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Heat-wave effects on greenhouse gas emissions from shallow lake mesocosms
Abstract
Shallow lakes are a key component of the global carbon cycle. It is, therefore, important to know how shallow lake ecosystems will respond to the current climate change. Global warming affects not only average temperatures, but also the frequency of heat waves (HW). The impact of extreme events on ecosystems processes, particularly greenhouse gas ( GHG ) emissions, is uncertain.
Using the world's longest‐running shallow lake experiment, we studied the effects of a simulated summer HW on the fluxes of carbon dioxide ( CO 2 ), methane ( CH 4 ) and nitrous oxide (N 2 O). The experimental mesocosms had been exposed to different temperature treatments and nutrient loading for 11 years prior to the artificial HW.
In general, there was an increase in total GHG emissions during the 1‐month artificial HW, with a significant increase in CO 2 , CH 4 and N 2 O being observed in the shallow lake mesocosms. No significant effect of the HW on CO 2 emissions could be traced, though, in the mesocosms with high nutrient levels. Furthermore, the data suggested that in addition to the direct effect of increased temperature on metabolic processes during the HW, biotic interactions exerted a significant control of GHG emissions. For example, at low nutrient levels, increased CO 2 emissions were associated with low macrophyte abundance, whereas at high nutrient levels, decreased phytoplankton abundance was linked to increased emissions of CO 2 and CH 4 .
In contrast to the observable heat‐wave effect, no clear general effect of the long‐term temperature treatments could be discerned over the summer, likely because the potential effects of the moderate temperature increase, applied as a press disturbance, were overridden by biotic interactions. This study demonstrates that the role of biotic interactions needs to be considered within the context of global warming on ecosystem processes.
... To elucidate the effect of the HW, we followed the method used in a study of HW effects on GHG in the same HW experiment [47]. In the models, HW had three levels: HW0, the pre-and post-HW period (i.e., June and August in A0, A2 and A2+); HW1, the HW period (July in A2 and A2+); HW2, the "Ambient summer" period (July in the A0 ), the latter because A0 was not heated the extra 5 °C during the HW period but A2 and A2+ were. ...
... The sign between brackets indicates whether the effect on the response variable was positive or negative. "0": the HW1 treatment did not differ significantly from the HW2 treatment, i.e., the effect of the HW did not differ from the effect of the A0 summer temperature (see "Materials and Methods" and Audet et al. [47]). After analyzing LN and HN data separately, we found increasing Chl-a and decreasing PO4 with increasing temperature in LN, while PO4 increased in HN (Table 2). ...
... mg L −1 , indicating that eutrophic systems in particular are highly sensitive to short-term changes in weather conditions under HW conditions. Concomitant with the decline in O2, an increase in the emission of methane was observed during the HW [47]. During low-O2 period, fish kills occurred in HN-A2 and HN-A2+ as well, with cascading effects on the zooplankton [35] and the phytoplankton communities [55]. ...
Global changes (e.g., warming and population growth) affect nutrient loadings and temperatures , but global warming also results in more frequent extreme events, such as heat waves. Using data from the world's longest-running shallow lake experimental mesocosm facility, we studied the effects of different levels of nutrient loadings combined with varying temperatures, which also included a simulated 1-month summer heat wave (HW), on nutrient and oxygen concentrations , gross ecosystem primary production (GPP), ecosystem respiration (ER), net ecosystem production (NEP) and bacterioplankton production (BACPR). The mesocosms had two nutrient levels (high (HN) and low (LN)) combined with three different temperatures according to the IPCC 2007 warming scenarios (unheated, A2 and A2 + 50%) that were applied for 11 years prior to the present experiment. The simulated HW consisted of 5 °C extra temperature increases only in the A2 and A2 + 50% treatments applied from 1 July to 1 August 2014. Linear mixed effect modeling revealed a strong effect of nutrient treatment on the concentration of chlorophyll a (Chl-a), on various forms of phosphorus and nitrogen as well as on oxygen concentration and oxygen percentage (24 h means). Applying the full dataset, we also found a significant positive effect of nutrient loading on GPP, ER, NEP and BACPR, and of temperature on ER and BACPR. The HW had a significant positive effect on GPP and ER. When dividing the data into LN and HN, temperature also had a significant positive effect on Chl-a in LN and on orthophosphate in HN. Linear mixed models revealed differential effects of nutrients, Chl-a and macrophyte abundance (PVI) on the metabolism variables , with PVI being particularly important in the LN mesocosms. All metabolism variables also responded strongly to a cooling-low irradiance event in the middle of the HW, resulting in a severe drop in oxygen concentrations, not least in the HN heated mesocosms. Our results demonstrate strong effects of nutrients as well as an overall rapid response in oxygen metabolism and BACPR to changes in temperature, including HWs, making them sensitive ecosystem indicators of climate warming.
... From those ecosystems, small lakes with surface areas <1 km² comprise the major proportion of lakes (Cael & Seekell, 2016;Verpoorter, Kutser, Seekell, & Tranvik, 2014). With respect to the amount of the Earth's surface that they cover, small lakes emit disproportionately high amounts of CH 4 and CO 2 , and their projected contribution to global warming is still under investigation (Audet et al., 2017;Downing, 2010;Holgerson & Raymond, 2016;Wik, Varner, Walter Anthony, Macintyre, & Bastviken, 2016;Yvon-Durocher, Hulatt, Woodward, & Trimmer, 2017). ...
... One topic with several uncertainties is how the ecosystems will recover after the reduction and/or blocking of DOC inputs from the catchment. One method used for assessing this question is the use of experimental enclosures in the ecosys- tems; they can achieve a realistic manipulation of the ecosystem ( Giling et al., 2017) to obtain insights into the functioning of lake metabolism at the ecosystem level, such as CH 4 and CO 2 cycling (Audet et al., 2017;Bogard et al., 2014;Casper, 1992aCasper, , 1992bCasper et al., 2003Casper et al., , 2005Davidson et al., 2015;Yvon-Durocher et al., 2017). Audet et al. (2017) and Davidson et al. (2015Davidson et al. ( , 2018 found that biotic interactions influenced greenhouse gas emissions more strongly than warming effects, while Yvon-Durocher et al. (2017) showed that long-term effects of warming increased CH 4 and CO 2 emissions derived from the change in microorganism metabo- lism and ecosystem dynamics. ...
... One method used for assessing this question is the use of experimental enclosures in the ecosys- tems; they can achieve a realistic manipulation of the ecosystem ( Giling et al., 2017) to obtain insights into the functioning of lake metabolism at the ecosystem level, such as CH 4 and CO 2 cycling (Audet et al., 2017;Bogard et al., 2014;Casper, 1992aCasper, , 1992bCasper et al., 2003Casper et al., , 2005Davidson et al., 2015;Yvon-Durocher et al., 2017). Audet et al. (2017) and Davidson et al. (2015Davidson et al. ( , 2018 found that biotic interactions influenced greenhouse gas emissions more strongly than warming effects, while Yvon-Durocher et al. (2017) showed that long-term effects of warming increased CH 4 and CO 2 emissions derived from the change in microorganism metabo- lism and ecosystem dynamics. In Lake Grosse Fuchskuhle, an acidic lake rich in DOC, CH 4 and CO 2 dynamics have been examined in sediments. ...
1. Although lakes are important sources of methane (CH4) and carbon dioxide (CO2) to the atmosphere contributing to global warming, their CH4 and CO2 emissions are rarely assessed. In particular, increasing inputs of terrestrial dissolved organic carbon (DOC) may affect gas dynamics and alter seasonal changes in gas production.
2. Here, we analysed variations in CH4 and CO2 dynamics in sub-basins of an acidic bog lake, which was artificially divided into four quarters three decades ago, leading to divergence in water chemistry and biology. In the divided lake, only the south-west basin (SW) received DOC inputs from an adjacent peat bog, while the north-east basin (NE) was hydrologically disconnected. A year-long determination of CH4 and CO2 production and emission patterns in the two contrasting basins exposed the indirect mechanisms by which DOC supply exercised control on greenhouse gas dynamics in this shallow lake.
3. In both basins, dissolved CH4 was negatively correlated with dissolved oxygen (O2) through the water column, suggesting that aerobic methanotrophy is an important regulator of CH4 emissions in this lake. In contrast, the amount of CO2 stored in oxic and anoxic layers was not significantly different between the basins, suggesting that O2 is not the most important driver of dissolved CO2.
4. Estimated total CH4 and CO2 emissions were 2.1 and 1.7 times lower in the NE basin than in the SW basin, with major CH4 and CO2 emissions occurring during the fall turnover. The differences in CH4 and CO2 emissions suggest that the hydro-physical properties, namely seasonal temperature, the duration of stratification and O2 availability, are the main drivers of CH4 and CO2 emissions to the atmosphere from small shallow lakes under the influence of DOC inputs under global warming pressure.
... The relationships between the selected explanatory variables (temperature, Chl-a, alkalinity, PVI) and the zooplankton community response variables (large and small Cladocera and Copepoda, Rotifera, taxon richness and evenness of zooplankton, and size diversity) were further evaluated in the COI analyses, which were run separately for HN and LN. Dissolved oxygen was excluded in this analysis as it generally was high (super-saturation except for a short term period during a cooling event during the middle of the heat wave, see [70]). Significant relationships (p < 0.001) were found, with RV-coefficients of 0.40 and with the variance explained by the first two axes of 94.0% and 5.1% for LN; for HN, RV-coefficients were much lower, 0.14, as was the variance explained by the first two axes, namely 67.4% and 26.2% ( Figure 5). ...
... However, fish kill in HN-A2 and not least in HN-A2+ enhanced the effect of the HW. Modest effects of the HW were also found for phytoplankton in the same experiment [51], whereas temperature changes had a substantial effect on the biomass of bacterioplankton, heterotrophic flagellates, ciliates [95], and macrophytes [13], oxygen metabolism (gross ecosystem production and ecosystem respiration), and bacterioplankton production [96], as well as on greenhouse gases dynamics [70]. These results suggest that microbial communities and ecosystem processes are more sensitive to short-term HWs than the larger-bodied and more slow-growing organisms, such as zooplankton, which are likely to exhibit a delayed or weak response, unless a major fish kill occurs. ...
Shallow lakes are globally the most numerous water bodies and are sensitive to external perturbations, including eutrophication and climate change, which threaten their functioning. Extreme events, such as heat waves (HWs), are expected to become more frequent with global warming. To elucidate the effects of nutrients, warming, and HWs on zooplankton community structure, we conducted an experiment in 24 flow-through mesocosms (1.9 m in diameter, 1.0 m deep) imitating shallow lakes. The mesocosms have two nutrient levels (high (HN) and low (LN)) crossed with three temperature scenarios based on the Intergovernmental Panel on Climate Change (IPCC) projections of likely warming scenarios (unheated, A2, and A2 + 50%). The mesocosms had been running continuously with these treatments for 11 years prior to the HW simulation, which consisted of an additional 5 • C increase in temperature applied from 1 July to 1 August 2014. The results showed that nutrient effects on the zooplankton community composition and abundance were greater than temperature effects for the period before, during, and after the HW. Before the HW, taxon richness was higher, and functional group diversity and evenness were lower in HN than in LN. We also found a lower biomass of large Cladocera and a lower zooplankton: phytoplankton ratio, indicating higher fish predation in HN than in LN. Concerning the temperature treatment, we found some indication of higher fish predation with warming in LN, but no clear effects in HN. There was a positive nutrient and warming interaction for the biomass of total zooplankton, large and small Copepoda, and the zooplankton: phytoplankton ratio during the HW, which was attributed to recorded HW-induced fish kill. The pattern after the HW largely followed the HW response. Our results suggest a strong nutrient effect on zooplankton, while the effect of temperature treatment and the 5 • C HW was comparatively modest, and the changes likely largely reflected changes in predation.
