the dynamics of C gas fluxes in fluvial networks. We acknowledge that our results represent a first approximation to better understand the role of flow discontinuities on C gas fluxes from fluvial networks. Further work on the temporal and spatial patterns of the C gas fluxes is needed to provide a more conclusive understanding.
... Aquatic photosynthesis in cyanobacteria and macrophytes absorbs carbon dioxide (CO2) from the atmosphere and consumes DIC and DIN in the water, converting them into organic C and N [34,35]. In this study, the TN, TPN and Chla contents in the water layers showed similar vertical variations in water depths of 0~3 m, especially in Meiliang Bay (Figures 2 and 4). ...
... Aquatic photosynthesis in cyanobacteria and macrophytes absorbs carbon dioxide (CO 2 ) from the atmosphere and consumes DIC and DIN in the water, converting them into organic C and N [34,35]. In this study, the TN, TPN and Chla contents in the water layers showed similar vertical variations in water depths of 0~3 m, especially in Meiliang Bay (Figures 2 and 4). ...
The metabolic activities of primary producers play an important role in the migration and transformation of carbon (C) and nitrogen (N) in aquatic environments. This study selected two typical areas in Taihu Lake, a cyanobacteria-dominant area (Meiliang Bay) and a macrophyte-dominant area (in the east area of the lake), to study the effects of cyanobacteria and macrophyte activities on C and N migration and transformation in aquatic environments. The results showed that total N and total particulate N concentrations in the water of the cyanobacteria-dominant area were much higher than those in the macrophyte-dominant area, which was mainly due to the assimilated intracellular N in cyanobacteria. Macrophyte activity drove a significantly higher release of dissolved organic C (DOC) in the water than that driven by cyanobacteria activity, and the DOC contents in the water of the macrophyte-dominant area were 2.4~4.6 times the DOC contents in the cyanobacteria-dominant area. In terms of the sediments, organic matter (OM), sediment total N and N species had positive correlations and their contents were higher in the macrophyte-dominant area than in the cyanobacteria-dominant area. Sediment OM contents in the macrophyte-dominant area increased from 4.19% to 9.33% as the sediment deepened (0~10 cm), while the opposite trend was presented in the sediments of the cyanobacteria-dominant area. Sediment OM in the macrophyte-dominant area may contain a relatively high proportion of recalcitrant OC species, while sediment OM in the cyanobacteria-dominant area may contain a relatively high proportion of labile OC species. Compared with the macrophyte-dominant area, there was a relatively high richness and diversity observed in the bacterial community in the sediments in the cyanobacteria-dominant area, which may be related to the high proportion of labile OC in the OM composition in its sediments. The relative abundances of most OC-decomposing bacteria, denitrifying bacteria, Nitrosomonas and Nitrospira were higher in the sediments of the cyanobacteria-dominant area than in the macrophyte-dominant area. These bacteria in the sediments of the cyanobacteria-dominant area potentially accelerated the migration and transformation of C and N, which may supply nutrients to overlying water for the demands of cyanobacteria growth. This study enhances the understanding of the migration and transformation of C and N and the potential effects of bacterial community structures under the different primary producer habitats.
... In inland aquatic ecosystems, the net photosynthetic uptake of dissolved inorganic carbon (DIC) by aquatic organisms is 0.233 Pg C/a (Liu et al., 2010). The biological carbon pump (BCP), which affects the uptake of DIC in carbonate weathering and forms autochthonous organic carbon (AOC) via aquatic photosynthesis, is enhanced by high concentrations of DIC in karst areas (Chen et al., 2017;Einsele et al., 2001;Lerman and Mackenzie, 2005;Liu, 2013;Maavara et al., 2017;Noges et al., 2016;Ternon et al., 2000;Yang et al., 2016). Previous studies estimated that AOC in karstic aquatic systems accounted for 45-65% of the total organic carbon (Yang et al., 2016;Huang et al., 2020;Sun et al., 2021). ...
... DOC accounts for more than 80% of the TOC, especially in clear karst water, and is the dominant organic component in karstic aquatic ecosystems. As such, DOC composition, formation, and transformation is important for calculating autochthonous DOC and accurately determining organic carbon sink in karstic aquatic ecosystems (Noges et al., 2016). The origin and composition of DOC is closely associated with microbes , but problem remains on critical microbial species and their formational processes of microbial sourced DOC. ...
Recalcitrant dissolved organic carbon (RDOC) resulting from microbial carbon (MCPs) holds promise as a relatively long-term natural carbon sink in marine environments. However, the RDOC formation mechanism remains uncertain in terrestrial aquatic systems. To determine the microbial impacts on autochthonous dissolved organic carbon (DOC), RDOC formation, and the critical influencing bacteria species, spatial changes in hydrochemistry, carbon isotopes, and microbial diversity were investigated in water samples from the karstic Lijiang River, southwest China. Samples were collected at various locations along the river system in May and July 2017. The biodegradable DOC (BDOC), RDOC, soil sourced DOC (SDOC), submerged aquatic vascular plant sourced DOC (PDOC) and microbial sourced DOC (MDOC) were calculated using the in-situ microbial incubation method, stable carbon isotopes and C/N ratio. RDOC accounted for 67% to 93% of DOC concentrations, measuring 1.3 mg/L and 1.2 mg/L in May and July, respectively. In May, BDOC concentrations increased by 0.05 mg/L from 0.18 mg/L to 0.23 mg/L, but decreased by 0.43 mg/L from 0.66 mg/L to 0.23 mg/L in July. The spatiotemporal variation of BDOC indicated photosynthesis was the main BDOC source and induced high autochthonous DOC formation, especially in May. However, RDOC was the dominant accumulation component in Lijiang River. MDOC increased by 0.86 mg/L from 0 to 0.86 mg/L in May and 0.78 mg/L from 0.10 mg/L to 0.88 mg/L in July, which was the dominant accumulated DOC and RDOC component. The abundance of Sporichthyaceae accounted for 3.4%–22.6% in May and Novosphingobium accounted for 3.5%–34.0% in July. These were the critical bacteria species induced MDOC formation, which were confirmed by their abundances in KEGG pathway modules determined by PICRUAST2. These results demonstrate that heterotrophic bacteria dominate autochthonous DOC and RDOC formation in the karst surface river, which is valuable for understanding organic carbon cycling in karstic aquatic systems.
... In temperate regions and boreal forest in areas of carbonate terrain, dissolved inorganic carbon is the dominant form of aquatic C Stets et al., 2009). Terrestrial carbonate weathering is considered a sink for CO 2 (Marcé et al., 2015;Noges et al., 2016). However, as our results indicate, the salinity and alkalinity budgets of the lake/ reservoir sediments under climate change differ from those of the catchment soils (Fig. 7). ...
... Therefore, carbonate dissolution may be an important process in highly alkaline lakes (pH ≥ 10), which would synchronously lower the sedimentary EC and pH and consuming CO 2 to produce HCO 3 − . Meanwhile, carbonate dissolution, which increases dissolved inorganic carbon concentrations, may represent a net atmospheric CO 2 sink as primary productivity consumes DIC, resulting in organic carbon burial Martin, 2017;Noges et al., 2016). To summarize, the studied alkaline lakes/reservoirs are sensitive recorders of climate change and regulators of the carbon budget, mediated by processes associated with changes in salinity and alkalinity. ...
The rising temperatures, increased evaporation, and altered precipitation patterns associated with global warming pose threats to aquatic ecosystems, especially the salinization of lake water and changes in the terrestrial carbon budget. We studied a series of samples of catchment soils, surface sediments, and sediment cores
from 51 lakes and reservoirs covering an extensive climatic range in northeastern China. Measurements included salinity indices (electrical conductivity and pH) and other physicochemical parameters, including magnetic properties and color (chroma). The results indicate that the occurrence of salt minerals and the salinity of the lake sediments are dominated by the arid climatic conditions of the region. This enabled us to develop climatic transfer functions between salinity, precipitation and evaporation, with potential applications in paleoclimatic research. As carbonates are the dominant salts in most of the studied lakes and reservoirs, past salinity variations are likely
reflected by changes in HCO3 - and CO3 2- concentrations, which provides the opportunty to study the response of water-CO2-carbonate interactions to climate change. Our findings emphasize the important role of alkaline lakes in carbon burial and carbon neutralization, in the context of ongoing global warming.
... High rates of calcite precipitation are often found during the summer season when calcite saturation peaks due to enhanced primary production that raises the pH [9][10][11][12]. The abundance of nucleation sites provided by autotrophic picoplankton, particularly picocyanobacteria, further facilitates calcite precipitation in the pelagic zones of lakes [13][14][15], which in some cases reach such intensities where lakes take on a milky appearance caused by the calcite crystals that can be seen from satellite imagery [16]. ...
... Calcification can have a significant effect on the C budget of a lake, simultaneously acting as a C sink to the sediments [10] and increasing the water CO2 partial pressure (pCO2) due to the CO2 release caused by the reaction of calcium carbonate formation [20,21]. However, calcification is not accounted for in C budgets or in lake models for two reasons. ...
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.
... Bicarbonate inputs to inland waters can thus be transformed to calcite (CaCO 3 ) and CO 2 (i.e. calcite precipitation; Deemer et al., 2020;Müller et al., 2016;Figure 4c), thereby simultaneously generating CO 2 emissions and inorganic C accumulation in the sediments (Einsele et al., 2001;Müller et al., 2006;Nõges et al., 2016). ...
... Although calcite precipitation can explain a significant proportion of CO 2 emissions in some lakes and reservoirs with high pH and alkalinity (Khan et al., 2020;Marcé et al., 2015), this still represents a small fraction of the total HCO − 3 pool and most of the transiting HCO − 3 will be transported to downstream systems (Knoll et al., 2013;Nõges et al., 2016). Bicarbonate can also be assimilated by algae (McConnaughey et al., 1994;Sand-Jensen et al., 2018) and aquatic plants, especially when CO 2 is limiting (Iversen et al., 2019). ...
