Azolla cultivation enables phosphate extraction from inundated former agricultural soils

To combat the global loss of wetlands and their essential functions, the restoration and creation of wetlands is imperative. However, wetland development is challenging when soils have been in prolonged agricultural use, often resulting in a substantial nutrient legacy, especially of phosphorous (P). Inundating these soils typically leads to P mobilization, resulting in poor water quality and low biodiversity recovery. As a potential novel means to overcome this challenge, we tested whether cultivation of the floating fern Azolla filiculoides could simultaneously extract and recycle P, and provide a commercial product. Azolla has high growth rates due to the nitrogen fixing capacity of its microbiome and is capable of luxury consumption of P. Azolla cultivation may also accelerate soil P mobilization and subsequent extraction by causing surface water anoxia and the release of iron-bound P. To test this approach, we cultivated Azolla on 15 P-rich former agricultural soils in an indoor mesocosm experiment. Soils were inundated and either left unvegetated or inoculated with A. filiculoides during two 8-week cultivation periods. Biomass was harvested at different intervals (weekly/monthly/bimonthly) to investigate the effect of harvesting frequency on oxygen (O2) and nutrient dynamics. We found that Azolla attained high growth rates only on soils with high mobilization of labile P, as plant cover did not reduce surface water O2 concentrations in the first phase after inundation. This concurred with low porewater iron to P ratios (<10) and high porewater P concentrations. A. filiculoides cultivation substantially reduced surface water nutrient concentrations and extracted P at rates up to 122 kg ha-1 yr-1. We conclude that rapid P extraction by A. filiculoides cultivation is possible on soils rich in labile P, offering new perspectives for wetland rehabilitation. Additional field trials are recommended to investigate long-term feasibility, seasonal variations, and the influence of potential grazers and pathogens.

Fe(II)Cl2 amendment suppresses pond methane emissions by stimulating iron-dependent anaerobic oxidation of methane

Aquatic ecosystems are large contributors to global methane (CH4) emissions. Eutrophication significantly enhances CH4-production as it stimulates methanogenesis. Mitigation measures aimed at reducing eutrophication, such as the addition of metal salts to immobilize phosphate (PO43-), are now common practice. However, the effects of such remedies on methanogenic and methanotrophic communities-and therefore on CH4-cycling-remain largely unexplored. Here, we demonstrate that Fe(II)Cl2 addition, used as PO43- binder, differentially affected microbial CH4 cycling-processes in field experiments and batch incubations. In the field experiments, carried out in enclosures in a eutrophic pond, Fe(II)Cl2 application lowered in-situ CH4 emissions by lowering net CH4-production, while sediment aerobic CH4-oxidation rates-as found in batch incubations of sediment from the enclosures-did not differ from control. In Fe(II)Cl2-treated sediments, a decrease in net CH4-production rates could be attributed to the stimulation of iron-dependent anaerobic CH4-oxidation (Fe-AOM). In batch incubations, anaerobic CH4-oxidation and Fe(II)-production started immediately after CH4 addition, indicating Fe-AOM, likely enabled by favorable indigenous iron cycling conditions and the present methanotroph community in the pond sediment. 16S rRNA sequencing data confirmed the presence of anaerobic CH4-oxidizing archaea and both iron-reducing and iron-oxidizing bacteria in the tested sediments. Thus, besides combatting eutrophication, Fe(II)Cl2 application can mitigate CH4 emissions by reducing microbial net CH4-production and stimulating Fe-AOM.

Experimental warming promotes CO2 uptake but hinders carbon incorporation toward higher trophic levels in cyanobacteria-dominated freshwater communities