... For lakes, the most direct and recognized impact of heatwaves includes an increase in lake surface water temperature (Woolway et al., 2020). This can also have a knock-on influence on the entire physical lake environment (Jankowski and Livingstone, 2006) and likewise can influence other key processes, such as oxygen dynamics, greenhouse gas emissions, and the presence of toxic substances (Audet et al., 2017;Bartosiewicz et al., 2016;Jankowski and Livingstone, 2006). Previous studies have also shown that heatwaves can lead to an increase in the occurrence of harmful cyanobacteria blooms Johnk et al., 2008;Paerl et al., 2011;Woolway et al., 2021c), which are already increasing in size, frequency and duration in many parts of the world due to ongoing climate change and anthropogenic nutrient enrichment (Ho et al., 2019;Hou et al., 2022;Huisman et al., 2018;Paerl and Huisman, 2008). ...
Heatwaves are increasing and expected to intensify in coming decades with global warming. However, direct evidence and knowledge of the mechanisms of the effects of heatwaves on harmful cyanobacteria blooms are limited and unclear. In 2022, we measured chlorophyll-a (Chla) at 20-s intervals based on a novel ground-based proximal sensing system (GBPSs) in the shallow eutrophic Lake Taihu and combined in situ Chla measurements with meteorological data to explore the impacts of heatwaves on cyanobacterial blooms and the potential relevant mechanisms. We found that three unprecedented summer heatwaves (July 4-15, July 22-August 16, and August 18-23) lasting a total of 44 days were observed with average maximum air temperatures (MATs) of 38.1 ± 1.9 °C, 38.7 ± 1.9 °C, and 40.2 ± 2.1 °C, respectively, and that these heatwaves were characterized by high air temperature, strong PAR, low wind speed and rainfall. The daily Chla significantly increased with increasing MAT and photosynthetically active radiation (PAR) and decreasing wind speed, revealing a clear promotion effect on harmful cyanobacteria blooms from the heatwaves. Moreover, the combined effects of high temperature, high PAR and low wind, enhanced the stability of the water column, the light availability and the phosphorus release from the sediment which ultimately boosted cyanobacteria blooms. The projected increase in heatwave occurrence under future climate change underscores the urgency of reducing nutrient input to eutrophic lakes to combat cyanobacteria growth and of improving early warning systems to ensure secure water management.
... 3). Current and expected reduced wind-speed in many parts of the world (atmospheric stilling)(Mölter et al., 2016) will accelerate lake thermal responses to warming and lengthen stratification(Woolway et al., 2019), further enhancing the risk of having low oxygen concentrations at the sediment surface in shallow lakes(Deng et al. 2018) and thus enhancing the risk of higher CH4 emissions.The alternative dominance by phytoplankton, submerged plants, and free-floating vegetation, that are likely under eutrophic conditions, can promote contrasting patterns in CO2 and other GHG fluxesAlmeida et al. 2016;Jeppesen et al. 2016;Audet et al. 2017). While all primary producers take up CO2 through photosynthesis and release CO2 through respiration, the effects on CH4 and N2O dynamics may differ among phytoplankton, submerged, free-floating, or emergent macrophytes. ...
NON FORMATED PUBLISHED VERSION
Feedbacks between climate change and eutrophication: revisiting the allied attack concept and how to strike back
Despite its well-established negative impacts on society and biodiversity, eutrophication continues to be one of the most pervasive anthropogenic influence along the freshwater to marine continuum. The interaction between eutrophication and climate change, particularly climate warming, was explicitly focused upon a decade ago in the paper by Moss et al. (2011), which called for an integrated response to both problems, given their apparent synergy. In this review, we summarise advances in the theoretical framework and empirical research on this issue and analyse the current understanding of the major drivers and mechanisms by which climate change can enhance eutophication, and vice versa, with a particular focus on shallow lakes. Climate change can affect nutrient loading, through changes at the catchment and landscape levels by affecting hydrological patterns and fire frequency, and through temperature effects on nutrient cycling. Biotic communities and their interactions can also be directly and indirectly affected by climate change, leading to an overall weakening of resilience to eutrophication impacts. Increasing empirical evidence now indicates several mechanisms by which eutrophying aquatic systems can increasingly act as important sources of greenhouse gases to the atmosphere, particularly methane. We also highlight potential feedbacks between eutrophication, cyanobacterial blooms, and climate change. Facing both challenges at the same time is more pressing than ever. Meaningful and strong measures at the landscape and water body levels are therefore required if we are to ensure ecosystem resilience and safe water supply, conserving biodiversity, and decreasing the carbon footprint of freshwaters.
... For example, heat waves directly affect photosynthesis and respiration [37]. Short-term heat waves in summer can promote nitrous oxide emissions [38]. Heat waves in autumn may prolong the growing season of primary producers and stimulate decomposers and, with climate warming, river ecosystems may become more heterotrophic and the degradation rate of recalcitrant carbon will become faster [39]. ...
Extreme climatic events, such as heat wave and large temperature fluctuations, are predicted to increase in frequency and intensity during the next hundred years, which may rapidly alter the composition and function of lake bacterial communities. Here, we conducted a year-long experiment to explore the effect of warming on bacterial metabolic function of lake water and sediment. Predictions of the metabolic capabilities of these communities were performed with FAPROTAX using 16S rRNA sequencing data. The results indicated that the increase in temperature changed the structure of bacterial metabolic functional groups in water and sediment. During periods of low temperature, the carbon degradation pathway decreased, and the synthesis pathway increased, under the stimulation of warming, especially under the conditions temperature fluctuation. We also observed that nitrogen fixation ability was especially important in the warming treatments during the summer season. However, an elevated temperature significantly led to reduced nitrogen fixation abilities in winter. Compared with the water column, the most predominant functional groups of nitrogen cycle in sediment were nitrite oxidation and nitrification. Variable warming significantly promoted nitrite oxidation and nitrification function in winter, and constant warming was significantly inhibited in spring, with control in sediments. Co-occurrence network results showed that warming, especially variable warming, made microbial co-occurrence networks larger, more connected and less modular, and eventually functional groups in the water column and sediment cooperated to resist warming. We concluded that warming changed bacterial functional potentials important to the biogeochemical cycling in the experimental mesocosms in winter and spring with low temperature. The effect of different bacteria metabolism functions in water column and sediment may change the carbon and nitrogen fluxes in aquatic ecosystems. In conclusion, the coupling response between different bacterial metabolic functions in water and sediment may improve the ability to mitigate climate change.
... During dry season, there was an increase in salinity in the urban area, leading to the generation of more electron acceptors and a consecutive decrease in methanogenesis. Lower CH 4 concentrations in the dry period could also be attributed to high respiration rates at the roots of the floating aquatic plants (supported by CH 4 -oxidising bacteria) such as water hyacinths (Attermeyer et al., 2016;Audet et al., 2017), while in the rainy season with stronger flushing capacity, macrophytes in the Saigon River were not as dense (Photo S2). Besides, Rosentreter et al., 2018 suggested that the distinct CH 4 difference between the two seasons in tropical estuaries was contributed by flushing capacity. ...
Estuaries are considered as important sources of the global emission of greenhouse gases (GHGs). Urbanized estuaries often experience eutrophication under strong anthropogenic activities. Eutrophication can enhance phytoplankton abundance, leading to carbon dioxide (CO2) consumption in the water column. Only a few studies have evaluated the relationship between GHGs and eutrophication in estuaries. Here we assessed the concentrations and fluxes of CO2, methane (CH4) and nitrous oxide (N2O) in combination with a suite of biogeochemical variables in four sampling campaigns over two years in a highly urbanized tropical estuary in Southeast Asia (the Saigon River Estuary, Vietnam). The impact of eutrophication on GHGs was evaluated through several statistical methods and interpreted by biological processes. The average concentrations of CO2, CH4 and N2O at the Saigon River in 2019–2020 were 3174 ± 1725 μgC-CO2 L⁻¹, 5.9 ± 16.8 μgC-CH4 L⁻¹ and 3.0 ± 4.8 μgN-N2O L⁻¹, respectively. Their concentrations were 13–18 times, 52–332 times, and 9–37 times higher than the global mean concentrations of GHGs, respectively. While CO2 concentration had no clear seasonal pattern, N2O and CH4 concentrations significantly differed between the dry and the rainy seasons. The increase in eutrophication status along the dense urban area was linearly correlated with the increase in GHGs concentrations. The presence of both nitrification and denitrification resulted in elevated N2O concentrations in this urban area of the estuary. The high concentration of CO2 was contributed by the high concentration of organic carbon and mineralization process. GHGs fluxes at the Saigon River Estuary were comparable to other urbanized estuaries regardless of climatic condition. Control of eutrophication in urbanized estuaries through the implantation of efficient wastewater treatment facilities will be an effective solution in mitigating the global warming potential caused by estuarine emissions.
... The alternative dominance by phytoplankton, submerged plants, and free-floating vegetation, likely growing under eutrophic conditions, can promote contrasting patterns in CO 2 and other GHG fluxes , Almeida et al. 2016, Audet et al. 2017. While all primary producers take up CO 2 through photosynthesis and release CO 2 through respiration, the effects on CH 4 and N 2 O dynamics may differ among phytoplankton and submerged, free-floating, or emergent macrophytes. ...
Despite its well-established negative impacts on society and biodiversity, eutrophication continues to be one of the most pervasive anthropogenic influence along the freshwater to marine continuum. The interaction between eutrophication and climate change, particularly climate warming, was explicitly focused upon a decade ago in the paper by Moss et al. (2011), which called for an integrated response to both problems, given their apparent synergy. In this review, we summarise advances in the theoretical framework and empirical research on this issue and analyse the current understanding of the major drivers and mechanisms by which climate change can enhance eutophication, and vice versa, with a particular focus on shallow lakes.
Climate change can affect nutrient loading, through changes at the catchment and landscape levels by affecting hydrological patterns and fire frequency, and through temperature effects on nutrient cycling. Biotic communities and their interactions can also be directly and indirectly affected by climate change, leading to an overall weakening of resilience to eutrophication impacts. Increasing empirical evidence now indicates several mechanisms by which eutrophying aquatic systems can increasingly act as important sources of greenhouse gases to the atmosphere, particularly methane. We also highlight potential feedbacks between eutrophication, cyanobacterial blooms, and climate change. Facing both challenges at the same time is more pressing than ever. Meaningful and strong measures at the landscape and water body levels are therefore required if we are to ensure ecosystem resilience and safe water supply, conserving biodiversity, and decreasing the carbon footprint of freshwaters.