Inland waters receive a significant quantity of carbon (C) from land. The fate of this C during transit, whether it is emitted to the atmosphere, accumulated in sediments or transported to the ocean, can considerably reshape the landscape C balance. However, these different fates of terrestrial C are not independent but are instead linked via several catchment and aquatic processes. Thus, according to mass conservation , any environmental change inducing a shift in a particular C fate should come at the expense of at least one other fate. Nonetheless, studies that have investigated C emission, accumulation and transport concertedly are scarce, resulting in fragmented knowledge of the role of inland waters in the global C cycle. Here, we propose a framework to understand how different C fates in aquatic systems are interlinked and covary under environmental changes. First, to explore how C fates are currently distributed in streams, rivers, reservoirs and lakes, we compiled data from the literature and show that 'C fate allocation' varies widely both within and among inland water systems types. Secondly, we developed a framework that integrates C fates in any inland water system by identifying the key processes underlying their linkages. Our framework places the partitioning between the different C forms, and how this is controlled by export from land, internal transformations and hydrology, as central to understanding C fate allocation. We argue that, by focusing on a single fate, studies could risk drawing misleading conclusions regarding how environmental changes will alter the role of inland waters in the global C cycle. Our framework thus allows us to holistically assess the consequences of such changes on coupled C fluxes, setting a foundation for understanding the contemporary and future fate of land-derived C in inland water systems. K E Y W O R D S carbon cycle, conceptual framework, coupled fluxes, global change, inland waters, terrestrial carbon fate
... Carbonate weathering rates are several orders of magnitude faster than those of silicates (Beaulieu et al., 2012;Dreybrodt, 1988;White and Brantley, 2003;Zeng et al., 2019b). With the biological carbon pump effect, in which there is aquatic photosynthetic uptake of carbonate weathering-derived DIC and burial of some of the resulting (autochthonous) organic carbon (OC) (Chen et al., 2017;Einsele et al., 2001;Lerman and Mackenzie, 2005;Liu et al., 2015;Maavara et al., 2017;Nõges et al., 2016;Ternon et al., 2000;Yang et al., 2016), the carbon sink via coupled carbonate weathering may be an important contributor to both short-term and long-term climate change (Liu et al., 2010(Liu et al., , 2011(Liu et al., , 2018. ...
... Lakes can be ideal locations for the preservation of autochthonous OC. Researchers have found that terrestrial lakes and reservoirs have significant potential as carbon sinks in the global carbon cycle (Anderson et al., 2013Battin et al., 2009;Buffam et al., 2011;Clow et al., 2015;Cole et al., 2007;Dean and Gorham, 1998;Dietz et al., 2015;Dong et al., 2012;Downing et al., 2008;Einsele et al., 2001;Gui et al., 2013;Heathcote and Downing, 2012;Heathcote et al., 2015;Huang et al., 2017Huang et al., , 2018Kastowski et al., 2011;Mendonça et al., 2016Mendonça et al., , 2017Nõges et al., 2016;Tranvik et al., 2009;Yu et al., 2015;Zhang et al., 2017Zhang et al., , 2018. OC burial in lacustrine sediments is characterized by comparatively rapid accumulation (Dean and Gorham, 1998;Cole et al., 2007;Mendonça et al., 2016) and a high preservation factor that on average is 50 times greater than that observed in the oceans (Einsele et al., 2001). ...
... As the lake lies within the hemiboreal region, it is covered with ice for 131 days a year, on average, and the snowmelt in spring causes seasonal high water. These water-level changes play a crucial role in Võrtsjärv ecosystem functioning and carbon-cycling processes (Nõges et al., 2003(Nõges et al., , 2016b. The northern part of the lake is, except for the shoreline, poorly vegetated and turbid (Secchi depth often \ 1 m in ice-free season) and constitutes the overwhelming part of Võrtsjärv surface area and volume. ...
... Group sampling and biomass measurement methods are described in Nõges et al. (2016a) for fish, phytoplankton, and zooplankton; and in Cremona et al. (2014b) and Kangur (2015) for macroinvertebrates. The detritus biomass was calculated from the organic carbon burial values of Nõges et al. (2016b) and then converted to wet mass with Brey's (2001) empirical relationship. ...
We modeled energy transfer and trophic position of fish, plankton, and macroinvertebrates and the relative importance of top-down versus bottom-up processes in Lake Võrtsjärv, a large shallow eutrophic lake in Estonia (northeastern Europe). We employed input values based on 37 years of biomass and fishing activity monitoring for calibrating the Ecopath with Ecosim (EwE) model. Energy flows from primary producers and detritus, represented by total system throughput, were nearly equal (51 and 49%, respectively). Simulation revealed that top-down and bottom-up forces were at play, metazooplankton was not efficiently grazing phytoplankton production, and a trophic cascade proceeded through macroinvertebrates rather than through zooplankton. Detritivory was responsible for the relatively low trophic position of Võrtsjärv fish compared to other lakes. Bottom-up processes were the main drivers for the dual, primary production- and detritus-based pathways in energy flow. Our findings suggest that the predicted biomass increase of cyanobacteria in shallow lakes in the future will strengthen the reliance of consumers on the detrital pathway at the expense of the primary production pathway.
... DIC (the sum of CO 2 (aq), H 2 CO 3 , HCO 3 − , and CO 3 2− , which occurs mainly as HCO 3 at 6.5 < pH < 10) in surface water systems, is consumed by aquatic phototrophs on the continents and in the ocean (Invers et al., 2001;Kahara and Vermaat, 2003;Cassar et al., 2004;Iglesias-Rodriguez et al., 2008;Pedersen et al., 2013;Liu et al., 2015;Yang et al., 2015;Noges et al., 2016;Yang et al., 2016;Chen et al., 2017;Liu et al., 2017): ...
... However, questions remain open about the sources of DOC and/or TOC: are the DOC and/or TOC in these rivers or lakes soil-derived (allochthonous) or formed in aquatic ecosystems (autochthonous) (Bianchi et al., 2004;Bianchi et al., 2007;Yang et al., 2016)? If autochthonous, the long-term increases in OC concentrations in the world's surface water systems may also be related to fertilization by increased DIC concentrations (Iglesias-Rodriguez et al., 2008;Yang et al., 2016) if light and/or other nutrients were not limiting factors, especially under increased nutrient additions by human activities, and thus they may be linked to CCW (Liu et al., 2010b;Liu et al., 2015;Yang et al., 2015;Noges et al., 2016;Yang et al., 2016;Chen et al., 2017;Liu et al., 2017). ...
Carbonate mineral weathering coupled with aquatic photosynthesis on the continents, herein termed coupled carbonate weathering (CCW), represents a current atmospheric CO2 sink of about 0.5 Pg C/a. Because silicate mineral weathering has been considered the primary geological CO2 sink, CCW's role in the present carbon cycle has been neglected. However, CCW may be helping to offset anthropogenic atmospheric CO2 increases as carbonate minerals weather more rapidly than silicates. Here we provide an overview of atmospheric CO2 uptake by CCW and its impact on global carbon cycling. This overview shows that CCW is linked to climate and land-use change through changes in the water cycle and water-born carbon fluxes. Projections of future changes in carbon cycling should therefore include CCW as linked to the global water cycle and land-use change.
... However, there is still little information about the relative contribution of these major sources to the CO 2 emissions from lotic and lentic waterbodies located within fluvial networks. Likewise, the role of other less known processes such as internal geochemical reactions of calcite precipitation usually occurring in alkaline waterbodies [Otsuki and Wetzel, 1974;Stets et al., 2009;Nõges et al., 2016] or photochemical mineralization of organic solutes [Amon and Benner, 1996;Cory et al., 2014; Vachon et al., 2016] on sustaining CO 2 supersaturation and emission in aquatic ecosystems is still largely undefined. ...
... Precipitation and dissolution of carbonate minerals may, respectively, produce or consume CO 2 in fluvial networks. Considering the high alkalinity of our fluvial network (mean=4.1 meq L -1 , n=144), we suggest that calcite precipitation may be a relevant process contributing to the CO 2 supersaturation and emission [Otsuki and Wetzel, 1974;Stets et al., 2009;Nõges et al., 2016]. However, further investigation is needed in order to understand ...
Inland waters are significant sources of carbon dioxide (CO2) to the atmosphere. CO2 supersaturation and subsequent CO2 emissions from inland waters can be driven by internal metabolism, external inputs of dissolved inorganic carbon (DIC) derived from the catchment and other processes (e.g., internal geochemical reactions of calcite precipitation or photochemical mineralization of organic solutes). However, the sensitivity of the magnitude and sources of CO2 emissions to fluvial network hydromorphological alterations is still poorly understood. Here, we investigated both the magnitude and sources of CO2 emissions from lotic (i.e., running waters) and lentic (i.e., stagnant waters associated to small dams) waterbodies of a Mediterranean fluvial network by computing segment-scale mass balances of CO2. Our results showed that sources other than internal metabolism sustained most (82%) of the CO2 emissions from the studied fluvial network. The magnitude and sources of CO2 emissions in lotic waterbodies were highly dependent on hydrology, with higher emissions dominated by DIC inputs derived from the catchment during high flows, and lower emissions partially fueled by CO2 produced biologically within the river during low flows. In contrast, CO2 emissions in lentic waterbodies were low, relatively stable over the time and the space, and dominated by DIC inputs from the catchment regardless of the different hydrological situations. Overall, our results stress the sensitivity of fluvial networks to human activities and climate change, and particularly highlight the role of hydromorphological conditions on modulating the magnitude and sources of CO2 emissions from fluvial networks.
... This balance considered hydrological and biogeochemical processes affecting dissolved inorganic carbon (DIC), DOC and POC. Nõges et al. (2016) showed that most of the carbon species flow through the lake without changing their category. ...
Lakes play a crucial role in the nutrient cycling of Earth, despite covering only a small fraction of the planet’s surface. Their interactions with their surrounding catchment areas significantly impact ecosystems and regulatory services. The connection between a lake and its catchment, especially the drainage ratio (catchment area to lake surface area), shapes the characteristics of lakes and their response to catchment processes. Within the catchment area, geological, land cover, and land use factors influence the composition of stream water that flows into the lake. These factors play a role in transporting various substances, both organic and inorganic, to the streams. Lakes act as dynamic filters, altering the chemical composition of water that flows through them. This study aims to investigate how a large, shallow lake impacts the quality of the river water as it passes through. It builds on an analysis of nutrient (carbon, nitrogen, phosphorus, silicon) fluxes into Lake Võrtsjärv, using six years of monthly monitoring data from five main inflows and the outflow. The research explores how catchment characteristics and hydrology affect nutrient concentrations and loadings into the lake, as well as the retention or release of substances by the lake. Findings reveal that catchment characteristics, such as land use and forest cover, significantly influence water quality parameters. Different inflows showed variations in water quality, and annual variations were observed, largely correlated with precipitation and discharge. Võrtsjärv plays a critical role in retaining or releasing nutrients, with varying impacts depending on the water budget of the lake. In years with a positive water balance, the lake retains all nutrients, whereas in dry years only inflowing N and P loads exceed their outflow. Overall, this study underscores the importance of lakes as integral components of catchment ecosystems, shedding light on their complex interactions with the environment and the implications for water quality. It emphasizes the need for careful consideration of land use and hydrological factors in managing and preserving these vital aquatic systems.
