Rapid shifts in methanotrophic bacterial communities mitigate methane emissions from a tropical hydropower reservoir and its downstream river

Authors

Paula C. J. Reis, Clara Ruiz-González, Sophie Crevecoeur, Cynthia Soued, Yves T. Prairie

Methane-oxidizing bacteria (MOB) present in the water column mitigate methane (CH₄) emissions from hydropower complexes to the atmosphere. By creating a discontinuity in rivers, dams cause large environmental variations, including in CH₄ and oxygen concentrations, between upstream, reservoir, and downstream segments. Although highest freshwater methanotrophic activity is often detected at low oxygen concentrations, CH₄ oxidation in well-oxygenated downstream rivers below dams has also been reported. Here we combined DNA and RNA high-throughput sequencing with microscopic enumeration (by CARD-FISH) and biogeochemical data to investigate the abundance, composition, and potential activity of MOB taxa from upstream to downstream waters in the tropical hydropower complex Batang Ai (Malaysia). High relative abundance of MOB (up to 61% in 16S rRNA sequences and 19% in cell counts) and enrichment of stable isotopic signatures of CH₄ (up to 0‰) were detected in the hypoxic hypolimnion of the reservoir and in the outflowing downstream river. MOB community shifts along the river-reservoir system reflected environmental sorting of taxa and an interrupted hydrologic connectivity in which downstream MOB communities resembled reservoir’s hypolimnetic communities but differed from upstream and surface reservoir communities. In downstream waters, CH₄ oxidation was accompanied by fast cell growth of particular MOB taxa. Our results suggest that rapid shifts in active MOB communities allow the mitigation of CH₄ emissions from different zones of hydropower complexes, including in quickly re-oxygenated rivers downstream of dams.

Eutrophication Drives Extreme Seasonal CO2 Flux in Lake Ecosystems

Authors

Ana M. Morales-Williams, Alan D. Wanamaker Jr., Clayton J. Williams & John A. Downing

Lakes process a disproportionately large fraction of carbon relative to their size and spatial extent, representing an important component of the global carbon cycle. Alterations of ecosystem function via eutrophication change the balance of greenhouse gas flux in these systems. Without eutrophication, lakes are net sources of CO₂ to the atmosphere, but in eutrophic lakes this function may be amplified or reversed due to cycling of abundant autochthonous carbon. Using a combination of high-frequency and discrete sensor measurements, we calculated continuous CO₂ flux during the ice-free season in 15 eutrophic lakes. We found net CO₂ influx over our sampling period in 5 lakes (− 47 to − 1865 mmol m⁻²) and net efflux in 10 lakes (328 to 11,755 mmol m⁻²). Across sites, predictive models indicated that the highest efflux rates were driven by nitrogen enrichment, and influx was best predicted by chlorophyll a concentration. Regardless of whether CO₂ flux was positive or negative, stable isotope analyses indicated that the dissolved inorganic carbon pool was not derived from heterotrophic degradation of terrestrial organic carbon, but from degradation of autochthonous organic carbon, mineral dissolution, and atmospheric uptake. Optical characterization of dissolved organic matter revealed an autochthonous organic matter pool. CO₂ influx was correlated with autochthony, while efflux was correlated with total nitrogen and watershed wetland cover. Our findings suggest that CO₂ uptake by primary producers during blooms can contribute to continuous CO₂ influx for days to months. Conversely, eutrophic lakes in our study that were net sources of CO₂ to the atmosphere showed among the highest rates reported in the literature. These findings suggest that anthropogenic eutrophication has substantially altered biogeochemical processing of carbon on Earth.