... Compared to the significance of this issue, studies in this field remain constrained by a lack of data. Thus, characterizing the CH 4 emission scenario from different variety of ecosystems has become an urgent need of the hour, as this would enable us to understand how these systems are going to respond in the future to the ongoing climate change (Negandhi et al. 2016;Audet et al. 2017). Keeping in view this background, the present chapter has focussed on delineating the CH 4 flux dynamics from the household ponds of the Indian Sundarbans Biosphere Reserve. ...
Small aquatic ecosystems like ponds and lakes have been found to emit a significant amount of CH4 towards the atmosphere and their role is worth inclusion in delineating the global CH4 budget. The Sundarbans Biosphere Reserve (SBR) of India, besides being the abode of the world’s largest mangrove forest, shelters almost 4.4 million people with a substantially high population density. The CH4 dynamics from several compartments of this biosphere reserve is studied in the recent past; however, the ponds are yet to receive any attention as such. The present chapter reports the variability of the partial pressure of CH4 in water [pCH4(water)] and the subsequent air–water CH4 fluxes from four different types of ponds situated within the SBR. One of these selected ponds is abandoned, and not used for any human purpose and another pond is well-maintained and not at all used for any human purpose. The rest of the two ponds are typical homestead ponds with varying degrees of anthropogenic disturbances. The results indicated that all four ponds acted as a source of CH4 towards the atmosphere; however, the rate of emission varied across the ponds. The most well-maintained least anthropogenically disturbed pond emitted CH4 at the lowest rate, whereas the dilapidated and abandoned pond, emitted the most. The other two ponds showed an intermediate range of fluxes. Water temperature showed a strong, positive, and statistically significant relationship with pCH4(water). The results indicate that the ongoing climate-change-induced rise in temperature can effectively enhance the CH4 emission rate from these ponds. However, the dissolved oxygen levels exhibited a significant negative relationship with pCH4(water), which indicates that if autotrophic conditions can be maintained through proper pond management practices, methanotrophy can negate the dominance of methanogens to a large extent.
... In the last decade, various studies have highlighted the effect of lake internal factors, such as primary producers (e.g. Almeida et al., 2016;Audet et al., 2017;Davidson et al., 2015;Ger et al., 2014), and of external drivers, such as eutrophication (e.g. Davidson et al., 2015;Hansson et al., 2012;Jeppesen et al., 2016;Yvon-Durocher et al., 2011), hydrological input (e.g. ...
• Shallow aquatic systems exchange large amounts of carbon dioxide (CO2) and methane (CH4) with the atmosphere. The production and consumption of both gases is determined by the interplay between abiotic (such as oxygen availability) and biotic (such as community structure and trophic interactions) factors.
• Fish communities play a key role in driving carbon fluxes in benthic and pelagic habitats. Previous studies indicate that trophic interactions in the water column, as well as in the benthic zone can strongly affect aquatic CO2 and CH4 net emissions. However, the overall effect of fish on both pelagic and benthic processes remains largely unresolved, representing the main focus of our experimental study.
• We evaluated the effects of benthic and pelagic fish on zooplankton and macroinvertebrates; on CO2 and CH4 diffusion and ebullition, as well as on CH4 production and oxidation, using a full-factorial aquarium experiment. We compared five treatments: absence of fish (control); permanent presence of benthivorous fish (common carps, benthic) or zooplanktivorous fish (sticklebacks, pelagic); and intermittent presence of carps or sticklebacks.
• We found trophic and non-trophic effects of fish on CO2 and CH4 emissions. Intermittent presence of benthivorous fish promoted a short-term increase in CH4 ebullition, probably due to the physical disturbance of the sediment. As CH4 ebullition was the major contributor to the total greenhouse gas (GHG) emissions, incidental bioturbation by benthivorous fish was a key factor triggering total carbon emissions from our aquariums.
• Trophic effects impacted GHG dynamics in different ways in the water column and the sediment. Fish predation on zooplankton led to a top-down trophic cascade effect on methane-oxidising bacteria. This effect was, however, not strong enough as to substantially alter CH4 diffusion rates. Top-down trophic effects of zooplanktivorous and benthivorous fish on benthic macroinvertebrates, however, were more pronounced. Continuous fish predation reduced benthic macroinvertebrates biomass decreasing the oxygen penetration depth, which in turn strongly reduced water–atmosphere CO2 fluxes while it increased CH4 emission.
• Our work shows that fish can strongly impact GHG production and consumption processes as well as emission pathways, through trophic and non-trophic effects. Furthermore, our findings suggest their impact on benthic organisms is an important factor regulating carbon (CO2 and CH4) emissions.
... Currently, a wide range of changes caused by global warming can be observed, especially in shallow lake ecosystems (Moss, 2012). The effects of global warming may include very hot periods in the summer or a complete lack of ice cover on water bodies during the winter (Cao et al., 2015;Audet et al., 2017;Nandini et al., 2019). The results of previous studies indicate that in natural lake ecosystems the share of smaller species increases in planktonic invertebrate communities, because the higher temperature causes an increase in mortality, mainly through pressure from top predators (Heckmann et al., 2012;Shurin et al., 2012;Rall et al., 2012;Zingel et al., 2018;Reczuga et al., 2018). ...
One of the effects of warming is earlier retreat of the ice cover or a complete lack of ice cover on water bodies in the winter. However, there is still no information on how climate warming affects the 24-hour dynamics of the planktonic microbial loop in winter. The aim of this investigation was to assess the diurnal dynamics of the taxonomic composition and abundance of microbial communities in experimentally reproduced conditions (samples from under the ice, +2, +4 and +8°C) and to analyse the relationships between components of the microbial loop in relation to physical and chemical parameters. Samples were taken in winter from three dystrophic reservoir. The biological and physicochemical parameters in the water were analysed at the start (day 0), 15 and end of the experiment (day 30) over a 24-hour cycle. The increase in temperature caused an increase in the numbers of predators (particularly testate amoebae and ciliates) and a reduction in the body size of individual populations. During the period with ice cover, marked dominance of mixotrophic testate amoeba (Hyalosphenia papilio) and ciliates (Paramecium bursaria) was observed, while the increase in temperature caused an increase in the proportion of bacterivorous ciliates (Cinetochilum margaritaceum).
... All of these metabolic process variables also responded to a cool and cloudy-low irradiance event in the middle of the heatwave, resulting in a severe drop in O 2 , not least in the heated mesocosms. In the same experiment, substantial effects associated with the heatwave were also found on the biomass of bacterioplankton, heterotrophic flagellates, and ciliates, respectively [25], and to some degree also on the greenhouse gas (GHG) dynamics [26]. The results from this experiment collectively indicated that microbial communities and ecosystem processes are more sensitive to a short-term heatwave than larger-bodied and more slow-growing organisms such as zooplankton [24]. ...
The Earth is facing a major change in climate due to ongoing global warming, and as a result of this warming, the occurrence of more extreme weather is also expected to in-crease. Accordingly, the impacts of extreme climatic events on lakes have been receiving more and more attention in recent years. Furthermore, advances in real-time high-resolution monitoring, together with an increase in the use of in-situ monitoring platforms on many lakes across the globe, now make it possible to track even the short-term effects of such events in lakes with differing characteristics and local climates. Such high-resolution data can also be used to better validate dynamic models of lake pro-cesses, models which are essential if the projected effects of global warming on lake eco-systems are to be quantified. Extreme climatic events include heatwaves, storms, extreme calm periods, sudden and intense rainfall, and droughts. These changes in local weather have the potential to result in physical, chemical, and biological changes within lakes. In deep lakes, for example, heatwaves and calm periods lead to stronger stratification and, if the lakes are nutrient enriched, often to higher levels of cyanobacterial biomass. Similar conditions can lead to temporary stratification of shallow lakes and this, in turn, can result in higher internal loading of phosphorus and thus eutrophication]. Storms often deepen the mixed layer depth and, combined with rain, it may enhance the input of dissolved and particu-late matter and ultimately increase eutrophication. Extreme drought may result in reduced water level, higher salinity, and higher nutrient concentration , and extreme rain may result in increases in the flux of dissolved organic carbon from catchments. Such effects may be of short-term duration, but if severe, they may have long-lasting con-sequences, particularly in lakes with longer retention times.
There is no universally accepted approach to define extreme events for lake studies. Extremes in climatic and hydrological datasets are generally defined by the use of thresholds based on, for example, a top or bottom percentile of the observed data from the site. This type of threshold approach does allow for a systematic identification and assessment of events but has the drawback that the absolute threshold values will differ de-pending on location and the local climate. Nevertheless, thresholds based on local meteorological data have been used to identify potential extreme events within lakes. Definitions based on within-lake physical changes can, however, be more complex due to inconsistency in the metrics used to define such extremes. Other commonly used and equally valid approaches focus on the effects of, for example, named storms or studies on extreme or episodic responses within the lakes, even though the meteorological conditions that trigger the response may not in itself be over any extreme threshold.
This Special Issue includes all of these approaches and addresses effects of extreme events based on field studies, controlled experiments, and modelling.
... However, the relationship between k and wind speed typically breaks down under low wind conditions, and in any event, this approach is not applicable for indoor greenhouse experiments. Mesocosms can also simulate the condition of slow-flow waterways or wetlands where gas exchanges across the air-water interface are affected by the turbulence induced by moving water (Audet et al., 2017;Messer et al., 2017). Moreover, the presence of emergent vegetation contributes to modify the common drivers of gas transfer at a small-scale, e.g., by attenuating wind speed above the water surface and affecting hydraulic parameters related to mixing and turbulence (Naden et al., 2006;Coates and Folkard, 2009;Poindexter and Variano, 2013). ...
Denitrification associated with emergent macrophytes is a pivotal process underlying the treatment performance of wetlands and slow-flow waterways. Laboratory scale experiments targeting N losses via denitrification in sediments colonized by emergent macrophytes require the use of mesocosms that are necessarily open to the atmosphere. Thus, the proper quantification of N2 effluxes relies on the accurate characterization of the air–water gas exchanges. In this study, we present a simple approach for direct measurements of the gas transfer velocity, in open-top mesocosms with Phragmites australis, by using argon as a tracer. Different conditions of water velocity (0, 1.5, 3, and 6 cm s-1) and temperature (8.5, 16, and 28 °C), were tested, along with, for the first time, the presence of emergent vegetation. The outcomes demonstrated that water velocity and temperature are not the only factors regulating aeration at the mesocosm scale. Indeed, the gas transfer velocity was systematically higher, in the range of 42−53%, in vegetated compared to unvegetated sediments. The increase of small-local turbulence patterns created within water parcels moving around plant stems translated into significant modifications of the reaeration process. The adopted approach may be used to improve the accuracy of denitrification measurements by N2 efflux-based methods in wetland and slow-flow waterway sediments colonized by emergent macrophytes. Moreover, the present outcomes may have multiple implications for whole-system metabolism estimations from which largely depend our understanding of biogeochemical dynamics in inland waters that have strong connections to worldwide issues, such as nitrate contamination and greenhouse gas emissions.