... Highly human-impacted eutrophic aquatic ecosystems vary in their nutrient concentrations, dissolved organic matter quality, primary productivity, and degree of watershed development [17][18][19][20], which can potentially exert an influence on carbon cycling in lakes (e.g., carbon transformation, burial, and CO 2 emissions) [21,22]. Recently, the impact of eutrophication on lake CO 2 variability has received increasing awareness and interest [23][24][25]. ...
Lakes are considered sentinels of terrestrial environmental change. Nevertheless, our understanding of the impact of catchment anthropogenic activities on nutrients and the partial pressure of carbon dioxide (pCO2, an important parameter in evaluating CO2 levels in water) is still restrained by the scarcity of long-term observations. In this study, spatiotemporal variations in nutrient concentrations (total nitrogen: TN, total phosphorus: TP, nitrate: NO3−–N, and ammonium: NH4+–N) pCO2 in Taihu Lake were analyzed from 1992 to 2006, along with the gross domestic product (GDP) and wastewater discharge (WD) of its catchment. The study area was divided into three zones to characterize spatial heterogeneity in water quality: the inflow river mouth zone (Liangxi River and Zhihugang River), transition zone (Meiliang Bay), and central Taihu Lake, respectively. It is abundantly obvious that external nutrient inputs from the catchment have a notable impact on the water parameters in Taihu Lake, because nutrient concentrations and pCO2 were substantially higher in the inflow river mouth zone than in the open water of Meiliang Bay and central Taihu Lake. The GDP and WD of Taihu Lake’s catchment were significantly and positively correlated with the temporal variation in nutrient concentrations and pCO2, indicating that catchment development activities had an impact on Taihu Lake’s water quality. In addition, pCO2 was negatively correlated with chlorophyll a and the saturation of dissolved oxygen, but positively correlated with nutrient concentrations (e.g., TN, TP, and NH4+–N) in inflow river mouth zone of Taihu Lake. The findings of this study reveal that the anthropogenic activities of the catchment not only affect the water quality of Taihu Lake but also the CO2 concentrations. Consequently, catchment effects require consideration when modeling and estimating CO2 emissions from the extensively human-impacted eutrophic lakes.
... The missing carbon sink associated with the strengthening of autochthonous production (AP) in inland ecosystems has been estimated to range from 0.38 to 1.8 Gt C yr − 1 with large uncertainties . AP is an important carbon sink and dissolved organic carbon (DOC) accounts for more than 80% of the total organic carbon (TOC), especially in karst lake water columns, DOC is the dominant indicator of autochthonous productivity in karstic aquatic ecosystems (Noges et al., 2016;He et al., 2022), which is generally neglected in the calculation of the carbon budget based on the assumption that DOC is readily mineralised by planktonic bacteria (Sawakuchi et al., 2017;Drake et al., 2018;Ran et al., 2021). Therefore, it is crucial to study the stability of DOC in inland waters to assess missing global carbon sinks around the world. ...
Biological carbon pump (BCP) in karst areas has received intensive attention for years due to their significant contribution to the global missing carbon sink. The stability of autochthonous dissolved organic matter (Auto-DOM) produced by BCP in karst aquatic ecosystems may play a critical role in the missing carbon sink. However, the source of dissolved organic matter (DOM) in inland waters and its consumption by planktonic bacteria have not been thoroughly examined. Recalcitrant dissolved organic matter (RDOM) may exist in karst aquatic ecosystem as in the ocean. Through the study of the chromophoric dissolved organic matter (CDOM) and the interaction between CDOM and the planktonic bacterial community under different land uses at the Shawan Karst Water-carbon Cycle Test Site, SW China, we found that C2, as the fluorescence component of Auto-DOM mineralised by planktonic bacteria, may have some of the characteristics of RDOM and is an important DOM source in karst aquatic ecosystems. The stability ratio (Fmax(C2/(C1+C2))) of Auto-DOM reached 89.6±6.71% in winter and 64.1±7.19% in spring. Moreover, correlation-based network analysis determined that the planktonic bacterial communities were controlled by different fluorescence types of CDOM, of which C1 (fresh Auto-DOM), C3 (conventional allochthonous DOM (Allo-DOM)) and C4 (the Allo-DOM mineralised by bacteria) were clustered in one module together with prevalent organic-degrading planktonic bacteria; C2 was clustered in another tightly combined module, suggesting specific microbial utilisation strategies for the C2 component. In addition, some important planktonic bacterium and functional genes (including chemotrophic heterotrophs and photosynthetic bacteria) were found to be affected by high Ca²⁺ and dissolved inorganic carbon (DIC) concentrations in karst aquatic ecosystems. Our research showed that Auto-DOM may be as an important carbon sink as the Allo-DOM in karst ecosystems, the former generally being neglected based on a posit that it is easily and first mineralized by planktonic bacteria.
... It accompanies HCO 3 − utilization by aquatic macrophytes (Eq. 2) that precipitate carbonate in alkaline bands along cell surfaces by active calcium extrusion (McConnaughey 1991;Nõges et al. 2016). Shallow lakes which are rich in submerged macrophytes may accomplish these ideal conditions because they stratify into distinct layers on a daily basis. ...
Nutrient enrichment in lakes due to municipal wastewater discharge and agricultural run-off leads to excessive growth of algae and aquatic macrophytes leading to their altered trophic states. This paper presents the effect of wastewater-induced nutrient enrichment on the anatomical changes and elemental profiling in three common aquatic macrophytes of freshwater lakes in India’s Central Gangetic Plain. It is observed that with increase in trophic state, biomineral depositions are seen in the leaf anatomy of aquatic macrophytes. Elemental variations in free-floating ( Eichhornia crassipes ), submerged ( Hydrilla verticillata ) and emergent ( Typha latifolia ) macrophytes collected from three different lakes with different catchment characteristics and trophic state using EDS (Energy-Dispersive X-Ray Spectroscopy) spectra show that with increasing trophic state, elemental constituent in the aquatic macrophytes also increases. The rhizome of Eichhornia crassipes showed the formation of calcium oxalate crystals in SEM (Scanning Electron Microscope) images and EDS analysis. Among macrophytes, floating and submerged macrophytes show a greater number of elemental constituents as compared to the emergent macrophytes. The findings of this study show that the anatomical and elemental responses of macrophytes are dependent both on the water quality and trophic state of the lakes. In-situ responses of macrophytes are based on their tolerance level against the pollution load and environmental changes. This study has important implications for understanding the response mechanism of macrophytes with changing water quality and increasing trophic state, which may help in proper management of freshwater ecosystem.
... However, in comparison with that in reservoirs, the minor variation amplitudes of DIC in rivers indicate that the dilution effect is not the key factor controlling the seasonal variations. Thus, the seasonal variations in DIC in reservoirs mainly contribute to the utilization of aquatic photosynthesis Müller et al., 2016;Nõges et al. 2016;Yang et al., 2020;Zeng et al., 2019). Here, we used Chl-a and DO to characterize aquatic photosynthesis. ...
Biological carbon pumps (BCPs) convert dissolved inorganic carbon (DIC) into autochthonous organic carbon (AOC), which is the key to form long-term stable carbonate weathering-related carbon sink. The DIC fertilization may increase the strength of BCP. As a phase of BCP, eutrophication is one of the major problems in surface water environments which shows poor water quality with harmful cyanobacteria blooms. It is generally believed that the controlling elements of eutrophication are nitrogen (N) and phosphorus (P), while the controlling elements of BCP also includes carbon (C). Meanwhile P removal by BCPs through the coprecipitation of P with calcite and Fe(III) oxyhydroxide colloids decreases its content in water bodies and prevent water from cyanobacteria eutrophication. In the present study, we examine the seasonal variations of general physiochemical parameters of the surface water, DIC, total N and total P concentrations, chlorophyll concentrations in three karst river-reservoir systems (PZR, PDR and HFR) in Guizhou Province, Southwest China. The phytoplankton community structure dynamics and the settling flux of the total P and P fractions in the settling particulate matter in PZR and HFR were also examined. It was found that: (1) the nutrient limitations of BCPs shifted from C-limitation to N- or P-limitation after the rivers were dammed; (2) P removal by BCPs reduced the total P concentration and increased the stoichiometric N:P ratio in surface waters; (3) P removal by BCPs alleviated the development of eutrophication by decreasing the relative abundance of Cyanobacteria. Our results demonstrate that the damming of a river may shift the nutrient limitation patterns of dammed karst rivers and the P removal by BCP may retard the development of water body into Cyanophyta-type eutrophication. This may have important implications for eutrophication control (i.e., strengthening BCP effect via DIC fertilization) in HCO3-Ca type surface water, especially in karst areas, which cover about 15% of the world land surface.
... Approximately 10-20 times more fast weathering rate of carbonate mineral compared to silicate (Meybeck, 1986) results in a greater CO 2 removal potential in the short timescale which makes carbonate weathering have a more significant role than silicate weathering in controlling the global carbon cycle (Liu et al., 2011;Gaillardet et al., 2019). Moreover, a growing body of new evidence suggests that coupled carbonate weathering with aquatic photosynthesis (Liu et al., 2010(Liu et al., , 2011(Liu et al., , 2018 may also generate longterm carbon sequestration via sedimentation and burial of autochthonous organic carbon (AOC) formed by the aquatic biological uptake of HCO 3 − (DIC) in natural surface waters (the 'biological carbon pump effect', Einsele et al., 2001;Lerman and Mackenzie, 2005;Hain et al., 2014;Ma et al., 2014;Nõges et al., 2016) in the following reaction:. ...
Continental rock weathering exerts a negative feedback on global warming by removing atmospheric CO2. Carbonate weathering contributes most of the dissolved inorganic carbon measured in inland waters, chiefly as HCO3⁻, though carbonate rock covers only a minor proportion of continental surface (~15%). The fast kinetics of carbonate weathering leads the trace carbonate to contribute greater HCO3⁻ loads than silicate even in silicate catchments. Thus, the uses of carbonate powders on continental surface with silicate via agricultural liming can potentially enhance the CO2 removal. Here, using an equilibrium modeling approach and results of the CMIP6 GCM model, we estimate the actual total CO2 removal (ATCR) by the weathering of both carbonate bedrock and carbonate-rich soils, at the global scale. The potential CO2 removal (PCR) which can be achieved by spreading carbonate powders on non‑carbonate lands is also estimated. The results show that the ATCR at present is approximately 0.166 Gt yr⁻¹, which is similar to the previous result estimated from the global river data base. The PCR is 0.843 Gt yr⁻¹, five times the ATCR. Future climate and land-use changes may strongly increase the ATCR. Therefore, we suggest that carbonate weathering enhancement by land-use changes can potentially help mitigate the current climate trends.