Shallow freshwaters can exchange large amounts of carbon dioxide (CO2) with the atmosphere and also store significant quantities of carbon (C) in their sediments. Current warming and eutrophication pressures might alter the role of shallow freshwater ecosystems in the C cycle. Although eutrophication has been widely associated to an increase in total phytoplankton biomass and particularly of cyanobacteria, it is still poorly understood how warming may affect ecosystem metabolism under contrasting phytoplankton community composition. We studied the effects of experimental warming on CO2 fluxes and C allocation on two contrasting natural phytoplankton communities: chlorophytes-dominated versus cyanobacteria-dominated, both with a similar zooplankton community with a potentially high grazing capacity (i.e., standardized density of large-bodied cladocerans). The microcosms were subject to two different constant temperatures (control and +4 °C, i.e., 19.5 vs 23.5 °C) and we ensured no nutrient nor light limitation. CO2 uptake increased with warming in both communities, being the strongest in the cyanobacteria-dominated communities. However, only a comparatively minor share of the fixed C translated into increased phytoplankton (Chl-a), and particularly a negligible share translated into zooplankton biomass. Most C was either dissolved in the water (DIC) or sedimented, the latter being potentially available for mineralization into DIC and CO2, or methane (CH4) when anoxic conditions prevail. Our results suggest that C uptake increases with warming particularly when cyanobacteria dominate, however, due to the low efficiency in transfer through the trophic web the final fate of the fixed C may be substantially different in the long run.

CO2, CH4, and N2O emissions from dredged material exposed to drying and zeolite addition under field and laboratory conditions

Dredging, the removal of sediment from water courses, is generally conducted to maintain their navigability and to improve water quality. Recent studies indicate that dredging can significantly reduce aquatic greenhouse gas (GHG) emissions. These studies, however, do not consider the potential emission from the dredged material (sludge) in the depot. In addition, it is unknown if and how GHG emissions from sludge depots can be reduced. Here we present spatiotemporal variations of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes, as well as environmental variables from a sludge depot located in the Netherlands. Measurements were conducted monthly from the time the depot was filled until the sludge was dry and the depot was abolished. We also experimentally assessed the GHG mitigation potential of 1) keeping the sludge permanently inundated, and 2) the addition of different amounts of zeolite to increase sludge nitrogen binding capacity to reduce N2O emissions. In the depot and in the laboratory, a decrease in moisture content coincided with increased CO2 and N2O emissions while CH4 emissions decreased. We observed that permanent inundation reduced emissions (∼4 times less CO2-eq than in drying sludge). Adding zeolite lowered N2O fluxes from permanently inundated sludge but did not reduce total GHG emissions. During the depot's operational period, average CO2, CH4, and N2O fluxes were 5078, 27, and 5 mg m-2 d-1, respectively. GHG emissions from drying sludge occurred mainly in the form of CO2 (73% of the total CO2-eq emissions), with average GHG emission rates comparable to those reported for ditches and ponds. We estimate that approximately 14 tons of CO2-eq were emitted from the 0.011 km2 depot, which contained ∼20,000 m3 of sludge, during its entire operational period, and we argue that more studies are needed, considering different sludge origins, to expand our understanding of sludge depots.

Phosphorus control and dredging decrease methane emissions from shallow lakes

Freshwater ecosystems are an important source of the greenhouse gas methane (CH₄), and their emissions are expected to increase due to eutrophication. Two commonly applied management techniques to reduce eutrophication are the addition of phosphate-binding lanthanum modified bentonite (LMB, trademark Phoslock©) and dredging, but their effect on CH₄ emissions is still poorly understood. Here, this study researched how LMB and dredging affected CH₄ emissions using a full-factorial mesocosm design monitored for 18 months. The effect was tested by measuring diffusive and ebullitive CH₄ fluxes, plant community composition, methanogen and methanotroph activity and community composition, and a range of physicochemical water and sediment variables. LMB addition decreased total CH₄ emissions, while dredging showed a trend towards decreasing CH₄ emissions. Total CH₄ emissions in all mesocosms were much higher in the summer of the second year, likely because of higher algal decomposition and organic matter availability. First, LMB addition lowered CH₄ emissions by decreasing P-availability, which reduced coverage of the floating fern Azolla filiculoides, and thereby prevented anoxia and decreased surface water NH₄⁺ concentrations, lowering CH₄ production rates. Second, dredging decreased CH₄ emissions in the first summer, possibly it removed the methanogenic community, and in the second year by preventing autumn and winter die-off of the rooted macrophyte Potamogeton cripsus. Finally, methanogen community composition was related to surface water NH₄⁺ and O₂, and porewater total phosphorus, while methanotroph community composition was related to organic matter content. To conclude, LMB addition and dredging not only improve water quality, but also decrease CH₄ emissions, mitigating climate change.