... The Intergovernmental Panel on Climate Change (IPCC) has predicted that the frequency of extreme climate events (heavy rainfall, droughts and heat waves) will increase in the future (Field et al., 2014), which, even if they are of short-term duration, may have significant effects on the structure and function of ecosystems, including lakes (Lesack et al., 2013;Neif et al., 2017;Audet et al., 2017). For example, changes in precipitation may affect nutrient concentrations and induce a sudden shift in the structure and function of temperate lake ecosystems (Coops et al., 2003;Alberta et al., 2014). ...
... Reduced pCO 2 occurs in summer when the water column is stable and primary production increases, whereas pCO 2 increases during fall as respiratory products in the hypolimnion are mixed into surface waters (Alin & Johnson, 2007;Ducharme-Riel et al., 2015;Marcé et al., 2015;Stets et al., 2009). These seasonal patterns can be disrupted by climatic or meteorological events such as passing storms or heat waves (Audet et al., 2017;Klug et al., 2012;Maberly, 1996) or be dampened in polymictic lakes where CO 2 exhibits more limited seasonal variation (Jonsson et al., 2003). ...
Spatiotemporal variation in climate and weather, allochthonous carbon loads, and autochthonous factors such as lake metabolism (photosynthesis and respiration) interacts to regulate atmospheric CO2 exchange of lakes. Understanding this interplay in diverse basin types at different timescales is required to adequately place lakes into the global carbon cycle and predict CO2 flux through space and time. We analyzed 18 years of data from seven moderately hard lakes in an agricultural prairie landscape in central Canada. We applied generalized additive models and sensitivity analyses to evaluate the roles of metabolic and climatic drivers in regulating CO2 flux at the intra-annual scale. At mean conditions with respect to other predictors, metabolic controls resulted in uptake of atmospheric CO2 when surface waters exhibited moderate primary production but released CO2 only when primary production was very low (<8 μg/L or when dissolved nitrogen was elevated (>2,000 μg/L), implying that respiratory controls offset photosynthetic CO2 uptake under these conditions. Climatically, dry conditions increased the likelihood of in-gassing, likely due to evaporative concentration of base cations and/or reduced allochthonous carbon loads. While more research is required to establish the relative importance of climate and metabolism at other timescales (diel, autumn/winter), we conclude that these hard fresh waters characteristic of continental interiors are mainly affected by metabolic drivers of pCO2 at daily-monthly timescales, are climatically controlled at interannual intervals, and are more likely to in-gas CO2 for a given level of algal abundance than are soft water, boreal ecosystems.
... Lofton et al. (2014) reported a linear response of methanotrophy to temperature increase under substrate-saturated conditions, and Duc et al. (2010) and Dunfield et al. (1993) have observed that substrate availability (i.e., CH 4 and O 2 ) positively affects methanotrophy rates in a stronger way than temperature. The response of methanogenesis and methanotrophy to temperature comprises a complicated matrix of factors, and its understanding is fundamental to predict how net CH 4 emission from aquatic ecosystems will respond to climate warming (Aben et al., 2017;Audet et al., 2017;Davidson et al., 2018;Marotta et al., 2014;Negandhi et al., 2016;Yvon-Durocher et al., 2014;Yvon-Durocher et al., 2017). ...
Net methane (CH4) emission from lakes depends on two antagonistic processes: CH4 production (methanogenesis) and CH4 oxidation (methanotrophy). It is unclear how climate warming will affect the balance between these processes, particularly among lakes of different trophic status. Here we show that methanogenesis is more sensitive to temperature than methanotrophy, and that eutrophication magnifies this temperature sensitivity. Using laboratory incubations of water and sediment from ten tropical, temperate and subarctic lakes with contrasting trophic states, ranging from oligotrophic to hypereutrophic, we explored the temperature sensitivity of methanogenesis and methanotrophy. We found that both processes presented a higher temperature sensitivity in tropical lakes, followed by temperate, and subarctic lakes; but more importantly, we found that eutrophication triggered a higher temperature sensitivity. A model fed by our empirical data revealed that increasing lake water temperature by 2 °C leads to a net increase in CH4 emissions by 101–183% in hypereutrophic lakes and 47–56% in oligotrophic lakes. We conclude that climate warming will tilt the CH4 balance towards higher lake emission and that this impact will be exacerbated by the eutrophication of the lakes.
... The likelihood of extremely hot summers has greatly increased over the last decades. Heat waves can severely affect the thermal regime of lakes and alter their ecosystems, and the growth of submerged macrophytes (Audet et al. 2017;Cao et al. 2015). Shallow lakes may be particularly sensitive to heat waves as their average water temperature will increase more quickly than in deeper water bodies (Mooij et al. 2008). ...
We analysed changes in the abundance, biomass and cell size of the microbial food web community (bacteria, heterotrophic nanoflagellates, ciliates) at contrasting nutrient concentrations and temperatures during a simulated heat wave. We used 24 mesocosms mimicking shallow lakes in which two nutrient levels (unenriched and enriched by adding nitrogen and phosphorus) and three different temperature scenarios (ambient, IPCC A2 scenario and A2 +%50) are simulated (4 replicates of each). Experiments using the mesocosms have been running un-interrupted since 2003. A 1-month heat wave was imitated by an extra 5 °C increase in the previously heated mesocosms (from 1st July to 1st August 2014). Changes in water temperature induced within a few days a strong effect on the microbial food web functioning, demonstrating a quick response of microbial communities to the changes in environment, due to their short generation times. Warming and nutrients showed synergistic effects. Microbial assemblages of heterotrophic nanoflagellates and ciliates responded positively to the heating, the increase being largest in the enriched mesocosms. The results indicate that warming and nutrients in combination can set off complex interactions in the microbial food web functioning.
... Reduced pCO 2 occurs in summer when the water column is stable and primary production increases, whereas pCO 2 increases during fall as respiratory products in the hypolimnion are mixed into surface waters (Alin & Johnson, 2007;Ducharme-Riel et al., 2015;Marcé et al., 2015;Stets et al., 2009). These seasonal patterns can be disrupted by climatic or meteorological events such as passing storms or heat waves (Audet et al., 2017;Klug et al., 2012;Maberly, 1996) or be dampened in polymictic lakes where CO 2 exhibits more limited seasonal variation (Jonsson et al., 2003). ...
Hardwater lakes in the North American Prairies exhibit high pH (>8) which enhances influx of CO2. At the scale of years-to-decades, variation in pH is the principle control of atmospheric CO2 exchange in systems rich in dissolved inorganic carbon. However, pH and pCO2 can also vary substantially over 24 h, suggesting important metabolic control of short-term CO2 fluxes. We analyzed temporal variation in pCO2 and enhanced CO2 diffusion in seven lakes for 20 years in the Qu'Appelle River basin (Saskatchewan, Canada) to quantify how lake metabolism (chlorophyll a, productivity) and climate (SOI, PDO) might interact to regulate pH and control CO2 exchange. Analysis of biweekly time series (May-Sept) using generalised additive models revealed that while variation in climate was the main factor regulating seasonal variation in pH, increasing algal biomass during summer also dependently (via pH) and independently (via pCO2 after accounting for pH) increased under-saturation of CO2. Together, these analyses suggest that while climatically-induced variation in pH regulates overall CO2 flux, photosynthesis by phytoplankton exerted a fine-scale control of atmospheric CO2 exchange at daily-to-monthly-timescales by increasing lake water pH.
We assessed how warm air temperatures, high solar radiation, and weak wind speeds might induce hypoxia in a shallow lake during a heat wave. We simulated bottom‐water dissolved oxygen concentrations and compared concentrations in 2022 with the average for the previous 30 years. We found that hypoxia was most sensitive to wind speeds. When the wind speed was low, convection was insufficient to prevent hypoxia, but there was no hypoxia if the wind speed equaled the average speed during the previous 30 years. However, if solar radiation and air temperatures equaled the respective averages during the previous 30 years, hypoxia did not occur, even if wind speeds were low. We conclude that the combined effects of weak winds and either high solar radiation or air temperatures induced hypoxia during the heat wave of 2022.
Lakes are important sources of greenhouse gases (GHGs) to the atmosphere. Factors controlling CO2, CH4 and N2O fluxes include eutrophication and warming, but the integrated influence of climate-warming-driven stratification, oxygen loss and resultant changes in bloom characteristics on GHGs are not well understood. Here we assessed the influence of contrasting meteorological conditions on stratification and phytoplankton bloom composition in a eutrophic lake, and tested for associated changes in GHGs inventories in both the shallow and deep waters, over three seasons (2010-2012). Atmospheric heatwaves had one of the most dramatic effects on GHGs. Indeed, cyanobacterial blooms that developed in response to heatwave events in 2012 enhanced both sedimentary CH4 concentrations (reaching up to 1mM) and emissions to the atmosphere (up to 8 mmol m−2 d−1). That summer, CH4 contributed 52% of the integrated warming potential of GHGs produced in the lake (in CO2 equivalents) as compared to between 34 and 39% in years without cyanobacterial blooms. High CH4 accumulation and subsequent emission in 2012 were preceded by CO2 and N2O consumption and under-saturation at the lake surface (uptakes at -30 mmol m−2 d−1 and -1.6 µmol m−2 d−1, respectively). Fall overturn presented a large efflux of N2O and CH4, particularly from the littoral zone after the cyanobacterial bloom. We provide evidence that, despite cooling observed at depth during relatively hot summers, CH4 emissions increased via stronger stratification due to surface warming, resulting in enhanced cyanobacterial biomass deposition and intensified bottom water anoxia. Our results, supported by recent literature reports, suggests a novel interplay between climate change effects on lake hydrodynamics that impacts both bloom characteristics and GHGs production in shallow eutrophic lakes. Given global trends of warming and enrichment, these interactive effects should be considered to more accurately predict the future global role of lakes in GHG emissions.
Climate change models predict a possible increase in air temperature of 2–8°C. This means that global warming will significantly affect the functioning of various types of hydrogenic ecosystems. However, the effect of the temperature increase on microbial loop function in small water bodies associated with peat ecosystems (peat pools and Sphagnum hollows) is still unknown. We used mesocosm experiments (control and treatments with a 2°C, 4°C and 8°C temperature increase) to determine the response of bacterioplankton, flagellates, testate amoebae and ciliates to simulated temperature changes, taking into account seasonal variation in the temperate climate zone. The simulated increase in climate warming increased the species richness of ciliates and the abundance of bacteria, flagellates and ciliates. In contrast, there was a decrease in the species number and abundance of testate amoebae, the top predators in peat ecosystems. The sensitivity of the various microbial groups to temperature was size‐dependent; large‐sized testate amoebae declined under warming. These shifts caused a decrease in the predator–prey mass ratio. An increase in the abundance of top predators promotes increased abundance of ciliates, and thus changes the architecture of the food web. At the same time, we observed the increase in phycoflora biomass thus can cancel the potential negative effects of warming on heterotrophic microbial activity. So, the potential effect of warming on the C budgets of peat pools and hollows is evident. A better understanding of what regulates microbial populations and activity in small reservoirs in peat bogs and unravelling of these fundamental mechanisms are particularly critical to more accurately predict how peat bogs will respond to climate disturbances.