... As shown in Figure 2, in karst water systems, higher DIC and increased NO 3 À concentration due to the enhanced nitrification and human activities can promote the growth of aquatic communities (Liu et al. 2018;Zeng et al. 2019). The growing amount of algae and microorganisms in water increases the consumption of DIC and NO 3 À through photosynthesis, as the conversion of DIC to OC by photosynthesis induces the consumption of NO 3 À , and therefore, reduces the NO 3 À concentration (Pedersen et al. 2013;Nõges et al. 2016;Liu et al. 2018). In addition, during this process, DIC and NO 3 À are converted to OM and O 2 is released, which contrasts with the traditional knowledge that CO 2 is released during carbonate precipitation (Jiang 2013). ...
Carbonate bedrock regions represent that 14% of Earth's continental surface and carbon (C) sink in karst water plays an important role in the global C cycle due to the CO2 consumption during carbonate mineral weathering. Intensive agriculture and urbanization have led to the excessive input of nitrogen (N) into aquatic systems, while the high concentrations of inorganic C in the karst water might affect the N cycle. This paper summarized the characteristics of water in karst regions and discussed the N transformation coupled with the C cycle in the condition of high Ca2+ content, high pH, and high C/N ratios. Carbonates can consume more atmospheric and pedologic CO2 than non-carbonates because of their high solubility and high rate of dissolution, resulting in the higher average CO2 sink in karst basins worldwide than that in non-karst basins. Therefore, carbonate mineral weathering and aquatic photosynthesis are the two dominant ways of CO2 absorption, which are termed as coupled carbonate weathering. As the alkalinity and high C/N content of karst water inhibit the denitrification and mineralization processes, the karst aquatic environment is also served as the N sink. HIGHLIGHTS
Karst aquatic systems contain high contents of DIC, Ca2+, Mg2+, and high pH.;
C–N cycles in the karst aquatic systems are mainly related with DIC and NO3−.;
Enhanced nitrification and DIC can promote aquatic communities growth.;
Atmospheric CO2 sink in carbonate area is high.;
... 河流水化学参数是研究地表水重要的地 球化学指标，可以指示物理、化学、生物过程以及人类活动等对地表水体的影响 [3] . Gibbs将地表水系统划 分为蒸发浓缩、岩石风化、降雨控制三类过程控制 [8] ; Hu等研究了中国两大河流的水化学特征，表明其主 要受到碳酸盐和蒸发岩溶解作用的影响 [9] ；Torres等在安第斯-亚马逊流域的水化学研究表明构造活动作用 增强岩石风化 [10] ；张飞等在青海湖的研究表明水化学受自生碳酸盐沉淀的影响 [11] ；De Montety等研究表明 水化学不仅受到碳酸盐溶解的控制，还受浮游植物光合作用的影响 [12][13][14][15][16][17][18][19] . 在地表水循环过程中无机碳和有 机碳的转化会影响水化学参数，可以利用水化学指标计算地表水体的二氧化碳分压(pCO 2 )和方解石饱 和指数(SIc) ，以此来判断水库水体是CO 2 源或汇以及方解石是否可能沉积，因此利用水库水化学变化可 以用来指示无机碳循环过程. ...
To explore the impact of changes in physical, chemical and biological processes on hydrochemistry and carbon cycle as induced by damming, we investigated the suspended particulate flux, the spatiotemporal variations of major cations and anions (Ca2+、Mg2+、Na+、K+、HCO3-、SO42-、Cl-) in three karst reservoirs of Guizhou Province, i.e. Puding reservoir, Pingzai reservoir and Hongfenghu reservoir, for a whole hydrological year. The results show that carbonates weathering and photosynthesis controlled the spatiotemporal variations of ion concentrations. The hydro-chemical type of Hongfenghu Lake is Ca-Mg-HCO3-SO4, while it’s Ca-HCO3-SO4 in Puding reservior and Pingzhai reservoir. During the summer time, as the result of carbonate precipitation induced by algal photosynthesis, the concentration of Ca2+, HCO3- and SiO2 in the surface water decreased by 25.31%~45.67%, 33.12%~51.18% and 48.55%~96.34%, respectively. Besides, our results also suggests that the Stoichiometric relationship among C、N、Si could be affected by photosynthesis. And that calcite precipitation regulated the Mg2+/Ca2+ and Sr2+/Ca2+ ratios in water column. The measured inorganic carbon fluxes through sediment traps were 48.05tC/km2、88.19tC/km2、17.56tC/km2 for Pingzhai Reservoir, Puding Reservoir and Hongfenghu reservoir in summer, while it’s 38.74~84.15tC/km2、37.62~164.34tC/km2、15.97~43.26tC/km2 based on calcium and 19.37~42.08tC/km2、18.81~82.17tC/km2、7.99~21.63tC/km2 based on HCO3-. The measured IC flux in Hongfenghu reservoir is well within the range of calculation based on calcium and HCO3- . However, the measured fluxes of IC in Pingzhai reservoir and Puding reservoir are higher than the calculated which may be induced by the larger allochthonous IC input corresponding to the stronger hydrological condition. Therefore, it’s possible to make use of the actual thermal stratification hydrochemistry.
... The climate of the area is controlled by the interaction of the Asian monsoon and the westerly jet (Zhang et al., 2014). The average monthly air temperature in the Ngoring Lake basin varies Liu et al., (2018), Nõges et al., (2016) and Wachniew and Różański, (1997)). Notes: 1. CH 2 O refers to organic carbon transformed from DIC via the biological carbon pump effect. ...
A critical problem in the study of global climate change is how to identify “missing” carbon sinks. The carbon sink resulting from the weathering of carbonate and silicate minerals may make a major contribution, and it is often coupled with aquatic photosynthesis (the biological carbon pump effect - ‘BCP’). However, the lack of relevant field studies hinders our understanding of rock weathering and the carbon cycle. To address this issue, we analyzed the pH, [HCO3⁻], carbon isotopic composition of dissolved inorganic carbon (δ¹³CDIC), total organic carbon (TOC), total nitrogen (TN), TOC/TN, and the stable carbon isotopic composition of TOC (δ¹³Corg) and CaCO3 (δ¹³Ccarb) in the modern surface sediments of Ngoring Lake, in the northeastern Qinghai-Tibet Plateau (QTP), China. The results show that the efficiency of aquatic photosynthesis may be an important factor affecting the δ¹³CDIC in Ngoring Lake. δ¹³Corg in the surface sediments is positively correlated with the δ¹³Ccarb of inorganic carbon produced within the lake, especially in shallow water (<10 m). In addition, the increase in the TOC content of the surface sediment samples is accompanied by an increase in the CaCO3 content. These observations suggest that the BCP effect is the major mechanism driving the observed synchronous increases in TOC and CaCO3 in the sediments, and the range of observed carbon isotope values. Furthermore, organic carbon in the surface sediments is characterized by a relatively low C/N ratio and δ¹³Corg, suggesting it is mainly autochthonous, and is transformed within the aquatic ecosystem from rock weathering-derived DIC via the BCP effect. Therefore, the BCP effect plays an important role in the stabilization of the rock weathering carbon sink. Our results emphasize the importance of the carbon sink of coupled rock weathering and aquatic photosynthesis in the evolution of the carbon cycle in lakes in the QTP.
... The relative abundances of autochthonous and allochthonous OC in lake sediments depend closely on variations in the environmental conditions within and around the lake (Choudhary et al., 2009). In terrestrial aquatic ecosystems, photosynthetic uptake of dissolved inorganic carbon or CO 2 transforms these carbon sources into autochthonous OM (Einsele, 2001;Lerman and Mackenzie, 2005;Nõges et al., 2016;Yang et al., 2016;Chen et al., 2017;Maavara et al., 2017;He et al., 2019). Allochthonous OC, on the other hand, represents terrigenous carbon sources such as vascular plant tissues, detritus from marsh vegetation, and upland sources (Bianchi, 2007;O'Reilly et al., 2014). ...
During the past century, many lacustrine environments have changed substantially at the ecosystem level as a result of anthropogenic activities. In this study, the distributions of n-alkane homologues, carbon isotopes (δ13Corg), organic carbon, and the C/N atomic ratio in two sediment cores from Fuxian Lake (Yunnan, southwest China) are used to elucidate the anthropogenic impacts on this deep, oligotrophic, freshwater lake. The carbon preference index (CPI) of long-chain components, average chain length (ACL), proportion of aquatic macrophytes (Paq), and terrigenous/aquatic ratios (TAR) show different temporal patterns that reflect variations in biological production. Notably, the n-alkane homologues are shown to be more sensitive to environmental changes than δ13Corg and the C/N ratio. Prior to the 1950s, minor variations in the sedimentary geochemical record were likely caused by climate changes, and they represent a natural stage of lake evolution. The onset of cultural eutrophication in Fuxian Lake occurred in the 1950s, when the n-alkane proxies collectively exhibited high-amplitude fluctuations but overall decreasing trends that coincided with population growth and related increases in land-use pressure. In the 21st century, Fuxian Lake has become even more eutrophic in response to human activities, as indicated by sharp increases in C/N ratio, Paq, δ13Corg, ACL, CPI, and TAR. Our findings provide robust molecular sedimentary evidence confirming that the environmental evolution of lakes in the Yunnan–Guizhou Plateau over the past century was closely associated with enhanced anthropogenic activities.
... However, little is known about the coupled C-N cycling in karst aquatic systems. In the coupled C -N cycling process, aquatic photosynthesis can consume DIC and NO 3 − , implying that DIC can be preserved in a relatively stable form by converting DIC into OC, while water quality can be enhanced by consuming NO 3 − (Liu et al., 2018;Nõges et al., 2016;Pedersen et al., 2013). Furthermore, the coupled C -N cycling could cause carbonate precipitation in karst aquatic systems. ...