Temperature response of aquatic greenhouse gas emissions differs between dominant plant types

Greenhouse gas (GHG) emissions from small inland waters are disproportionately large. Climate warming is expected to favor dominance of algae and free-floating plants at the expense of submerged plants. Through different routes these functional plant types may have far-reaching impacts on freshwater GHG emissions in future warmer waters, which are yet unknown. We conducted a 1,000 L mesocosm experiment testing the effects of plant type and warming on GHG emissions from temperate inland waters dominated by either algae, free-floating or submerged plants in controls and warmed (+4 °C) treatments for one year each. Our results show that the effect of experimental warming on GHG fluxes differs between dominance of different functional plant types, mainly by modulating methane ebullition, an often-dominant GHG emission pathway. Specifically, we demonstrate that the response to experimental warming was strongest for free-floating and lowest for submerged plant-dominated systems. Importantly, our results suggest that anticipated shifts in plant type from submerged plants to a dominance of algae or free-floating plants with warming may increase total GHG emissions from shallow waters. This, together with a warming-induced emission response, represents a so far overlooked positive climate feedback. Management strategies aimed at favouring submerged plant dominance may thus substantially mitigate GHG emissions.

Cross-continental importance of CH4 emissions from dry inland-waters

Despite substantial advances in quantifying greenhouse gas (GHG) emissions from dry inland waters, existing estimates mainly consist of carbon dioxide (CO₂) emissions. However, methane (CH₄) may also be relevant due to its higher Global Warming Potential (GWP). We report CH₄ emissions from dry inland water sediments to i) provide a cross-continental estimate of such emissions for different types of aquatic systems (i.e., lakes, ponds, reservoirs, and streams) and climate zones (i.e., tropical, continental, and temperate); and ii) determine the environmental factors that control these emissions. CH₄ emissions from dry inland waters were consistently higher than emissions observed in adjacent uphill soils, across climate zones and in all aquatic systems except for streams. However, the CH₄ contribution (normalized to CO₂ equivalents; CO₂-eq) to the total GHG emissions of dry inland waters was similar for all types of aquatic systems and varied from 10 to 21%. Although we discuss multiple controlling factors, dry inland water CH₄ emissions were most strongly related to sediment organic matter content and moisture. Summing CO₂ and CH₄ emissions revealed a cross-continental average emission of 9.6 ± 17.4 g CO₂-eq m^-2 d^-1 from dry inland waters. We argue that increasing droughts likely expand the worldwide surface area of atmosphere-exposed aquatic sediments, thereby increasing global dry inland water CH₄ emissions. Hence, CH₄ cannot be ignored if we want to fully understand the carbon (C) cycle of dry sediments.

Better assessments of greenhouse gas emissions from global fish ponds needed to adequately evaluate aquaculture footprint

While providing protein for a fast-growing human population, the ongoing boom in global aquaculture comes with environmental costs. Particularly, the intense greenhouse gas (GHG) emissions reported for several aquaculture systems are a source of concern. Still, we argue that actual emissions could be multiple times higher than currently thought. Most studies supporting existing estimates solely rely on measurements of water-atmosphere diffusive fluxes of GHG, whereas methane (CH₄) and nitrous oxide (N₂O) emissions during drainage and refilling and CH₄ bubbles emerging from sediments are largely ignored. Yet, abundant evidence for similar aquatic ecosystems suggests that these largely unaccounted emission pathways may be responsible for a large share of annual GHG emissions. Uncertainties from overlooking important emission pathways may have serious consequences, including incorrect advice on mitigation strategies and overly optimistic assessments of the GHG footprint of cultured freshwater fish. To ensure a low-carbon future for global aquaculture, we contend that GHG assessments in fish-farming ponds must extend beyond the focus on diffusive water-atmosphere fluxes and include all emission pathways and possible carbon burial in the sediment. In parallel, we call for a better understanding of the biological, microbiological and physical drivers of aquaculture emissions to effectively support mitigation strategies to minimize the footprint of this nutritionally valuable protein source.