The aquatic microbiota is involved in crucial ecosystem functions. Thus, investigating the effects of global warming on these organisms is highly relevant, especially given the numerous climatic changes expected by the end of the century. In this study, we used an experimental approach and high-throughput sequencing to evaluate the shortterm effect of warming predicted by different future scenarios in the composition of the planktonic freshwater bacteria and microeukaryotes, and to verify if the same effects occur for each trophic level separately (autotrophic, heterotrophic and mixotrophic). Our experiment demonstrated that the composition for eukaryotes and prokaryotes based on DNA metabarcoding is affected by the increase in temperature and these have a similar pattern of response to warming. This highlights the temperature importance in structuring the communities of different groups. Modifications in the communities were observed through the substitution of specific taxa, which occurred mainly in warmer levels. Changes in community composition were also identified when trophic levels were assessed separately. Mixotrophic eukaryotes organisms are more sensitive to warming, modifying the patterns of composition with an increase in temperature of 2 °C. Microeukaryotes and heterotrophic bacteria were more resistant, with alterations in the communities composition visualized only in higher warming levels. The composition of autotrophic organisms was not affected by the increase in water temperature in any of the biological classifications evaluated, although the richness of eukaryotic autotrophic has decreased with warming. Our results contribute to predict how different biological levels and trophic groups of the aquatic microbiota respond to global warming. This approach is relevant because warming leads to changes in community composition and affects ecosystem processes essentials to the aquatic environment.
In 2018, Europe experienced the warmest May–October (Northern Hemisphere warm season) since air temperature records began. In this study, we ran model simulations for 46 557 lakes across Europe to investigate the influence of this heatwave on surface water temperature. We validated the model with satellite-derived lake surface temperatures for 115 lakes from 1995 to 2018. Using the validated model, we demonstrated that, during May–October 2018, mean and maximum lake surface temperatures were 1.5 and 2.4 °C warmer than the base-period average (1981–2010). A lake model experiment demonstrated that, on average, the increase in air temperature was the dominant driver of surface water temperature change. However, in some lake regions, other meteorological forcing had a greater influence. Notably, higher than average solar radiation and lower than average wind speed exacerbated the influence of the heatwave on lake surface temperature in many regions, particularly Fennoscandia and Western Europe. To place our results in the context of projected 21st century climate change, we then ran the lake model with input data from state-of-the-art climate model projections under 3 emissions scenarios. Under the scenario with highest emissions (Representative Concentration Pathway 8.5), we demonstrated that by the end of the 21st century, the lake surface temperatures that occurred during the heatwave of 2018 will become increasingly common across many lake regions in Europe.
Hutchinson and Löffler's (1956) classification of lakes based on the seasonal thermal mixing regime has become a cornerstone of any analysis of lakes as elements of the earth surface. Until now however the lake classification has lacked a physically sound quantitative criterion distinguishing between two fundamental lake types: thermally stratified during a large portion of the year (mono- and dimictic) and predominantly mixed to the bottom (polymictic). Using the mechanistic balance between potential and kinetic energy we review the different formulations of the Richardson number to derive a generalized scaling for seasonal stratification in a closed lake basin. The scaling parameter is the critical mean basin depth, Hcrit, that delineates lakes that mix regularly from those that stratify seasonally based on lake water transparency, lake length, and an annual mean estimate for the Monin-Obukhov length. We validate the scaling on available data of lakes worldwide using logistic regression. The scaling criterion consistently described the mixing regime significantly better than either the conventional unbounded basin scaling or a simple depth threshold. Thus, the generalized scaling is universal for freshwater lakes and allows the seasonal mixing regime to be estimated without numerically solving the heat transport equations.
Increasing air temperatures may result in stronger lake stratification, potentially altering nutrient and biogenic gas cycling. We assessed the impact of climate forcing by comparing the influence of stratification on oxygen, nutrients, and global-warming potential (GWP) of greenhouse gases (the sum of CH4, CO2, and N2O in CO2 equivalents) emitted from a shallow productive lake during an average versus a heat-wave year. Strong stratification during the heat wave was accompanied by an algal bloom and chemically enhanced carbon uptake. Solar energy trapped at the surface created a colder, isolated hypolimnion, resulting in lower ebullition and overall lower GWP during the hotter-than-average year. Furthermore, the dominant CH4 emission pathway shifted from ebullition to diffusion, with CH4 being produced at surprisingly high rates from sediments (1.2–4.1 mmol m–2 d–1). Accumulated gases trapped in the hypolimnion during the heat wave resulted in a peak efflux to the atmosphere during fall overturn when 70% of total emissions were released, with littoral zones acting as a hot spot. The impact of climate warming on the GWP of shallow lakes is a more complex interplay of phytoplankton dynamics, emission pathways, thermal structure, and chemical conditions, as well as seasonal and spatial variability, than previously reported.
Elevated temperatures and extreme climatic events, such as heat waves, can negatively affect submerged macrophytes. Here, we investigated how submerged macrophytes adapted to three different temperatures: 1) ambient, 2) ca. +3 oC and 3) ca. +4.5 oC responded to a heat wave. After ten years of adaptation, the shoots of two species of submerged macrophytes, Elodea canadensis and Potamogeton crispus, were collected from each of the three temperature treatments and transferred to the two heated treatments for one month. Thereafter, the two heated treatments were exposed to a one-month heat wave with an additional 5 oC temperature increase. For P. crispus, total biomass did not differ among the plants adapted to the different temperatures or between the two heated treatments for the whole duration of the experiment. Plants adapted to the highest temperatures, however, produced a larger number of smaller turions before the heat wave and allocated less biomass to elongation before and after the heat wave. As to E. canadensis, the plants adapted to higher temperatures had higher total biomass before and during the heat wave and allocated more biomass to roots and leaves during the heat wave. Most indicators (e.g. length and biomass) of the macrophyte performance measured during the experiment did not differ between the two heated treatments. In summary, after the ten-year adaptation to higher temperatures, the submerged macrophytes showed adaptive changes in growth and asexual reproduction and responded in a complex way to the heat wave depending on species, growth status and adaptation temperature.
Many freshwater lakes are supersaturated in CO2 with respect to the atmosphere. This concentration gradient implies a net flux of CO2 from the water to the air. The actual rate of gas exchange is governed by both this concentration gradient and the gas transfer coefficient, k. To directly measure k, we added the chemically and biologically inert gas, sulfur hexaflouride (SF6), to the epilimnion of Minor Lake, New Hampshire, a small (15 ha), low-wind softwater lake. k was independent of wind speed over the 50-d summer stratification period and averaged 2.65 +/- 0.12 cm h(-1) (95% CI; normalized to a Schmidt number of 600); k(800) was better correlated to precipitation events than it was to wind speed. Our data support the idea that gas exchange across the air-water interface is largely independent of wind at low wind speeds. The surface water of Mirror Lake was persistently supersaturated in CO2 with respect to the atmosphere. During a 3.5-year period the partial pressure of CO2 in the surface waters of the lake averaged 726 +/- 39 mu atm (95% CI) and showed substantial seasonal variation (360-2,000 mu atm). Diel and day-to-day variation in CO2 were very small compared to the CO2 pool. We combined our estimates of k with weekly measurements of the partial pressure of CO2 to estimate CO2 gas exchange in the lake. Mirror Lake released from 26 to 50 g C m(-2) to the atmosphere each year, depending on the method of calculating k. Atmospheric CO2 exchange is a large term in the C economy of the lake-the most conservative gas flux estimate is about four times as large as outflow plus seepage of total dissolved inorganic carbon and 1.5 times as large as the export of dissolved organic C from the lake.
Lakes can be sources or sinks of carbon, depending on local conditions. Recent studies have shown that the CO 2 efflux increases when lakes recover from eutrophication, mainly as a result of a reduction in phytoplankton biomass, leading to less uptake of CO 2 by producers. We hypothesised that lake restoration by removal of coarse fish (biomanipulation) or invasion of mussels would have a similar effect. We studied 14–22 year time series of five temperate Danish lakes and found profound effects on the calculated CO 2 efflux of major shifts in ecosystem structure. In two lakes, where limited colonisation of submerged macrophytes occurred after biomanipulation or invasion of zebra mussels (Dreissena polymorpha), the efflux increased significantly with decreasing phytoplankton chlorophyll a. In three lakes with major interannual variation in macrophyte abundance, the efflux declined with increasing macrophyte abundance in two of the lakes, while no relation to macrophytes or chlorophyll a was found in the third lake, likely due to high groundwater input to this lake. We conclude that clearing water through invasive mussels or lake restoration by bioma-nipulation may increase the CO 2 efflux from lakes. However, if submerged macrophytes establish and form dense beds, the CO 2 efflux may decline again.
Temperature alone explains a great amount of variation in sediment organic carbon (OC) mineralization. Studies on decomposition of soil OC suggest that (1) temperature sensitivity differs between the fast and slowly decomposition OC and (2) over time, decreasing soil respiration is coupled with increase in temperature sensitivity. In lakes, autochthonous and allochthonous OC sources are generally regarded as fast and slowly decomposing OC, respectively. Lake sediments with different contributions of allochthonous and autochthonous components, however, showed similar temperature sensitivity in short-term incubation experiments. Whether the mineralization of OC in lake sediments dominated by allochthonous or autochthonous OC has different temperature sensitivity in the longer term has not been addressed. We incubated sediments from two boreal lakes that had contrasting OC origin (allochthonous versus autochthonous), and OC characteristics (C/N ratios of 21 and 10) at 1, 3, 5, 8, 13, and 21°C for five months. Compared to soil and litter mineralization, sediment OC mineralization rates were low in spite of low apparent activation energy (Ea). The fraction of the total OC pool that was lost during five months varied between 0.4 and 14.8%. We estimate that the sediment OC pool not becoming long-term preserved was degraded with average apparent turnover times between 3 and 32 years. While OC mineralization was strongly dependent on temperature as well as on OC composition and origin, temperature sensitivity was similar across lakes and over time. We suggest that the temperature sensitivity of OC mineralization in lake sediments is similar across systems within the relevant seasonal scales of OC supply and degradation.
Fresh waters make a disproportionately large contribution to greenhouse gas (GHG) emissions, with shallow lakes being particular hotspots. Given their global prevalence, how GHG fluxes from shallow lakes are altered by climate change may have profound implications for the global carbon cycle. Empirical evidence for the temperature dependence of the processes controlling GHG production in natural systems is largely based on the correlation between seasonal temperature variation and seasonal change in GHG fluxes. However, ecosystem-level GHG fluxes could be influenced by factors, which whilst varying seasonally with temperature are actually either indirectly related (e.g. primary producer biomass) or largely unrelated to temperature, for instance nutrient loading. Here, we present results from the longest running shallow-lake mesocosm experiment which demonstrate that nutrient concentrations override temperature as a control of both the total and individual GHG flux. Furthermore, testing for temperature treatment effects at low and high nutrient levels separately showed only one, rather weak, positive effect of temperature (CH4 flux at high nutrients). In contrast, at low nutrients, the CO2 efflux was lower in the elevated temperature treatments, with no significant effect on CH4 or N2 O fluxes. Further analysis identified possible indirect effects of temperature treatment. For example, at low nutrient levels increased macrophyte abundance was associated with significantly reduced fluxes of both CH4 and CO2 for both total annual flux and monthly observation data. As macrophyte abundance was positively related to temperature treatment, this suggests the possibility of indirect temperature effects, via macrophyte abundance, on CH4 and CO2 flux. These findings indicate that fluxes of GHGs from shallow lakes may be controlled more by factors indirectly related to temperature, in this case nutrient concentration and the abundance of primary producers. Thus, at ecosystem scale response to climate change may not follow predictions based on the temperature dependence of metabolic processes. This article is protected by copyright. All rights reserved.