The cycling of carbon (C) and nitrogen (N) in karst aquatic systems has been shown to be closely related, with coupled control of dissolved organic carbon (DOC) concentrations through the metabolic pathways of subaquatic communities. However, the coupled CN cycling involving in the transformation of dissolved inorganic carbon (DIC) into DOC has not been well-explored. In this study, we chose the Lijiang River, a typical karst aquatic system in Southwest China as our study area and documented its diurnal and seasonal variations in terms of several hydrochemical and isotopic parameters to identify how to couple cycling for C and N. The results of the Bayesian stable isotope-mixing model showed that approximately 50% and 72% of the total DOC formed in summer and winter, respectively, represented autochthonous organic carbon in the Lijiang River. Diurnal monitoring results revealed that DIC and NO3⁻ transformations were primarily controlled by metabolic processes (photosynthesis and respiration) of subaquatic communities, accompanying DOC formation, in the Lijiang River. The consumption of DIC and NO3⁻ by aquatic photosynthesis was in the ratio of 9:1 (mol/mol) to produce autochthonous DOC, accompanying the enriched δ¹³CDIC, δ¹⁵N-NO3⁻ and δ¹⁸O-NO3⁻, with a daily variation of 7.9‰, 10.6‰ and 11.2‰, respectively. On the diurnal scale, 6.2% of the total DIC and 7.1% of the total NO3⁻ were consumed by metabolic processes of subaquatic communities and these values were consistent with their corresponding values on the interannual scale. However, the proportions of DIC and NO3⁻ utilized in the dry season were higher than those in the wet season. Approximately 1.18 × 10⁷ kg C/yr of DIC and 1.64 × 10⁶ kg N/yr of NO3⁻ were converted into organic matter by the aquatic photosynthesis, with 80% and 79% of the total DIC and NO3⁻ consumption respectively occurring in the wet season. Furthermore, the coupled CN cycling involving DIC and NO3⁻ can promote the production of autochthonous DOC, constituting a relatively long-term natural C and N sinks in karst aquatic systems.
... Furthermore, compared to paleolake systems, modern lake systems are subjected to impacts from human activities, such as volume variations and eutrophication (Nõges et al., 2008;Scheffer et al., 2001;Smith, 2005;Tao et al., 2015). Therefore, understanding how such activities influence the depositional process of carbonate in lake systems has important implications in environmental decision-making and lake management with respect to the carbon cycle (Nõges et al., 2016). ...
Plain Language Summary
The mechanisms that control the deposition, on the bottom of lakes, of carbonate matter that forms within the lakes themselves, are important for the carbon cycles at regional and global scales. Here we present data of carbonate content and carbon isotope composition from the past ~150 years recorded at Lake Wuliangsu, where the historical hydrological and ecological conditions have been well studied. We first investigate the factors that control the deposition of carbonate matter and calculate how much of it derives from recycled organic carbon using a carbon isotope mass‐balance model. Furthermore, we compile published data from lake systems across the globe and we incorporate them into our framework, so as to seek a better understanding of organic carbon recycling in a global perspective. Finally, we find that the size of the lake (area and depth) and the lake stratification play a key role in determining the contribution of recycled organic carbon to the overall carbonate deposition.
... Hence, traditional carbon budget models may underestimate carbonate weathering-related carbon sink (Meybeck 1993). Recently, research has shown that dissolved inorganic carbon (DIC) (including CO 2 (aq), H 2 CO 3 , HCO 3 − , and CO 3 2− , and mainly as HCO 3 − when water pH is 6.5-10) can be preserved in the relatively stable form of organic carbon (OC) by aquatic photosynthesis in the surface water systems (Pedersen et al. 2013;Nõges et al. 2016;Liu et al. 2017). Carbonate weathering coupled with aquatic photosynthesis could contribute to the long-term carbon sink on the continents according to the following equation, and represents atmospheric CO 2 sink of about 0.5 Pg C/year in the surface water systems, including rivers, lakes, and wetlands (Liu et al. 2018): ...
One of the most important questions in the science of global carbon cycle is how to balance the atmospheric CO2 budget. However, there is a large terrestrial carbon sink is missing. The locations, magnitudes, variations, and mechanisms responsible for this terrestrial missing carbon sink are uncertain and the focus of much continuing debate. In order to provide a basis for the assessment of carbonate weathering-related carbon sink, this study investigated the spatial and temporal variations of dissolved inorganic carbon (DIC) and particulate organic carbon (POC), and used stable carbon isotopes and hydrochemical data to quantitatively estimate the proportion of DIC transformation to POC and analyze the potential influence of the transformation of DIC into OC on the carbon isotope composition in the Lijiang River (typical karst surface water), Southwest China. The results showed that DIC concentrations are high in winter and low in summer and increase from upstream to downstream. POC concentrations are high in summer and low in spring. δ13CPOC and δ13CDIC are negatively correlated in the Lijiang River. The δ13CDIC values are high in spring and low in summer, compared with the estimated δ13CDIC(cal.); the observed δ13CDIC values have a maximum positive shift in spring and similar in the summer; the δ13CPOC values decrease from upstream to downstream, and are high in summer and low in spring. These observations indicate that the isotopic compositions of both DIC and POC are significantly affected by aquatic photosynthesis that transforms DIC into OC. In-river primary production contribute 12–54% of POC, and the contributions of carbonate rock-sourced DIC to the riverine POC range from 7 to 30%, which suggests an important sink of atmospheric CO2 in river systems and should be taken into account in global carbon cycle.
... The DIC in surface water is consumed and transformed into OC by aquatic photosynthesis in marine and terrestrial ecosystems (Invers et al. 2001;Kahara and Vermaat 2003;Liu et al. 2008;Sun et al. 2011;Pedersen et al. 2013;Yang et al. 2015;Nõges et al. 2016;Chen et al. 2017). The process of aquatic photosynthesis can be approximated to the following standard equation: ...
The coupled carbonate weathering represents a significant carbon sink and can be controlled by the riverine hydrochemical variations. However, magnitudes, variations, and mechanisms responsible for the carbon sink produced by coupled carbonate weathering are unclear. In view of this, temperature, pH, dissolved oxygen, turbidity, electrical conductivity, and discharge of the Xijiang River at Wuzhou Hydrologic Gauging Station was recorded during October 2013 to September 2015 to elucidate the temporal variations in riverine hydrochemistry and their controlling mechanisms. To obtain the complete carbon sink flux (CSF) produced by coupled carbonate weathering with terrestrial aquatic photosynthesis in the river basin, the fluxes of dissolved inorganic carbon (DIC), autochthonous organic carbon (AOC, sourced from the transformation of DIC via aquatic photosynthesis), and sedimentary AOC were all considered. The results show that seasonal hydrochemical variations in the Xijiang River were related not only to dilution effects but also aquatic primary production. These results demonstrate that the variations in discharge caused by rainfall played a dominant role in controlling the variations in the CSF due to the chemostatic behavior of DIC and dissolved organic carbon (DOC). The CSF of the Xijiang River produced by coupled carbonate weathering was calculated as 11.06 t C km⁻² a⁻¹, including DIC carbon sink flux of 6.56 t C km⁻² a⁻¹, AOC flux (FAOC) of 2.25 t C km⁻² a⁻¹, and sedimentary AOC flux (FSAOC) of 2.25 t C km⁻² a⁻¹. The FAOC and FSAOC together accounted for approximately 69% of DIC carbon sink flux, or approximately 41% of the CSF, indicating that the riverine AOC flux may be high and must be considered in the estimation of rock weathering-related carbon sinks.
... The DIC that accumulates in the aphotic hypolimnion as a result of dissolution of precipitated calcite remains unavailable to epilimnetic productivity until autumn overturn fully mixes the lake again (McConnaughey et al. 1994;Weyhenmeyer et al. 2012). Even in well-mixed shallow lakes, calcification can decrease DIC concentrations during summer because carbonate dissolution in sediments can become restricted by high pH in the entire water column (Nõges et al. 2016). Within dense stands of charophytes and rooted plants, the DIC decrease can be particularly extensive due to high calcification rates within the stands and limited exchange with the free water (Kufel and Kufel 2002). ...
In the original publication, the subscript number 2 were incorrectly added without the subscript format as ‘CO2’ and ‘O2’ in Figs. 4 and 5 legends.
... nitrogen [N] and phosphorus [P]) availability (see Dodds and Cole, 2007). Hydrologically driven inputs of inorganic C from the catchment (Weyhenmeyer et al., 2015;Wilkinson et al., 2016) and carbonate precipitation may also affect CO 2 saturation (Nõges et al., 2016;Stets et al., 2017). These processes are sensitive to large-scale climate-related factors such as variations in temperature (Sand-Jensen and Staehr, 2007;Marotta et al., 2009a) and precipitation (Sobek et al., 2005), and also to physical, biological and anthropogenic properties of the landscape surrounding aquatic systems and their catchments, such as topography, productivity, land use, area, and morphometry (Vanni et al., 2005;Ferland et al., 2012;Maberly et al., 2012;Staehr et al., 2012). ...
The role of tropical lakes and reservoirs in the global carbon cycle has received increasing attention in the past decade, but our understanding of its variability is still limited. The metabolism of tropical systems may differ profoundly from temperate systems due to the higher temperatures and wider variations in precipitation. Here, we investigated the spatial and temporal patterns of the variability in the partial pressure of carbon dioxide (pCO2) and its drivers in a set of 102 low-latitude lakes and reservoirs that encompass wide gradients of precipitation, productivity and landscape properties (lake area, perimeter-to-area ratio, catchment size, catchment area-to-lake area ratio, and types of catchment land use). We used multiple regressions and structural equation modeling (SEM) to determine the direct and indirect effects of the main in-lake variables and landscape properties on the water pCO2 variance. We found that these systems were mostly supersaturated with CO 2 (92% spatially and 72% seasonally) regardless of their trophic status and landscape properties. The pCO2 values (9–40,020 μatm) were within the range found in tropical ecosystems, and higher (p b 0.005) than pCO2 values recorded from high-latitude ecosystems. Water volume had a negative effect on the trophic state (r = −0.63), which mediated a positive indirect effect on pCO2 (r = 0.4), representing an important negative feedback in the context of climate change-driven reduction in precipitation. Our results demonstrated that precipitation drives the pCO2 seasonal variability, with significantly higher pCO2 during the rainy season (F=16.67; p<0.001), due to two potential main mechanisms: (1) phytoplankton dilution and (2) increasing inputs of terrestrial CO2 from the catchment. We conclude that at low latitudes, precipitation is a major climatic driver of pCO2 variability by influencing volume variations and linking lentic ecosystems to their catchments.
... The DIC that accumulates in the aphotic hypolimnion as a result of dissolution of precipitated calcite remains unavailable to epilimnetic productivity until autumn overturn fully mixes the lake again (McConnaughey et al. 1994;Weyhenmeyer et al. 2012). Even in well-mixed shallow lakes, calcification can decrease DIC concentrations during summer because carbonate dissolution in sediments can become restricted by high pH in the entire water column (Nõges et al. 2016). Within dense stands of charophytes and rooted plants, the DIC decrease can be particularly extensive due to high calcification rates within the stands and limited exchange with the free water (Kufel and Kufel 2002). ...