Contribution of plant-induced pressurized flow to CH4 emission from a Phragmites fen

The widespread wetland species Phragmites australis (Cav.) Trin. ex Steud. has the ability to transport gases through its stems via a pressurized flow. This results in a high oxygen (O2) transport to the rhizosphere, suppressing methane (CH4) production and stimulating CH4 oxidation. Simultaneously CH4 is transported in the opposite direction to the atmosphere, bypassing the oxic surface layer. This raises the question how this plant-mediated gas transport in Phragmites affects the net CH4 emission. A field experiment was set-up in a Phragmites-dominated fen in Germany, to determine the contribution of all three gas transport pathways (plant-mediated, diffusive and ebullition) during the growth stage of Phragmites from intact vegetation (control), from clipped stems (CR) to exclude the pressurized flow, and from clipped and sealed stems (CSR) to exclude any plant-transport. Clipping resulted in a 60% reduced diffusive + plant-mediated flux (control: 517, CR: 217, CSR: 279 mg CH4 m-2 day-1). Simultaneously, ebullition strongly increased by a factor of 7-13 (control: 10, CR: 71, CSR: 126 mg CH4 m-2 day-1). This increase of ebullition did, however, not compensate for the exclusion of pressurized flow. Total CH4 emission from the control was 2.3 and 1.3 times higher than from CR and CSR respectively, demonstrating the significant role of pressurized gas transport in Phragmites-stands.

Global patterns and determinants of lake macrophyte taxonomic, functional and phylogenetic beta diversity

Documenting the patterns of biological diversity on Earth has always been a central challenge in macroecology and biogeography. However, for the diverse group of freshwater plants, such research program is still in its infancy. Here, we examined global variation in taxonomic, functional and phylogenetic beta diversity patterns of lake macrophytes using regional data from six continents. A data set of ca. 480 lake macrophyte community observations, together with climatic, geographical and environmental variables, was compiled across 16 regions worldwide. We (a) built the very first phylogeny comprising most freshwater plant lineages; (b) exploited a wide array of functional traits that are important to macrophyte autoecology or that relate to lake ecosystem functioning; (c) assessed if different large-scale beta diversity patterns show a clear latitudinal gradient from the equator to the poles using null models; and (d) employed evolutionary and regression models to first identify the degree to which the studied functional traits show a phylogenetic signal, and then to estimate community-environment relationships at multiple spatial scales. Our results supported the notion that ecological niches evolved independently of phylogeny in macrophyte lineages worldwide. We also showed that taxonomic and phylogenetic beta diversity followed the typical global trend with higher diversity in the tropics. In addition, we were able to confirm that species, multi-trait and lineage compositions were first and foremost structured by climatic conditions at relatively broad spatial scales. Perhaps more importantly, we showed that large-scale processes along latitudinal and elevational gradients have left a strong footprint in the current diversity patterns and community-environment relationships in lake macrophytes. Overall, our results stress the need for an integrative approach to macroecology, biogeography and conservation biology, combining multiple diversity facets at different spatial scales.

Global CO2 emissions from dry inland waters share common drivers across ecosystems

Many inland waters exhibit complete or partial desiccation, or have vanished due to global change, exposing sediments to the atmosphere. Yet, data on carbon dioxide (CO2) emissions from these sediments are too scarce to upscale emissions for global estimates or to understand their fundamental drivers. Here, we present the results of a global survey covering 196 dry inland waters across diverse ecosystem types and climate zones. We show that their CO2 emissions share fundamental drivers and constitute a substantial fraction of the carbon cycled by inland waters. CO2 emissions were consistent across ecosystem types and climate zones, with local characteristics explaining much of the variability. Accounting for such emissions increases global estimates of carbon emissions from inland waters by 6% (~0.12 Pg C y-1). Our results indicate that emissions from dry inland waters represent a significant and likely increasing component of the inland waters carbon cycle.