This article is protected by copyright. All rights reserved.
Global climate is changing rapidly, and the degree to which natural populations respond genetically to these changes is key to predicting ecological responses. So far, no study has documented evolutionary changes in the thermal tolerance of natural populations as a response to recent temperature increase. Here, we demonstrate genetic change in the capacity of the water flea Daphnia to tolerate higher temperatures using both a selection experiment and the reconstruction of evolution over a period of forty years derived from a layered dormant egg bank. We observed a genetic increase in thermal tolerance in response to a two-year ambient +4 °C selection treatment and in the genotypes of natural populations from the 1960s and 2000s hatched from lake sediments. This demonstrates that natural populations have evolved increased tolerance to higher temperatures, probably associated with the increased frequency of heat waves over the past decades, and possess the capacity to evolve increased tolerance to future warming.
An accurate description of the abundance and size distribution of lakes is critical to quantifying limnetic contributions to the global carbon cycle. However, estimates of global lake abundance are poorly constrained. We used high-resolution satellite imagery to produce a GLObal WAter BOdies database (GLOWABO), comprising all lakes greater than 0.002 km2. GLOWABO contains geographic and morphometric information for ~117 million lakes with a combined surface area of about 5 million km2, which is 3.7% of the Earth’ non-glaciated land area. Large and intermediate-sized lakes dominate the total lake surface area. Overall, lakes are less abundant, but cover a greater total surface area relative to previous estimates based on statistical extrapolations. The GLOWABO allows for the global-scale evaluation of fundamental limnological problems, providing a foundation for improved quantification of limnetic contributions to the biogeochemical processes at large scales.
Inland water sediments receive large quantities of terrestrial organic matter1, 2, 3, 4, 5 and are globally important sites for organic carbon preservation5, 6. Sediment organic matter mineralization is positively related to temperature across a wide range of high-latitude ecosystems6, 7, 8, 9, 10, but the situation in the tropics remains unclear. Here we assessed temperature effects on the biological production of CO2 and CH4 in anaerobic sediments of tropical lakes in the Amazon and boreal lakes in Sweden. On the basis of conservative regional warming projections until 2100 (ref. 11), we estimate that sediment CO2 and CH4 production will increase 9–61% above present rates. Combining the CO2 and CH4 as CO2 equivalents (CO2eq; ref. 11), the predicted increase is 2.4–4.5 times higher in tropical than boreal sediments. Although the estimated lake area in low latitudes is 3.2 times smaller than that of the boreal zone, we estimate that the increase in gas production from tropical lake sediments would be on average 2.4 times higher for CO2 and 2.8 times higher for CH4. The exponential temperature response of organic matter mineralization, coupled with higher increases in the proportion of CH4 relative to CO2 on warming, suggests that the production of greenhouse gases in tropical sediments will increase substantially. This represents a potential large-scale positive feedback to climate change.
Variation in terrestrial net primary production (NPP) with climate is thought to originate from a direct influence of temperature and precipitation on plant metabolism. However, variation in NPP may also result from an indirect influence of climate via growing season length, plant age, stand biomass, and local adaptation. To identify the relative importance of direct and indirect climate effects, we (i) extend metabolic scaling theory to link hypothesized climate influences with NPP; and (ii) assess hypothesized relationships using a global compilation of ecosystem woody plant biomass and production data. Intriguingly, age and biomass explained most of the variation in production while temperature and precipitation explained almost none, suggesting climate indirectly (not directly) influences production. Further, our theory shows that variation in NPP is governed by a common scaling relationship, suggesting that global change models that incorporate the mechanisms governing this relationship can improve predictions of future ecosystem functioning
Forests strongly affect climate through the exchange of large amounts of atmospheric CO2. The main drivers of spatial variability in net ecosystem production (NEP) on a global scale are, however, poorly known. As increasing nutrient availability increases the production of biomass per unit of photosynthesis and reduces heterotrophic respiration in forests, we expected nutrients to determine carbon sequestration in forests. Our synthesis study of 92 forests in different climate zones revealed that nutrient availability indeed plays a crucial role in determining NEP and ecosystem carbon-use efficiency (CUEe; that is, the ratio of NEP to gross primary production (GPP)). Forests with high GPP exhibited high NEP only in nutrient-rich forests (CUEe = 33 ± 4%; mean ± s.e.m.). In nutrient-poor forests, a much larger proportion of GPP was released through ecosystem respiration, resulting in lower CUEe (6 ± 4%). Our finding that nutrient availability exerts a stronger control on NEP than on carbon input (GPP) conflicts with assumptions of nearly all global coupled carbon cycle–climate models, which assume that carbon inputs through photosynthesis drive biomass production and carbon sequestration. An improved global understanding of nutrient availability would therefore greatly improve carbon cycle modelling and should become a critical focus for future research.
This study investigated the potential local CH4 sink in various plant parts as a boundary environment of CH4 emission and consumption. By comparing CH4 consumption activities in cultures inoculated with parts from 39 plant species, we observed significantly higher consumption of CH4 associated with aquatic plants than other emergent plant parts such as woody plant leaves, macrophytic marine algae, and sea grass. In situ activity of CH4 consumption by methanotrophs associated with different species of aquatic plants was in the range of 3.7-37 μmol·h(-1)·g(-1) dry weight, which was ca 5.7-370-fold higher than epiphytic CH4 consumption in submerged parts of emergent plants. The qPCR-estimated copy numbers of the particulate methane monooxygenase-encoding gene pmoA were variable among the aquatic plants and ranged in the order of 10(5)-10(7) copies·g(-1) dry weight, which correlated with the observed CH4 consumption activities. Phylogenetic identification of methanotrophs on aquatic plants based on the pmoA sequence analysis revealed a predominance of diverse gammaproteobacterial type-I methanotrophs, including a phylotype of a possible plant-associated methanotroph with the closest identity (86-89%) to Methylocaldum gracile.
1930 Climate change may have profound eff ects on phosphorus (P) transport in streams and on lake eutrophication. Phosphorus loading from land to streams is expected to increase in northern temperate coastal regions due to higher winter rainfall and to a decline in warm temperate and arid climates. Model results suggest a 3.3 to 16.5% increase within the next 100 yr in the P loading of Danish streams depending on soil type and region. In lakes, higher eutrophication can be expected, reinforced by temperature-mediated higher P release from the sediment. Furthermore, a shift in fi sh community structure toward small and abundant plankti-benthivorous fi sh enhances predator control of zooplankton, resulting in higher phytoplankton biomass. Data from Danish lakes indicate increased chlorophyll a and phytoplankton biomass, higher dominance of dinophytes and cyanobacteria (most notably of nitrogen fi xing forms), but lower abundance of diatoms and chrysophytes, reduced size of copepods and cladocerans, and a tendency to reduced zooplankton biomass and zooplankton:phytoplankton biomass ratio when lakes warm. Higher P concentrations are also seen in warm arid lakes despite reduced external loading due to increased evapotranspiration and reduced infl ow. Th erefore, the critical loading for good ecological state in lakes has to be lowered in a future warmer climate. Th is calls for adaptation measures, which in the northern temperate zone should include improved P cycling in agriculture, reduced loading from point sources, and (re)-establishment of wetlands and riparian buff er zones. In the arid Southern Europe, restrictions on human use of water are also needed, not least on irrigation. O n average, global surface temperatures have increased by about 0.74°C over the past 100 yr (Trenberth et al., 2007), with the majority of the increase (0.55°C) occurring over the past 30 yr. We may expect marked changes to occur in the global climate during this century (IPCC, 2007). Increasingly reliable regional climate projections are available for many regions of the world, but fewer projections are available for many developing countries than for the developed world (Christensen et al., 2007). Th e warming generally increases the spatial variability of precipitation with reduced rainfall in the subtropics and increases at higher latitudes and in parts of the tropics. Th e changes in temperature and rainfall lead to changes in agricul-tural land use and management, including changes in soil cultivation and in the rates and timing of fertilization (Howden et al., 2007). Th ese changes have cascading eff ects on the P cycling, directly and indirectly, that aff ect the aquatic environment. Th e direct eff ects are related to the increased temperatures, increased rainfall intensity, and changes in winter rainfall that are expected to enhance the P loading to freshwaters in the temperate zone (IPCC, 2007) and the Arctic (Arctic Climate Impact Assessment, 2002) and to reduce the loading, but not the concentrations, in streams and freshwater lakes in the Mediterranean region. However, a few quantitative studies are avail-able (Chang, 2004; Andersen et al., 2006). Th e indirect eff ects are related to changes in the choice of crops, crop rotations, use of catch crops, and agricultural practices, including tillage and fertilization. In northern temperate areas, new heat-demanding, warm-season crops (e.g., maize and sunfl ower) will replace many of the present grain cereals and oilseed crops (Olesen and Bindi, 2002). At the same time, changes occur in planting and harvesting times (Olesen, 2005) and in fertilization rates and strategies (Olesen et al., 2007). Crop rotation must be adapted to changes in crop choices, in crop maturing, and in the need to control weeds, pests, and diseases. Th is will aff ect the amount of P released to freshwaters and its seasonal pattern. More-Abbreviation: TP, total phosphorus.
Eutrophication resulting from high nutrient loading has been the paramount environmental
problem for lakes world-wide for the past four decades. Efforts are being made
in many parts of the world to reduce external nutrient loading via improved wastewater
treatment or diversion of nutrient-rich inflows. However, even after a reduction of the
external phosphorus loading, the effects obtained may be unsatisfactory. This may
reflect an insufficient reduction in the external nutrient loading to effectively limit phytoplankton
growth. However, the lack of success may also be due to chemical or biological
within-lake inertia preventing or delaying improvements. To overcome the
resilience and thereby reinforce recovery, a number of physico-chemical and biological
restoration methods have been developed.
In this chapter, we describe recent developments of biological restoration methods
related to eutrophication, their short-term and long-term effects, and discuss the
possibility of using combined physico-chemical and biological methods to improve
the long-term stability of restoration and to reduce restoration costs. As comprehensive
reviews of the effect of fish manipulation in cold temperate lakes are numerous, for
these waterbodies, we highlight recent results, including effects on biodiversity and
metabolism, and present new approaches of biomanipulation. Our particular focus is,
however, directed at biomanipulation in warm lakes and on combined treatments
which are far less well described in the literature.
One of the major impediments to the integration of lentic ecosystems into global environmental analyses has been fragmentary data on the extent and size distribution of lakes, ponds, and impoundments. We use new data sources, enhanced spatial resolution, and new analytical approaches to provide new estimates of the global abundance of surface-water bodies. A global model based on the Pareto distribution shows that the global extent of natural lakes is twice as large as previously known (304 million lakes; 4.2 million km 2 in area) and is dominated in area by millions of water bodies smaller than 1 km2. Similar analyses of impoundments based on inventories of large, engineered dams show that impounded waters cover approximately 0.26 million km2. However, construction of low-tech farm impoundments is estimated to be between 0.1 % and 6% of farm area worldwide, dependent upon precipitation, and represents >77,000 km 2 globally, at present. Overall, about 4.6 million km2 of the earth's continental "land" surface (>3%) is covered by water. These analyses underscore the importance of explicitly considering lakes, ponds, and impoundments, especially small ones, in global analyses of rates and processes. © 2006, by the American Society of Limnology and Oceanography, Inc.