Aquatic research on primary production and carbon dynamics often ignores calcification that supports photosynthesis by producing protons and forming CO2. Calcification prevents detrimental pH rise, but causes greater decrease of dissolved inorganic carbon (DIC). Concurrent DIC replenishment is therefore essential to maintain high photosynthesis. Here we show a mean daily DIC loss of 40% (26% to photosynthesis and 14% to calcification) in surface waters during summer periods in a shallow charophyte-lake and replenishment of the DIC pool by respiration and carbonate dissolution in the bottom waters followed by nocturnal mixing. The daytime DIC assimilation in organic matter relative to oxygen production in surface waters was close to 1.0 (molar ratio), while total DIC loss markedly exceeded oxygen production because of calcification. Our results suggest that photosynthesis would rapidly become carbon limited if permanent stratification prevented transfer of DIC from bottom waters to surface waters or if permanent mixing prevented CO2 accumulation conducive to carbonate dissolution in bottom waters. This vertical transport of DIC effectively functions as a physical and biological pump supporting high metabolism in charophyte-dominated shallow lakes with recurring daily stratification and mixing.
... The total contribution of Phacotus lenticularis on the epilimnic mean carbonate concentration per square meter reached up to 21.5% during peak concentrations and on average6% over the whole investigation period (7 th June-3 rd November 2015). Taking into account that sedimentation is only responsible for about 20% of total current C-turnover from inland waters (Tranvik et al., 2009), and that at least half of the C is precipitated in the form of organic carbon compounds (Noges et al., 2016), carbonate precipitation by Phacotus lenticularis represents a small but notable part of C-turnover in lakes. In hard-water lakes, it should certainly be considered in the assessment of lake carbon cycling. ...
The importance of carbonate precipitation by phytoplankton in fresh water lakes has not been sufficiently considered in global carbon cycles and climate change scenarios. The objective of this study was to determine the influence of the calcifying bivalved phytoflagellate Phacotus lenticularis (Ehrenberg) Deising 1866 on the total calcite precipitation in five European hard-water lakes. For this purpose, an accurate mass determination of single Phacotus lenticularis shells was required. We developed a novel methodological approach to precisely determine the volume and mass of the calcified shells. Focused ion beam (FIB) techniques were employed to investigate internal structural features. Thin layer cross-sections of the shell profiles were reproduced and perforation as well as the crystalline structure of the calcite plates were monitored. 3D-shell models were computed by 360° rotation of the shell cross-sections using a Accepted Article www.jlimnol.it CAD 3D imaging software to calculate precise volumes and estimate realistic masses. In contrast to previous estimates, we determined a 2.8-fold higher shell mass of 0.86 ng CaCO3 (standard deviation SD = 0.18) for the highly massive shells at a mean volume per individual of 334.1 µm³ (SD = 70). An initial shell porosity of less than 5% was derived from thin layer cross-section images, resulting in a presumed mean shell density of 0.0026 ng µm−3. The shell diameter was significantly influenced by the lake's origin. The shells from each lake displayed substantial variations in diameter and shape. The pores in the shells showed two variations. Wider pore canals penetrated the whole shell wall, whereas small, elongated pores were located along the interspaces between calcite crystals with tabular habit. The approximate average dimensions of these calcite plates were 1.0 × 1.6 × 0.2 µm. The mean lateral wall thickness at the rim and centre of the shell were 1.98 µm (SD = 0.42) and 0.79 µm (SD = 0.17), respectively. The average carbonate precipitation by Phacotus lenticularis in relation to the total epilimnetic suspended calcite precipitation was 6.0% in the oligotrophic lake Großer Ostersee (Bavaria, Germany). During the growing season, Phacotus lenticularis contributed up to 21% of the particulate calcium carbonate in the epilimnion. These findings suggest that Phacotus lenticularis should be considered in the assessment of hard-water lake carbon cycling.
... Ecological monitoring on Lake Võrtsjärv dates back to the early 1960s and the study methods are described thoroughly by Nõges et al. (2010), Nõges, Cremona et al. (2016a), and Nõges, Järvalt et al. (2016b). Unless stated otherwise, all biomasses were given in wet weights (ww). ...
We aimed at quantifying the importance of limnological variables in the decadal rise of cyanobacteria biomass in shallow hemiboreal lakes. We constructed estimates of cyanobacteria (blue-green algae) biomass in a large, eutrophic lake (Estonia, Northeastern Europe) from a database comprising 28 limnological variables and spanning more than 50 years of monitoring. Using a dual-model approach consisting in a boosted regression trees (BRT) followed by a generalized least squares (GLS) model, our results revealed that six variables were most influential for assessing the variance of cyanobacteria biomass. Cyanobacteria response to nitrate concentration and rotifer abundance was negative, whereas it was positive to pH, temperature, cladoceran and copepod biomass. Response to total phosphorus (TP) and total phosphorus to total nitrogen ratio was very weak, which suggests that actual in-lake TP concentration is still above limiting values. The most efficient GLS model, which explained nearly two thirds (r2 = 0.65) of the variance of cyanobacteria biomass included nitrate concentration, water temperature and pH. The very high number of observations (maximum n = 525) supports the robustness of the models. Our results suggest that the decadal rise of blue-green algae in shallow lakes lies in the interaction between cultural eutrophication and global warming which bring in-lake physical and chemical conditions closer to cyanobacteria optima.
... Tranvik et al. (2009) suggested that up to 50% of the carbon buried in sediment lakes is mineralized into CO 2 and CH 4 , whereas Hanson et al. (2004) estimated that only 26% of that carbon is mineralized. These estimates can be highly variable as they depend on the specific physical, chemical and biological characteristics of each lake and on the different pathways of organic matter transformation, including aer-obic and anaerobic respiration in both benthic and pelagic habitats (Ask et al., 2009), methane production and oxidation (Bastviken et al., 2011), and inorganic carbon mineralization through calcite precipitation (Noges et al., 2016). In addition, the role that lakes will play in a warmer climate is uncertain. ...
... During spring, LH was therefore a sink of CO 2 , which has been previously reported in other lakes with alkaline pH and under-saturated C CO2 (Bartosiewicz et al., 2015;McGinnis et al., 2015). The CO 2 uptake observed in LH might be explained by intensive phytoplankton growth (Nõges et al., 2016) or CO 2 conversion to soluble bicarbonate/carbonate and the subsequent precipitation (Wanninkhof and Knox, 1996). Assuming that each of the three campaigns was equally representative of one third of the year, the average CO 2 emissions in LH were estimated to be 21.14 mmol m −2 d −1 , with total annual CO 2 emissions of 7.68 mol m − 2 (5547 kg for the entire lake surface). ...
The sub-Antarctic Magellanic ecoregion is a part of the world where ecosystems have been understudied and where the CH4 cycling and emissions in lakes has not ever been reported. To fill that knowledge gap, a lake and a reservoir located at 53°S were selected and studied during three campaigns equally distributed over one year. Among the parameters measured were CH4 and CO2 emissions, as well their dissolved concentrations in the water column, which were determined with high spatial resolution. No ebullition was observed and the CH4 flux ranged from 0.0094 to 4.47 mmol m− 2 d− 1 while the CO2 flux ranged from − 22.95 to 35.68 mmol m− 2 d− 1. Dissolved CH4 concentrations varied over more than four orders of magnitude (0.025–128.75 μmol L− 1), and the dissolved carbon dioxide ranged from below the detection limit of our method (i.e., 0.15 μmol L− 1) to 379.09 μmol L− 1. The high spatial resolution of the methods used enabled the construction of bathymetric maps, surface contour maps of CH4 and CO2 fluxes, and transect contour maps of dissolved oxygen, temperature, and dissolved greenhouse gases. Overall, both lakes were net greenhouse gas producers and were not significantly different from temperate lakes located at a similar northern latitudes (53°N), except that ebullition was never observed in the studied sub-Antarctic lakes.
... Cremona et al. (2014) explained that the C cycle in Võrtsjärv is equally driven by hydrological factors, particularly seasonal water level changes, and by biogeochemical in-lake reactions which include also CO 2 and CH 4 release to the atmosphere. Annually about 12% of carbon entering the lake was buried to the sediment, 8% released to the atmosphere, while 80% left through the outflow (Nõges et al., 2016b). Seasonally, C outflow exceeded the inflow during most of the year, except the spring flood period and late autumn when peak loads of both DIC and DOC entered the lake. ...
This study is the first attempt at the European scale to make an inventory of ecosystem services (ESS) of a large lake. We analysed a set of ESS indicators against the annual mean values of environmental parameters for 2006–2013. According to principal component analysis, the trophic state- and hydrology-related factors explained about 70% of the environmental variability of the lake and showed strong relationships with some ESS. Among the provisioning ESS, the annual eel catch and the total fish catch were positively related to different eutrophication indicators while the catches of pike, bream, and burbot depended rather on hydrological factors. Reed harvesting efficiency was related to the lake’s water level. The indicators of regulating, maintenance, and cultural ESS showed very high variability in different years, the latter depending on socio-economic conditions rather than environmental factors. We discovered numerous trade-offs between ESS benefitting from higher trophic state or regulated water level of the lake and the goals of good ecological status of the lake. Our analysis showed a clear need for rules prioritizing life supporting regulatory services against other ESS.
... Inland waters including gravel pit lakes play an important role in the carbon cycle since they are extremely active sites for transport, transformation, and storage of considerable amounts of carbon received from the terrestrial environment despite a limited surface area (Nõges et al., 2016;McDonald et al., 2013;Marcé et al., 2015;Tranvik et al., 2009). Many processes are recognized to play a role in carbonate-bicarbonate equilibrium of natural lakes (e.g. ...