 

Phytoplankton in extreme environments: importance and consequences of habitat permanency

Authors

Judit Padisák & Luigi Naselli-Flores

There is hardly any sunshine exposed surface on this Earth, be it water or terrain, which would not support some biota. Still, many habitats offer harsh conditions requiring specialized physiological adaptations to survive. These environments are referred to as extremes; often inhabited by extremophilic organisms. In this review, characteristic species and assemblage properties of phytoplankton inhabiting extreme environments (especially lakes and pools where planktic life is potentially possible and independently of their origin) in terms of alkalinity, acidity, DOC, salinity, temperature, light and mixing regime will be outlined. Lakes characterized by more than a single extreme are common (e.g. saline + alkaline; acidic + high DOC + high metal content + low light). At the edge of extremes (e.g. pH of 1; salinity over ~ 100–150 g l⁻¹) single species with appropriate physiological adaptation are selected and the phytoplankton is often dominated by a single species (monodominant) setting compositional diversity to zero. Under less extreme conditions permanent equilibria may persist; in many cases over several years in contrast to „average” lakes where equilibria are rare and ephemeral. Food webs depending on „extreme phytoplankton” are often atypical for example because the microbial loop is of prior importance or because birds are top predators.

 

Physiological and biochemical responses of Microcystis aeruginosa to phosphine (PH3) under elevated CO2

Authors

Xiaojun Niu, Dongqing Zhang, Runyuan Zhang, Qi Song, Yankun Li, Mo Wang

Phosphine (PH₃) is an important factor driving the outbreak of cyanobacterial blooms that produce toxic microcystin threating human health. To clarify the physiological and biochemical responses of cyanobacteria to PH₃ under elevated CO₂ concentration, Microcystis aeruginosa was used in the coupling treatment of 1000 ppmv CO₂ and PH₃ at different concentrations respectively. The chlorophyll a (Chl-a), carotenoid, net photosynthetic rate and total protein of M. aeruginosa exhibited evidently increasing tendency under the coupling treatment of 1000 ppmv CO₂ and PH₃ at different concentrations (7.51 × 10⁻³, 2.48 × 10⁻², 7.51 × 10⁻² mg/L). The coupling treatments resulted in the higher concentrations of Chl-a and carotenoid of M. aeruginosa, compared to those in the control and the treatment with CO₂ alone, and their enhancement increased with the increase in PH₃ concentrations. The total antioxidant capacity (T-AOC) in the coupling treatment with CO₂ and PH₃ of 2.48 × 10⁻² mg/L and 7.51 × 10⁻³ mg/L showed increasing tendency, compared to the treatment with PH₃ alone. Additionally, the coupling treatment with 1000 ppmv CO₂ and PH₃ also altered the pH and DO level in the culture medium. In this regard, the coupling treatment with CO₂ and PH₃ at an appropriate concentration can enhance the resistance of M. aeruginosa to PH₃ toxicity and is beneficial to the reproduction of M. aeruginosa, presumably resulting in potential for the outbreak of cyanobacteria bloom. Given the concern about global warming and the increase in atmospheric CO₂ level, our research laid a foundation for the scientific understanding of the correlation between PH₃ and cyanobacteria blooms.

Carbon and nutrients transfer from primary producers to lake sediments – a stoichiometric approach

Authors

Lech Kufel, Małgorzata Strzałek, Elżbieta Biardzka, Marcin Becher

We aimed to demonstrate different input of organic and inorganic carbon, nitrogen and phosphorus from three main groups of primary producers (phytoplankton, charophytes and vascular submerged macrophytes) to respective lake sediments. Studies were carried out in one eutrophic and two mesotrophic lakes. Samples of sediments were taken from profundal and from littoral zones, the latter divided into such overgrown by charophytes and others covered by vascular submerged macrophytes. We applied a stoichiometric approach to illustrate variable functional carbon to nutrients relationships. Among profundal sediments, the lowest organic to inorganic carbon ratio was found in sediments from the eutrophic lake due to precipitation of calcium carbonate during algal blooms. Extremely low inorganic carbon input to profundal sediment of one of the mesotrophic lakes may be explained by low phytoplankton production but also by dissolution of once deposited calcium carbonates. Charophyte-dominated littoral sediments contained significantly more inorganic carbon than other littoral and profundal sediments. Comparison of stoichiometric ratios between plant standing crop and underlying littoral sediments showed significant enrichment of sediments in nitrogen manifested by reduction of organic carbon to total nitrogen ratio during plant decomposition taking place both in charophyte and in vascular plant stands. We also attempted to divide phosphorus pool in sediments into organic P and calcium-bound P present in charophyte stands and in profundal sediments of eutrophic lake. In the former, calcium-bound P was estimated at 17–19% of the total P pool while in profundal sediments it amounted 42% of the total P. This difference suggests that calcium carbonate settling during algal blooms in a eutrophic lake may be more effective in P trapping than calcite encrustations covering charophyte plants in littoral sites. In conclusions, we underline the need of considering often neglected inorganic fractions of carbon and phosphorus to get better insight into carbon and nutrient burial in lake sediments.