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.
1. Shallow lakes may switch from a state dominated by submerged macrophytes to a phytoplankton-dominated state when a critical nutrient concentration is exceeded. We explore how climate change may affect this critical nutrient concentration by linking a graphical model to data from 83 lakes along a large climate gradient in South America.
2. The data indicate that in warmer climates, submerged macrophytes may tolerate more underwater shade than in cooler lakes. By contrast, the relationship between phytoplankton biomass [approximated by chlorophyll-a (chl-a) or biovolume] and nutrient concentrations did not change consistently along the climate gradient. In warmer climates, the correlation between phytoplankton biomass and nutrient concentrations was overall weak, especially at low total phosphorus (TP) concentrations where the chl-a/ TP ratio could be either low or high.
3. Although the enhanced shade tolerance of submerged plants in warmer lakes might promote the stability of their dominance, the potentially high phytoplankton biomass at low nutrient concentrations suggests an overall low predictability of climate effects.
4. We found that near-bottom oxygen concentrations are lower in warm lakes than in cooler lakes, implying that anoxic P release from eutrophic sediment in warm lakes likely causes higher TP concentrations in the water column. Subsequently, this may lead to a higher phytoplankton biomass in warmer lakes than in cooler lakes with similar external nutrient loadings.
5. Our results indicate that climate effects on the competitive balance between submerged macrophytes and phytoplankton are not straightforward.
Dominance by cyanobacteria hampers human use of lakes and reservoirs worldwide. Previous studies indicate that excessive nutrient loading and warmer conditions promote dominance by cyanobacteria, but evidence from global scale field data has so far been scarce. Our analysis, based on a study of 143 lakes along a latitudinal transect ranging from subarctic Europe to southern South America, shows that although warmer climates do not result in higher overall phytoplankton biomass, the percentage of the total phytoplankton biovolume attributable to cyanobacteria increases steeply with temperature. Our results also reveal that the percent cyanobacteria is greater in lakes with high rates of light absorption. This points to a positive feedback because restriction of light availability is often a consequence of high phytoplankton biovolume, which in turn may be driven by nutrient loading. Our results indicate a synergistic effect of nutrients and climate. The implications are that in a future warmer climate, nutrient concentrations may have to be reduced substantially from present values in many lakes if cyanobacterial dominance is to be controlled.
The thawing and subsidence of frozen peat mounds (palsas) in permafrost landscapes results in the formation of organic-rich thermokarst lakes. We examined the effects of palsa degradation on CH4 and CO2 emissions by comparing thermokarst lakes at two peatland locations in subarctic Québec, Canada: in the northern discontinuous permafrost region, and in southern sporadic permafrost where palsas are more rapidly degrading. The lakes were shallow (< 3 m) but stratified at both sites, and most had anoxic bottom waters. The surface waters at both sites were supersaturated in CH4 and CO2, and to a greater extent in the southern lakes, where the surface CH4 concentrations were up to 3 orders of magnitude above air equilibrium. Concentrations of CH4 and CO2 increased by orders of magnitude with depth in the southern lakes, however these gradients were less marked or absent in the North. Strong CH4 and CO2 emissions were associated with gas ebullition, but these were greatly exceeded by diffusive fluxes, in contrast to thermokarst lakes studied elsewhere. Also unusual relative to other studies to date, the surface concentrations of both gases increased as a linear function of water column depth, with highest values over the central, deepest portion of the lakes. Radiocarbon dating of ebullition gas samples showed that the CH4 had 14C-ages from 760 yr to 2005 yr before present, while the CO2 was consistently younger. Peatland thermokarst lakes may be an increasingly important source of greenhouse gases as the southern permafrost limit continues to shift northwards. © 2016 Association for the Sciences of Limnology and Oceanography.
Effects of global warming on the physical, chemical, ecological structure and function and biodiversity of freshwater ecosystems are not well understood and there are many opinions on how to adapt aquatic environments to global warming in order to minimize the negative effects of climate change. Climatic Change and Global Warming of Inland Waters presents a synthesis of the latest research on a whole range of inland water habitats - lakes, running water, wetlands - and offers novel and timely suggestions for future research, monitoring and adaptation strategies. A global approach, offered in this book, encompasses systems from the arctic to the Antarctic, including warm-water systems in the tropics and subtropics and presents a unique and useful source for all those looking for contemporary case studies and presentation of the latest research findings and discussion of mitigation and adaptation throughout the world. Edited by three of the leading limnologists in the field this book represents the latest developments with a focus not only on the impact of climate change on freshwater ecosystems but also offers a framework and suggestions for future management strategies and how these can be implemented in the future.
Floating macrophytes, including water hyacinth (Eichhornia crassipes), are dominant invasive organisms in tropical aquatic systems, and they may play an important role in modifying the gas exchange between water and the atmosphere. However, these systems are underrepresented in global datasets of greenhouse gas (GHG) emissions. This study investigated the carbon (C) turnover and GHG emissions from a small (0.6 km2) water-harvesting lake in South India and analysed the effect of floating macrophytes on these emissions. We measured carbon dioxide (CO2) and methane (CH4) emissions with gas chambers in the field as well as water C mineralization rates and physicochemical variables in both the open water and in water within stands of water hyacinths. The CO2 and CH4 emissions from areas covered by water hyacinths were reduced by 57% compared with that of open water. However, the C mineralization rates were not significantly different in the water between the two areas. We conclude that the increased invasion of water hyacinths and other floating macrophytes has the potential to change GHG emissions, a process that might be relevant in regional C budgets.
Inland waters are an important component of the global carbon cycle. Although they contribute to greenhouse gas emissions, estimates of carbon processing in these waters are uncertain. The global extent of very small ponds, with surface areas of less than 0.001 km2, is particularly difficult to map, resulting in their exclusion from greenhouse gas budget estimates. Here we combine estimates of the lake and pond global size distribution, gas exchange rates, and measurements of carbon dioxide and methane concentrations from 427 lakes and ponds ranging in surface area from 2.5 m2 to 674 km2. We estimate that non-running inland waters release 0.583 Pg C yr-1. Very small ponds comprise 8.6% of lakes and ponds by area globally, but account for 15.1% of CO2 emissions and 40.6% of diffusive CH4 emissions. In terms of CO2 equivalence, the ratio of CO2 to CH4 flux increases with surface area, from about 1.5 in very small ponds to about 19 in large lakes. The high fluxes from very small ponds probably result from shallow waters, high sediment and edge to water volume ratios, and frequent mixing. These attributes increase CO2 and CH4 supersaturation in the water and limit efficient methane oxidation. We conclude that very small ponds represent an important inland water carbon flux.
ABSTRACT: Elevated temperatures and extreme climatic events, such as heat waves, can negatively
affect submerged macrophytes. Here, we investigated how submerged macrophytes
adapted to 3 different temperatures (ambient, ca. +3°C and ca. +4.5°C) responded to a heat wave.
After 10 yr of adaptation, the shoots of 2 species of submerged macrophytes, Elodea canadensis
and Potamogeton crispus, were collected from each of the 3 temperature treatments and transferred
to 2 heated treatments for 1 mo. The 2 heated treatments were then exposed to a 1 mo heat
wave with an additional 5°C temperature increase. For P. crispus, total biomass did not differ
among the plants adapted to the different temperatures or between the 2 heated treatments for
the duration of the experiment. Plants adapted to the highest temperatures, however, produced a
larger number of smaller turions before the heat wave and allocated less biomass to elongation
before and after the heat wave. For E. canadensis, the plants adapted to higher temperatures had
higher total biomass before and during the heat wave and allocated more biomass to roots and
leaves during the heat wave. Most indicators (e.g. length and biomass) of macrophyte performance
measured during the experiment did not differ between the 2 heated treatments. In summary,
after the 10 yr adaptation to higher temperatures, the submerged macrophytes showed
adaptive changes in growth and asexual reproduction and responded in a complex way to the
heat wave depending on species, growth status and adaptation temperature.
Eight empirical equations for the relationship between photosynthetic rate and surface light were examined in Danish streams. A hyperbolic relationship described data with good precision, whereas a linear relationship was not sufficient. Verification of the model included time series analysis of the one step ahead prediction errors between predicted and measured oxygen concentrations.
Socio-economic stress from the unequivocal warming of the global climate system could be mostly felt by societies through weather and climate extremes. The vulnerability of European citizens was made evident during the summer heatwave of 2003 (refs,) when the heat-related death toll ran into tens of thousands. Human influence at least doubled the chances of the event according to the first formal event attribution study, which also made the ominous forecast that severe heatwaves could become commonplace by the 2040s. Here we investigate how the likelihood of having another extremely hot summer in one of the worst affected parts of Europe has changed ten years after the original study was published, given an observed summer temperature increase of 0.81 K since then. Our analysis benefits from the availability of new observations and data from several new models. Using a previously employed temperature threshold to define extremely hot summers, we find that events that would occur twice a century in the early 2000s are now expected to occur twice a decade. For the more extreme threshold observed in 2003, the return time reduces from thousands of years in the late twentieth century to about a hundred years in little over a decade.
Water quality is largely dependent on the structure and function of food webs in aquatic ecosystems. Food webs can be controlled by resource limitation and by predation. Recently it has been widely accepted that both control mechanisms are closely coupled. Therefore, top-down control of the food web by maniuplating fish stocks should be applied with caution as the sole management tool. Long-term stability of top-down control depends on numerous factors and it seems to increase with increasing resource limitation. Therefore, combined bottom-up/top-down control is regarded to be the most promising strategy in water-quality management. -from Author
Limitations of linear regression applied on ecological data. - Things are not always linear additive modelling. - Dealing with hetergeneity. - Mixed modelling for nested data. - Violation of independence - temporal data. - Violation of independence spatial data. - Generalised linear modelling and generalised additive modelling. - Generalised estimation equations. - GLMM and GAMM. - Estimating trends for Antarctic birds in relation to climate change. - Large-scale impacts of land-use change in a Scottish farming catchment. - Negative binomial GAM and GAMM to analyse amphibian road killings. - Additive mixed modelling applied on deep-sea plagic bioluminescent organisms. - Additive mixed modelling applied on phyoplankton time series data. - Mixed modelling applied on American Fouldbrood affecting honey bees larvae. - Three-way nested data for age determination techniques applied to small cetaceans. - GLMM applied on the spatial distribution of koalas in a fragmented landscape. - GEE and GLMM applied on binomial Badger activity data.