Inland waters are crucial in the carbon cycle, contributing significantly to the global CO 2 fluxes. Carbonate lakes may act as both sources and sinks of CO 2 depending on the interactions between the amount of dissolved inorganic carbon (DIC) inputs, lake metabolisms, and geochemical processes. It is often difficult to distinguish the dominant mechanisms driving CO 2 dynamics and their effects on CO 2 emissions. This study was undertaken in three groundwater-fed carbonate-rich lakes in central Spain (Ruidera Lakes), severely polluted with nitrates from agricultural overfertilization. Diel and seasonal (summer and winter) changes in CO 2 concentration (C CO2 ) DIC, and CO 2 emissions-(F CO2 )-, as well as physical and chemical variables, including primary production and phytoplanktonic chlorophyll- a were measured. In addition, δ ¹³ C-DIC, δ ¹³ C-CO 2 in lake waters, and δ ¹³ C of the sedimentary organic matter were measured seasonally to identify the primary CO 2 sources and processes. While the lakes were consistently C CO2 supersaturated and F CO2 was released to the atmosphere during both seasons, the highest C CO2 and DIC were in summer (0.36–2.26 µmol L –1 ). Our results support a strong phosphorus limitation for primary production in these lakes, which impinges on CO 2 dynamics. External DIC inputs to the lake waters primarily drive the C CO2 and, therefore, the F CO2 . The δ ¹³ C-DIC signatures below –12‰ confirmed the primary geogenic influence on DIC. As also suggested by the high values on the calcite saturation index, the Miller-Tans plot revealed that the CO 2 source in the lakes was close to the signature provided by the fractionation of δ ¹³ C-CO 2 from calcite precipitation. Therefore, the main contribution behind the C CO2 values found in these karst lakes should be attributed to the calcite precipitation process, which is temperature-dependent according to the seasonal change observed in δ ¹³ C-DIC values. Finally, co-precipitation of phosphate with calcite could partly explain the observed low phytoplankton production in these lakes and the impact on the contribution to increasing greenhouse gas emissions. However, as eutrophication increases and the soluble reactive phosphorus (SRP) content increases, the co-precipitation of phosphate is expected to be progressively inhibited. These thresholds must be assessed to understand how the CO 3 ²⁻ ions drive lake co-precipitation dynamics. Carbonate regions extend over 15% of the Earth’s surface but seem essential in the CO 2 dynamics at a global scale.
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.
Groundwater discharge to headwater streams and concomitant terrestrially derived CO2 input play a key role in stream CO2 evasion. However, previous studies rarely examined the transport and transformation of terrestrially derived CO2 in headwater streams, thereby limiting our understanding of stream CO2 evasion. Here, we firstly used a numerical flow model to quantify groundwater discharge rate along a 43 km headwater stream (semiarid northwest China) by combining stream flow rate, radon-222 isotopic data, and then estimated internal metabolism through reach-scale dissolved O2 mass balance and carbonate buffering through carbon budget for the stream. Modeling results indicate that groundwater discharge rates were highly variable (2.17–7.03 m² d⁻¹, mean ± 1 standard deviation: 4.48 ± 1.25 m² d⁻¹). Groundwater was supersaturated with CO2 at all sites (5.45 ± 3.87 mg C L⁻¹). Our results also show that diffuse groundwater discharge was a major control on terrestrial carbon input to the stream. Terrestrial CO2 input (3.92 ± 2.03 g C m⁻² d⁻¹) contributed more CO2 to the stream than internal metabolism (1.74 ± 2.33 g C m⁻² d⁻¹). However, most terrestrially derived CO2 was transformed into HCO3⁻ through carbonate buffering after entering the stream, inhibiting stream CO2 evasion. This carbonate buffering process depends on the hydrochemistry and proportion of mixing waters. Overall, our study provides a bottom-up and holistic perspective to understand the transport and transformation of terrestrially derived CO2 in headwater streams.
Lakes process a disproportionately large fraction of carbon relative to their size and spatial extent, representing an important component of the global carbon cycle. Alterations of ecosystem function via eutrophication change the balance of greenhouse gas flux in these systems. Without eutrophication, lakes are net sources of CO2 to the atmosphere, but in eutrophic lakes this function may be amplified or reversed due to cycling of abundant autochthonous carbon. Using a combination of high-frequency and discrete sensor measurements, we calculated continuous CO2 flux during the ice-free season in 15 eutrophic lakes. We found net CO2 influx over our sampling period in 5 lakes (− 47 to − 1865 mmol m−2) and net efflux in 10 lakes (328 to 11,755 mmol m−2). Across sites, predictive models indicated that the highest efflux rates were driven by nitrogen enrichment, and influx was best predicted by chlorophyll a concentration. Regardless of whether CO2 flux was positive or negative, stable isotope analyses indicated that the dissolved inorganic carbon pool was not derived from heterotrophic degradation of terrestrial organic carbon, but from degradation of autochthonous organic carbon, mineral dissolution, and atmospheric uptake. Optical characterization of dissolved organic matter revealed an autochthonous organic matter pool. CO2 influx was correlated with autochthony, while efflux was correlated with total nitrogen and watershed wetland cover. Our findings suggest that CO2 uptake by primary producers during blooms can contribute to continuous CO2 influx for days to months. Conversely, eutrophic lakes in our study that were net sources of CO2 to the atmosphere showed among the highest rates reported in the literature. These findings suggest that anthropogenic eutrophication has substantially altered biogeochemical processing of carbon on Earth.
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.
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
The flux of atmospheric/soil CO 2 consumed by chemical weathering of the continents ( F CO2 ) can be estimated from river fluxes of bicarbonates. Using published data for 232 small monolithologic watersheds, empirical relationships between F CO2 and runoff ( Q ) have been determined for the major rock types outcropping on the continents. For validation, the models fitted to these relationships are applied to the Garonne, Congo and Amazon river basins to calculate the average CO 2 consumption on these large river basins; the model results are close to previous estimates based on field measurements. The model (GEM-CO 2 : Global Erosion Model for CO 2 fluxes) is then applied at the global scale and allows the determination of latitudinal variations of the consumption of atmospheric/soil CO 2 by chemical erosion. The main results show that the consumption of CO 2 is mainly localized in the Northern hemisphere, because of large continental area with a high proportion of carbonate rocks, and in equatorial regions which are very humid. Finally, the mean annual CO 2 consumption for the whole continents amount to 0.26 Gt C y -1 , and 72% of this flux is removed in the Northern hemisphere. DOI: 10.1034/j.1600-0889.47.issue1.23.x
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.
Biogenic calcite precipitation is the removal of calcite (CaCO 3) from the epilimnion to the sediments of hard-water lakes during summer stratification, caused by increased pH during algal blooms and by nucleation of calcite crystals on surfaces of micro-algae. Although this phenomenon has been studied for decades, details of the underlying mechanisms are still debated. Using results of approximately 70,000 alkalinity measurements from 13 hard-water Swiss lakes (each with approximately 30 yr to 50 yr of monitoring), we demonstrate that (i) calcium carbonate-buffered lakes act as alkalinity sinks during summer stratification but act as CO 2 sources during and immediately after spring overturn; (ii) as the alkalinity concentration ([Alk mix ]) and the total phosphorus concentration ([TP mix ]) at spring overturn increase, increasingly more alkalinity is lost from the epilimnion during summer stratification; (iii) [Alk mix ] is determined by the lake's discharge-weighted average inflow concentration ([Alk in ]), flushing rate, mean depth and [TP mix ]; and (iv) [Alk in ] depends on the mineralogy and the land use affecting in-soil nitrification of ammonia and subsequent calcite dissolution in the catchment.
The spring waters of the Sierra Nevada result from the attack of high CO2 soil waters on typical igneous rocks and hence can be regarded as nearly ideal samples of a major water type. Their compositions are consistent with a model in which the primary rock-forming silicates are altered in a closed system to soil minerals plus a solution in steady-state equilibrium with these minerals. Isolation of Sierra waters from the solid alteration products followed by isothermal evaporation in equilibrium with the earth's atmosphere should produce a highly alkaline Na-HCO3-CO3 water; a soda lake with calcium carbonate, magnesium hydroxysilicate, and amorphous silica as precipitates.
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.
The influence of functional group specific production and respiration patterns on a lake's metabolic balance remains poorly investigated to date compared to whole-system estimates of metabolism. We employed a summed component ecosystem approach for assessing lake-wide and functional group-specific metabolism (gross primary production (GPP) and respiration (R)) in shallow and eutrophic Lake Võrtsjärv in central Estonia during three years. Eleven functional groups were considered: piscivorous and benthivorous fish; phyto-, bacterio-, proto- and metazooplankton; benthic macroinvertebrates, bacteria and ciliates; macrophytes and their associated epiphytes. Metabolism of these groups was assessed by allometric equations coupled with daily records of temperature and hydrology of the lake and measurements of food web functional groups biomass. Results revealed that heterotrophy dominated most of the year, with a short autotrophic period observed in late spring. Most of the metabolism of the lake could be attributed to planktonic functional groups, with phytoplankton contributing the highest share (90% of GPP and 43% of R). A surge of protozooplankton and bacterioplankton populations forming the microbial loop caused the shift from auto- to heterotrophy in midsummer. Conversely, the benthic functional groups had overall a very small contribution to lake metabolism. We validated our ecosystem approach by comparing the GPP and R with those calculated from O2 measurements in the lake. Our findings are also in line with earlier productivity studies made with 14C or chlorophyll a (chl-a) based equations. Ideally, the ecosystem approach should be combined with diel O2 approach for investigating critical periods of metabolism shifts caused by dynamics in food-web processes.
The influence of inland water bodies on the global carbon cycle and the great potential for long-term carbon burial in them is an important component of global limnology. We used paleolimnological methods to estimate changes in carbon burial rates through time in a suite of natural lakes in the US state of Iowa which has watersheds that have been heavily modified over the last 150 years. Our results show increasing carbon burial for all lakes in our study as agriculture intensified. Our estimates of carbon burial rates, before land clearance, are similar to the published worldwide averages for nutrient-poor lakes. In nearly all the cases, burial rates increased to very high levels (up to 200 g C m−2 y−1) following agricultural development. These results support the idea that the increased autochthonous and allochthonous carbon flux, related to anthropogenic change, leads to higher rates of carbon burial. Further, these results imply that the fraction of global carbon buried by lakes will be increasingly important in the future if worldwide trends in anthropogenic eutrophication continue.
It is widely accepted that chemical weathering of Ca–silicate rocks could potentially control long-term climate change by providing feedback interaction with atmospheric CO2 drawdown by means of precipitation of carbonate, and that in contrast weathering of carbonate rocks has not an equivalent impact because all of the CO2 consumed in the weathering process is returned to the atmosphere by the comparatively rapid precipitation of carbonates in the oceans. Here, it is shown that the rapid kinetics of carbonate dissolution and the importance of small amounts of carbonate minerals in controlling the dissolved inorganic C (DIC) of silicate watersheds, coupled with aquatic photosynthetic uptake of the weathering-related DIC and burial of some of the resulting organic C, suggest that the atmospheric CO2 sink from carbonate weathering may previously have been underestimated by a factor of about 3, amounting to 0.477Pg C/a. This indicates that the contribution of silicate weathering to the atmospheric CO2 sink may be only 6%, while the other 94% is by carbonate weathering. Therefore, the atmospheric CO2 sink by carbonate weathering might be significant in controlling both the short-term and long-term climate changes. This questions the traditional point of view that only chemical weathering of Ca–silicate rocks potentially controls long-term climate change.