Regulation of carbon dioxide and methane in small agricultural reservoirs: optimizing potential for greenhouse gas uptake

Authors

Jackie R. Webb, Peter R. Leavitt, Gavin L. Simpson, Helen M. Baulch, Heather A. Haig, Kyle R. Hodder, and Kerri Finlay

Small farm reservoirs are abundant in many agricultural regions across the globe and have the potential to be large contributing sources of carbon dioxide (CO₂) and methane (CH₄) to agricultural landscapes. Compared to natural ponds, these artificial waterbodies remain overlooked in both agricultural greenhouse gas (GHG) inventories and inland water global carbon (C) budgets. Improved understanding of the environmental controls of C emissions from farm reservoirs is required to address and manage their potential importance in agricultural GHG budgets. Here, we conducted a regional-scale survey (∼ 235 000 km²) to measure CO₂ and CH₄ surface concentrations and diffusive fluxes across 101 small farm reservoirs in Canada’s largest agricultural area. A combination of abiotic, biotic, hydromorphologic, and landscape variables were modelled using generalized additive models (GAMs) to identify regulatory mechanisms. We found that CO₂ concentration was estimated by a combination of internal metabolism and groundwater-derived alkalinity (66.5 % deviance explained), while multiple lines of evidence support a positive association between eutrophication and CH₄ production (74.1 % deviance explained). Fluxes ranged from −21 to 466 and 0.14 to 92 mmol m⁻² d⁻¹ for CO₂ and CH₄, respectively, with CH₄ contributing an average of 74 % of CO₂-equivalent (CO₂-e) emissions based on a 100-year radiative forcing. Approximately 8 % of farm reservoirs were found to be net CO₂-e sinks. From our models, we show that the GHG impact of farm reservoirs can be greatly minimized with overall improvements in water quality and consideration to position and hydrology within the landscape.

Methane emission offsets carbon dioxide uptake in a small productive lake

Authors

Dominic Vachon Timon Langenegger Daphne Donis Stan E. Beaubien Daniel F. McGinnis

Here, we investigate the importance of net CH₄ production and emissions in the carbon (C) budget of a small productive lake by monitoring CH₄, CO₂, and O₂ for two consecutive years. During the study period, the lake was mostly a net emitter of both CH₄ and CO₂, while showing positive net ecosystem production. The analyses suggest that during the whole study period, 32% ± 26% of C produced by net ecosystem production was ultimately converted to CH₄ and emitted to the atmosphere. When converted to global warming potential, CH₄ emission (in CO₂ equivalents) was about 3–10 times higher than CO₂ removal from in‐lake net ecosystem production over 100‐yr and 20‐yr time frames, respectively. Although more work in similar systems is needed to generalize these findings, our results provide evidence of the important greenhouse gas imbalance in human‐impacted aquatic systems.

 

Effect of eutrophication and humification on nutrient cycles and transfer efficiency of matter in freshwater food webs

Authors

Maciej Karpowicz, Piotr Zieliński, Magdalena Grabowska, Jolanta Ejsmont-Karabin, Joanna Kozłowska & Irina Feniova