As a result of the Intergovernmental Panel on Climate Change reports, it is now widely accepted that global warming is a fact. It is also recognized that the extent of the effects will differ at different locations, being stronger at high latitudes and less strong at lower latitudes (IPCC 2007). According to a set of regional climate models, Denmark can expect an overall warmer climate and particularly higher temperatures in late summer and winter (van Roosmalen et al . 2010). An overall wetter climate can be expected, with more precipitation during winter, but likely lower precipitation during summer (van Roosmalen et al . 2010). Such changes will have profound effects on discharge, nutrient input, temperature, water column stability (in lakes), nutrient dynamics as well as biological structure and dynamics. In this chapter we show that Danish freshwaters are already facing changes that can be attributed to the recent warming during the past two decades. We use long-term data from 18 streams and between 20 and 250 lakes.
Carbon mass balance studies of 20 small, forested catchments and seven
lakes on the Precambrian Shield in central Ontario during a 12-year
period have shown that most carbon in the study lakes is derived from
terrestrial sources, primarily peatlands, and that carbon retained by
lakes (total inputs less loss via outflow from the lake) is partitioned
between lake sediments and the atmosphere. The partitioning of retained
carbon is a function of lake alkalinity: the ratio of evaded/sediment
carbon increases with decreasing alkalinity. These carbon flux
relationships were applied to the global boreal forest biome to evaluate
the role of aquatic systems with respect to carbon fluxes and pools
within the biome. We calculate that approximately 66 Tg of organic and
inorganic carbon are exported annually from the boreal forest biome to
adjacent surface waters of which 14 to 36 Tg reach coastal waters. The
remainder is either evaded to the atmosphere (12 to 21 Tg
yr-1) or stored in lake sediments (18 to 31 Tg
yr-1). Approximately 120 Pg of carbon may be stored in boreal
lake sediments, a conservative estimate based on an accumulation period
of 5,000 years and a size comparable to recent boreal pool estimates of
419 Pg for peatlands and 64 Pg for plant biomass. Hence the amount of
total carbon stored in the boreal forest biome may be significantly
larger because of storage in lake sediments.
N2O emissions were measured monthly for 1 year using the static chamber method along the littoral and pelagic zones of Meiliang Bay in hyper-eutrophic Taihu Lake. The results indicated that littoral zones were the “hotspots” of N2O emissions (−278∼2101μgN2Om−2h−1). While the littoral zone accounted for only 5.4% of the area of Meiliang Bay, the N2O emissions from the littoral zones were about 43.6% of total emissions from the bay. The importance of spatial variation on N2O emissions was demonstrated by dividing the lake into four unique zones through cluster analysis. The eulittoral zone was the key area of N2O emissions with an annual 5% trimmed mean 429.5μgN2Om−2h−1, following by supralittoral (138.8μgN2Om−2h−1), infralittoral (98.9μgN2Om−2h−1), and pelagic zones (15.7μg N2Om−2h−1). Further, N2O emissions showed a seasonal trend. Excessive N2O emissions during algal bloom periods suggested that algae played a significant role in the emission of N2O.
Shallow lakes are likely to be strongly impacted by climate changes and, in particular, by increased tempera- tures. To enable realistic experimental studies of the effects of higher temperatures on in-lake processes and dynamics, technologically advanced systems are required. This paper presents design details, operating charac- teristics, and background information on a currently operating experimental flow-through mesocosm system that allows investigation of the interactions between simulated climate warming and eutrophication and their impacts on biological structure and ecosystem processes in shallow lakes. We use 24 mesocosms to combine three temperature scenarios (one unheated and two heated relative to the Intergovernmental Panel on Climate Change climate scenario A2 and A2 + 50%, respectively) and two nutrient levels (enriched and nonenriched). Planktivorous fish (male sticklebacks, Gasterosteus aculeatus) are stocked in accordance with the nutrient level. The water residence time is regulated by the semicontinuous addition of water and is approximately 2.5 mo in each mesocosm. For heating, we use electrically powered heating elements. The heating system has performed well over 16 mo of continuous heating, and seasonal and diurnal temperature variations of the unheated refer- ence mesocosms were paralleled well by the heated mesocosms. The performance of the flow-through system and the heating technique are discussed with special emphasis on strengths, limitations, and potential improvements of the system. To illustrate the performance of the system and its potential, we present data for selected periods on total phosphorus retention in the mesocosms and system primary production and respiration.
Exchange of CO2 under low turbulence conditions and high pH can be enhanced by hydration reactions of CO, with hydroxide ions and water molecules in the boundary layer. A series of field experiments was performed on several lakes, including alkaline closed-basin lakes, using enclosures (helmets) to study the enhancement process in nature. In addition, the enhancement of CO, exchange was studied in laboratory experiments with freshwater and seawater. The results of the experiments are compared with published theoretical calculations. Within the experimental uncertainties and shortcomings of the chemical enhancement models, reasonable agreement was observed between experimental and theoretical results for seawater. The experiments indicate, in accordance with theory, that chemical enhancement has a minor effect on air-sea gas exchange of CO, under average oceanic turbulence conditions. However, for the equatorial CO, source regions, with high temperatures and low winds, the calculated CO, enhancement amounts to 4-8% of the total exchange. The observations on lakes show poorer agreement with models, which is attributed to experimental uncertainty and poor characterization of the chemistry of the lake waters. The experiments show that chemical enhancement of CO, is ubiquitous for the alkaline closed-basin lakes with enhancements of up to a factor of three. Air-water gas transfer for slightly soluble gases is re- tarded in the frst several hundred microns of the liquid (Liss 1983). Slightly soluble gases arc defined as those that have Ostwald solubility coefficients (0) of < 10, where p is the volum,e of gas at temperature T and partial pres- sure
Detailed gas exchange measurements from two circular and one linear wind/wave tunnels are presented. Heat, He, CH4, CO2, Kr, and Xe have been used as tracers. The experiments show the central importance of waves for the water-side transfer process. With the onset of waves the Schmidt number dependence of the transfer velocity k changes from k ~ Sc-2/3 to k ~ Sc-1/2 indicating a change in the boundary conditions at the surface. Moreover, energy put into the wave field by wind is transferred to near-surface turbulence enhancing gas transfer. The data show that the mean square slope of the waves is the best parameter to characterize the free wavy surface with respect to water-side transfer processes.
The rates of transfer of radon from sea to air estimated by Peng et al. (1979) from an extensive series of observations of radon profiles made during the Geosecs oceanographical cruises, are reexamined in relation to wind speed dependence. It is concluded that there is a significant increase with wind speed, but the extent of the increase is uncertain. At 7 m s-1, the transfer velocity is indicated to be some 33%; greater than the BOMEX value of Broecker and Peng (1974) — a reasonably close agreement These rates exceed the theoretical smooth-surface value by a factor of two or three. It is shown that little of this excess can be attributed to the surface dilation effect of capillary waves.
In summer 2003 central Europe suffered an unusually severe heat wave, with air temperatures similar to those predicted for an average summer during the late 21st century. We use a unique set of over half a century of lake data from two lakes in Switzerland to determine the effect of the 2003 heat wave on water temperature and oxygen conditions in order to assess how temperate lakes will react when exposed to the increased ambient summer air temperatures that will be encountered in a generally warmer world and to test the predictions of relevant simulation models. In both lakes, surface temperature and thermal stability in summer 2003 were the highest ever recorded, exceeding the long-term mean by more than 2.5 standard deviations. The extremely high degree of thermal stability resulted in extraordinarily strong hypolimnetic oxygen depletion. These results are consistent with the predictions of the simulation models. Additionally, the results indicate that climatic warming will increase the risk of occurrence of deep-water anoxia, thus counteracting long-term efforts that have been undertaken to ameliorate the effects of anthropogenic eutrophication.
Daily integrals of primary productivity were measured for 369 days in the macrophyte- dominated Gryde River (NW Jutland, Denmark) between July 1979 and September 1980. All seasons were distinct, with high primary productivity during summer, a gradual transition in autumn, low primary productivity in winter, and a rapid transition to summer conditions during April. Those seasons corresponded with the condition of the macrophytes, with high biomass and surface area during summer, a gradual decline in autumn, senescence during winter, and development of new shoots during April and May. The remarkably constant relationship between daily integrals of primary productivity and light (in comparison with that in rivers without macrophytes) could be described during both summer and winter by a hyperbolic equation, with more shade adaptation but less maximum photosynthesis during winter than during summer. The hyperbolic relationship held for both oxygen- and carbon- based measurements. The mean value for the photosynthetic quotient (P-Q.) was close to one, suggesting little transport of oxygen and inorganic carbon to or from roots. We could not discern effects on primary productivity of other variables such as temperature, which seemed unimportant in comparison to light.
Measured Bunsen solublllty coefficients reported In the
ilterature are used to derlve functlons that permlt accurate
calculatlon of the concentratlon of methane, carbon
monoxide, and hydrogen In water and sea water at
equilibrium wlth the normal atmosphere. Bunsen
coefflclents are fltted to equations establlshed by Welss
which give Bunsen coefflclents as functions of
temperature and sailnlty. Tables of Bunsen coefficients
coverlng the temperature range -2 to +30 O C and the
saiinlty range 0-40 parts per thousand are calculated for
each gas from the fltted equatlons. The data are also
fltted to an atmospheric equilibrlum solublllty function,
which has a form slmilar to the Bunsen coefficient
equation, but which Includes the atmospheric gas
concentratlon as a variable. Coefficients for thls equation
are glven to allow calculatlon of the concentratlon of
dlssolved methane, carbon monoxide, and hydrogen in
equliibrium wlth moist air at 1 atm total pressure In unlts
of nL/L, nmol/L, nL/kg, and nmol/kg sea water.
The solubility of nitrous oxide in pure water and seawater has been measured microgasometrically over the range 0–40°C. The data have been corrected for nonideality and are fitted to equations in temperature and salinity of the form used previously to fit the solubilities of other gases. The fitted values have a precision of 0.1% and an estimated accuracy of 0.3%. The nonideal behavior of nitrous oxide—air mixtures is discussed, and the solubility of atmospheric nitrous oxide is presented in parametric form. A similar parametric representation for the solubility of atmospheric carbon dioxide is given in the Appendix.
1. Climate warming is expected to change respiration in shallow lakes but to an extent that depends on nutrient state.
2. We measured sediment respiration (SR) over the season in the dark on intact sediment cores taken from a series of flow-through, heated and unheated, nutrient-enriched and unenriched mesocosms. The natural seasonal temperature cycle ranged from 2 to 20 °C in the unheated mesocosms. In the heated mesocosms, the temperature was raised 4–6 °C above ambient temperatures, depending on season, following the A2 climate change scenario downscaled to the local position of the mesocosms, but enlarged by 50%. We further measured ecosystem respiration (ER) in the mesocosms based on semi-continuous oxygen measurements.
3. SR changed over the season and was approximately ten times higher in summer than in winter. SR showed no clear response to warming in the nutrient-enriched treatment, while it increased with warming in the unenriched mesocosms which also had lower fish densities.
4. ER was not affected by artificial warming or nutrient enrichment, but it was ten times higher in summer than in winter.
5. SR contributed 24–32% to ER. The SR:ER ratio was generally stimulated by warming and was higher in winter than in summer, especially in the nutrient-enriched mesocosms.
6. Our results indicate that climate warming may lead to higher SR, especially in clear, macrophyte-dominated systems. Moreover, the contribution of SR to ER will increase with higher temperatures, but decrease as the winters get shorter.