This study explores the variability in concentrations of dissolved CH4 and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated representatively in the boreal zone, where the mean annual air temperature ranges from −2.8 to 5.9°C. We examined how lake CH4 dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH4 concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH4 concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. CH4 content in the hypolimnion was related to oxygen and nutrient concentrations, and lake depth or area. The surface water CH4 concentration was related to the depth or area of lake. Methane close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. Median CH4 release from randomly selected lakes was 45 mmol m−2 a−1. Shallow lakes had the highest median CH4 effluxes, with the clear shallow lake type having the smallest median. Our data, combined with other studies, suggest that lake surface area could be used for an approximation of CH4 release from lakes. Shallow small lakes common in boreal and arctic landscapes may have disproportional significance with respect to CH4 release.
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.
Sulphate deposition has decreased by about 60% in the Nordic countries
since the early 1980s. Nitrogen deposition has been roughly constant
during the past 20 years, with only a minor decrease in the late 1990s.
The resulting changes in the chemistry of small lakes have been followed
by national monitoring programmes initiated in the 1980s in Finland (163
lakes), Norway (100 lakes) and Sweden (81 lakes). These lakes are partly
a subset from the survey of 5690 lakes in the Northern European lake
survey of 1995. Trend analyses on data for the period 1990-1999 show
that the non-marine sulphate concentrations in lakes have decreased
significantly in 69% of the monitored lakes. Changes were largest in
lakes with the highest mean concentrations. Nitrate concentrations, on
the other hand, were generally low and showed no systematic changes.
Concentrations of non-marine base cations decreased in 26% of the lakes,
most probably an ionic-strength effect due to the lower concentrations
of mobile strong-acid anions. Acid neutralising capacity increased in
32% of the lakes. Trends in recovery were in part masked by large
year-to-year variations in sea-salt inputs and by increases in total
organic carbon concentrations. These changes were most probably the
result of climatic variations. Nordic lakes, therefore, show clear signs
of recovery from acidification. Recovery began in the 1980s and
accelerated in the 1990s. Reductions in sulphur deposition are the major
"driving force" in the process of recovery from acidification. Further
recovery can be expected in the next 10 years if the Gothenburg protocol
on emissions of acidifying pollutants is implemented.
Middle and Upper Pleistocene till beds with stratified glaciofluvial and glaciolacustrine deposits constitute the structure of the drumlins in the Saadjärve Drumlin Field. The only interglacial deposits of the area belong to the Karuküla Formation (Holsteinian) at the distal end of the drumlin field. The differences in grain-size distribution, mineralogical and chemical composition are regarded as confirmation of the crucial role of compositional characteristics in determining the stratigraphy of these tills. Analysis shows that the Upper Sangaste (Elsterian) till, the oldest in the area, as well as the Valgjärve (Lower Weichselian) and Võrtsjärve (Upper Weichselian) tills have distinct compositional characteristics. The Lower and Upper Ugandi (Saale glaciation) tills have no lithological differences; thus there may be only one Saalian till in the area. The stratigraphy and distribution of tills in the Saadjärve Drumlin Field proves an accretionary origin of these drumlins. They started to form during Saale glaciation and the formation process lasted for several glacial advances.
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.
We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments. © 2009, by the American Society of Limnology and Oceanography, Inc.
Biostratigraphic diatom analyses were carried out on a short sediment core from the large shallow-water Lake Võrtsjärv, Estonia,
in order to relate the diatom composition to the instrumental water level record. We dated the sediment core by radiometric
methods (210Pb, 137Cs, 241Am) and spheroidal fly-ash particle abundance chronology and evaluated the statistical significance of the relationships between
the percentage of planktonic diatoms and the water level continuously monitored since 1871. Before the 1960s, the percentage
of planktonic diatoms in the sediment showed quite strong positive relationship to water level. The impact of eutrophication
after the 1960s presumably masked the influence of water level changes on the diatom community. In addition, statistical analysis
of the upper part of the sediment core (1970—present day) together with measured limnological parameters of the lake showed
that water transparency had the strongest influence on diatoms, while temperature, pH and alkalinity had lesser impacts. Our
study shows that the planktonic:periphytic diatom ratio in the sediment can be used to track overall trends of the lake-level
changes in Lake Võrtsjärv before the onset of cultural eutrophication; however, the results have to be interpreted carefully,
taking into consideration other possible limnological factors such as water transparency, nutrients and wind.
Recent, parallel developments in the study of freshwater and marine ecosystems have provided evidence that net heterotrophic
systems (those in which respiratory organic matter destruction exeeds photosynthetic production) are more prevalent than hitherto
believed, including most rivers, oligo- to mesotrophic lakes and some oligotrophic regions of the ocean. In parallel, these
aquatic ecosystems have been shown to act as CO2 sources to the atmosphere, as expected from the heterotrophic nature of the communities they contain. The prevalence of net
heterotrophic aquatic ecosystems indicates that they must receive significant inputs of organic carbon from adjacent ecosystems,
assigning an important role to the lateral exchanges of carbon between land and aquatic ecosystems, between coastal and open
ocean ecosystems, as well as internal redistribution within large or complex aquatic ecosystems in determining their metabolic
status and the gaseous exchange with the atmosphere. The examination of the carbon budget of ecosystems requires, therefore,
an integrative approach that accounts for exchanges between compartments often studied in isolation. These recent findings
conform a new paradigm of the functioning of aquatic ecosystems, and the metabolic connectivity between ecosystems in the
biosphere.
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.
Partial pressures of CO2 and CH4 were measured directly or calculated from pH and alkalinity or DIC measurements for 25 lakes and 4 rivers on the North Slope
of Alaska. Nearly all waters were super-saturated with respect to atmospheric pressures of CO2 and CH4. Gas fluxes to the atmosphere ranged from −6.5 to 59.8 mmol m−2 d−1 for CO2 and from 0.08 to 1.02 mmol m−2 d−1 for CH4, and were uncorrelated with latitude or lake morphology. Seasonal trends include a buildup of CO2 and CH4 under ice during winter, and often an increased CO2 flux rate in August due to partial lake turnover. Nutrient fertilization experiments resulted in decreased CO2 release from a lake due to photosynthetic uptake, but no change in CO2 release from a river due to the much faster water renewal time. In lakes and rivers the groundwater input of dissolved CO2 and CH4 is supplemented by in-lake respiration of dissolved and particulate carbon washed in from land. The release of carbon from
aquatic systems to the atmosphere averaged 24 g C m−2 y−1, and in coastal areas where up to 50% of the surface area is water, this loss equals frac 1/5 to 1/2 of the net carbon accumulation
rates estimated for tundra.
This study explores the variability in concentrations of dissolved CH(4) and annual flux estimates in the pelagic zone in a statistically defined sample of 207 lakes in Finland. The lakes were situated in the boreal zone, in an area where the mean annual air temperature ranges from - 2.8 to 5.9 degrees C. We examined how lake CH(4) dynamics related to regional lake types assessed according to the EU water framework directive. Ten lake types were defined on the basis of water chemistry, color, and size. Lakes were sampled for dissolved CH(4) concentrations four times per year, at four different depths at the deepest point of each lake. We found that CH(4) concentrations and fluxes to the atmosphere tended to be high in nutrient rich calcareous lakes, and that the shallow lakes had the greatest surface water concentrations. Methane concentration in the hypolimnion was related to oxygen and nutrient concentrations, and to lake depth or lake area. The surface water CH(4) concentration was related to the depth or area of lake. Methane concentration close to the bottom can be viewed as proxy of lake status in terms of frequency of anoxia and nutrient levels. The mean pelagic CH(4) release from randomly selected lakes was 49 mmol m(-2) a(-1). The sum CH(4) flux (storage and diffusion) correlated with lake depth, area and nutrient content, and CH(4) release was greatest from the shallow nutrient rich and humic lakes. Our results support earlier lake studies regarding the regulating factors and also the magnitude of global emission estimate. These results propose that in boreal region small lakes have higher CH(4) fluxes per unit area than larger lakes, and that the small lakes have a disproportionate significance regarding to the CH(4) release.
Sequestering atmospheric CO2 is necessitated by its present concentration of 385 ppm and increasing at the rate of 2 ppm y−1. Increase in atmospheric emission of CO2 with the attendant global warming and environmental degradation are driven by global energy demand. In comparison with the emission of 300 Pg C between 1850 and 2000, total emission during the 21st century is estimated at 950 to 2195 Pg, with an annual rate of emission of 20 to 35 Pg C y−1. Reduction of CO2 atmospheric loading can be achieved by biological, chemical and technological options through either reducing or sequestering emissions. This article describes technological options of sequestering atmospheric CO2 into other global pools. Geologic sequestration involves underground storage of industrially emitted CO2 into the geosphere for long-term and secure storage. Liquefied CO2 is injected about 1000 m below the ground surface either in stable porous rocks, oil wells, coal beds, or saline aquifers. Co-injecting CO
Inland waters (lakes, reservoirs, streams, and rivers) are often substantial methane (CH4) sources in the terrestrial landscape. They are, however, not yet well integrated in global greenhouse gas (GHG) budgets.
Data from 474 freshwater ecosystems and the most recent global water area estimates indicate that freshwaters emit at least
103 teragrams of CH4 year−1, corresponding to 0.65 petagrams of C as carbon dioxide (CO2) equivalents year−1, offsetting 25% of the estimated land carbon sink. Thus, the continental GHG sink may be considerably overestimated, and
freshwaters need to be recognized as important in the global carbon cycle.
Chemical weathering and the subsequent export of carbonate alkalinity (HCO3– + CO3–2) from soils to rivers account for significant amounts of terrestrially sequestered atmospheric CO2. We show here that during the past half-century, the export of this alkalinity has increased dramatically from North America's
largest river, the Mississippi. This increased export is in part the result of increased flow resulting from higher rainfall
in the Mississippi basin. Subcatchment data from the Mississippi suggest that the increase in the export of alkalinity is
also linked to amount and type of land cover. These observations have important implications for the potential management
of carbon sequestration in the United States.
Data on the partial pressure of carbon dioxide (CO2) in the surface waters from a large number of lakes (1835) with a worldwide distribution show that only a small proportion
of the 4665 samples analyzed (less than 10 percent) were within ±20 percent of equilibrium with the atmosphere and that most
samples (87 percent) were supersaturated. The mean partial pressure of CO2 averaged 1036 microatmospheres, about three times the value in the overlying atmosphere, indicating that lakes are sources
rather than sinks of atmospheric CO2. On a global scale, the potential efflux of CO2 from lakes (about 0.14 x 1015 grams of carbon per year) is about half as large as riverine transport of organic plus inorganic carbon to the ocean. Lakes
are a small but potentially important conduit for carbon from terrestrial sources to the atmospheric sink.