We evaluated how eutrophication and humification impacted nutrient cycles and the efficiency of carbon transfer in the planktonic food webs of 12 lakes in north-eastern Poland that differed in trophic state. Our results indicated that the effectiveness of carbon transfer between phytoplankton and zooplankton varied from 0.0005% to 0.14%, which is much lower than the theoretical 10%. The highest efficiency of carbon transfer occurred in the mesotrophic lakes due to the higher hypolimnetic zooplankton production, while the lowest efficiency was observed in the dystrophic lakes and in one eutrophic lake that was dominated by cyanobacteria. Inedible algae (e.g. Gonyostomum semen) and cyanobacteria appeared to be the main factors reducing the efficiency of the transfer of matter in pelagic food webs. The results of our study showed that plankton communities are a key component of the nutrient cycle in freshwater food webs. Phytoplankton were a very effective nitrogen sink, and in the mesotrophic lakes, up to 99% of the total nitrogen was sequestrated in phytoplankton. As a result, there was a depletion of inorganic nitrogen in the upper water layer. Furthermore, zooplankton were an important phosphorus sink, thus significantly influencing the nutrient cycles.

Emission of greenhouse gases from French temperate hydropower reservoirs

Authors

Vincent Chanudet, Jérémie Gaillard, Johan Lambelain, Maud Demarty, Stéphane Descloux, Jim Félix-Faure, Alain Poirel & Etienne Dambrine

The emission of CO₂ and CH₄ by diffusion, bubbling and downstream was measured in ten reservoirs representative of the diversity of French hydropower reservoirs in 2016. In all reservoirs, higher fluxes were measured in summer than in spring and winter. Low fluxes were measured in alpine reservoirs as compared to run-of-the-river and storage reservoirs. The low temperatures as well as the low organic matter input from the watershed explained this observation. Bubbling was higher in run-of-the-river reservoirs, as compared to storage reservoirs. This was related to a higher ratio between the length of wooded river network in the watershed, and the reservoir surface area. This ratio was considered as a proxy for allochthonous particulate organic matter input per reservoir surface unit and its accumulation in the sediments. In the larger storage reservoirs, this preferential sedimentation area was limited to the river-reservoir transition zone, the extent of which is primarily a function of reservoir hydrodynamic and morphological parameters. Conversely, the long water residence time in deep storage reservoirs favoured greenhouse gas (GHG) accumulation in the bottom water and diffusion and downstream pathways as compared to bubbling. Classical drivers of GHG emissions in large reservoirs partly failed to explain our measurements, especially for bubbling which seemed to be primarily controlled by allochthonous particulate organic matter input per reservoir surface area. This may results from the small size and the large diversity of the studied reservoirs as compared to the larger systems classically used for global estimates.

Spatial and Temporal Variability of Nutrient Dynamics and Ecosystem Metabolism in a Hyper-eutrophic Reservoir Differ Between a Wet and Dry Year

Authors

Tanner J. Williamson, Michael J. Vanni & William H. Renwick

Climate change alters hydrologic regimes, including their variability. Effects will be pronounced in aquatic ecosystems, where resource subsidies (e.g., nutrients, carbon) drive key ecosystem processes. However, we know little about how changing hydrologic regimes will modulate the spatiotemporal dynamics of lake biogeochemistry and ecosystem metabolism. To address this, we quantified ecosystem metabolism and nutrient dynamicsat high spatial resolution in Acton Lake, a hyper-eutrophic reservoir in the Midwestern US. We captured two consecutive growing seasons with markedly different watershed discharge and nutrient loading. Temporal variability often exceeded spatial variability in both wet and dry years. However, relative spatial variability was higher in the dry year, suggesting that internal processes are more important in structuring spatial dynamics in dry years. Strikingly, marked differences in watershed discharge and nutrient loading between years produced relatively small differences in many lake metrics, suggesting resilience to hydrologic variability. We found little difference in gross primary productivity between wet and dry years, but ecosystem respiration was higher in the wet year, shifting net ecosystem production below zero. Discrete storm events produced strong, yet ephemeral and spatially explicit effects, reflective of the balance of stream input and discharge over the dam. Increases in limiting nutrients were restricted to near stream inlets and returned to pre-storm baseline within days. Ecosystem metabolism was suppressed during storm events, likely due to biomass flushing. Understanding how changing hydrologic regimes will mediate spatiotemporal dynamics of ecosystem metrics is paramount to preserving the ecological integrity and ecosystem services of lakes under future climates.