Sediment drying-rewetting cycles enhance greenhouse gas emissions, nutrient and trace element release, and promote water cytogenotoxicity

Increased periods of prolonged droughts followed by severe precipitation events are expected throughout South America due to climate change. Freshwater sediments are especially sensitive to these changing climate conditions. The increased oscillation of water levels in aquatic ecosystems causes enhanced cycles of sediment drying and rewetting. Here we experimentally evaluate the effects of induced drought followed by a rewetting event on the release of carbon dioxide (CO₂), methane (CH₄), nutrients (nitrogen and phosphorus), and trace elements (iron, manganese, and zinc) from the sediment of a tropical reservoir in southeastern Brazil. Furthermore, we used bulb onions (Allium cepa) to assess the potential cytogenotoxicity of the water overlying sediments after rewetting. We found peaks in CO₂ and CH₄ emissions when sediments first transitioned from wet to dry, with fluxes declining as sediments dried out. CO₂ emissions peaked again upon rewetting, whereas CH₄ emissions remained unaltered. Our experiment also revealed average increases by up to a factor of ~5000 in the release rates of nutrients and trace elements in water overlying sediments after rewetting. These increased release rates of potentially toxic compounds likely explain the lower replication of Allium cepa cells (up to 22% reduction) exposed to water overlying sediments after rewetting. Our findings suggest that increased events of drought followed by rewetting may lead to a range of changes in freshwater ecosystems, including nutrient enrichment, increased toxicity following resuspension of contaminants, and higher emission of greenhouse gases to the atmosphere.

Hidden treasures: Human-made aquatic ecosystems harbour unexplored opportunities

Artificial water bodies like ditches, fish ponds, weirs, reservoirs, fish ladders, and irrigation channels are usually constructed and managed to optimize their intended purposes. However, human-made aquatic systems also have unintended consequences on ecosystem services and biogeochemical cycles. Knowledge about their functioning and possible additional ecosystem services is poor, especially compared to natural ecosystems. A GIS analysis indicates that currently only ~ 10% of European surface waters are covered by the European Water Framework directive, and that a considerable fraction of the excluded systems are likely human-made aquatic systems. There is a clear mismatch between the high possible significance of human-made water bodies and their low representation in scientific research and policy. We propose a research agenda to build an inventory of human-made aquatic ecosystems, support and advance research to further our understanding of the role of these systems in local and global biogeochemical cycles as well as to identify other benefits for society. We stress the need for studies that aim to optimize management of human-made aquatic systems considering all their functions and to support programs designed to overcome barriers of the adoption of optimized management strategies.

The Water Hyacinth Microbiome: Link Between Carbon Turnover and Nutrient Cycling

Water hyacinth (WH), a large floating plant, plays an important role in the biogeochemistry and ecology of many freshwaters globally. Its biogeochemical impact on wetland functioning is strongly mediated by the microbiome associated with its roots. However, little is known about the structure and function of this WH rhizobiome and its relation to wetland ecosystem functioning. Here, we unveil the core and transient rhizobiomes of WH and their key biogeochemical functions in two of the world's largest wetlands: the Amazon and the Pantanal. WH hosts a highly diverse microbial community shaped by spatiotemporal changes. Proteobacteria lineages were most common, followed by Actinobacteria and Planctomycetes. Deltaproteobacteria and Sphingobacteriia predominated in the core microbiome, potentially associated with polysaccharide degradation and fermentation of plant-derived carbon. Conversely, a plethora of lineages were transient, including highly abundant Acinetobacter, Acidobacteria subgroup 6, and methanotrophs, thus assuring diverse taxonomic signatures in the two different wetlands. Our findings point out that methanogenesis is a key driver of, and proxy for, community structure, especially during seasonal plant decline. We provide ecologically relevant insights into the WH microbiome, which is a key element linking plant-associated carbon turnover with other biogeochemical fluxes in tropical wetlands.

Effects of seasonality, trophic state and landscape properties on CO2 saturation in low-latitude lakes and reservoirs

The role of tropical lakes and reservoirs in the global carbon cycle has received increasing attention in the past decade, but our understanding of its variability is still limited. The metabolism of tropical systems may differ profoundly from temperate systems due to the higher temperatures and wider variations in precipitation. Here, we investigated the spatial and temporal patterns of the variability in the partial pressure of carbon dioxide (pCO₂) and its drivers in a set of 102 low-latitude lakes and reservoirs that encompass wide gradients of precipitation, productivity and landscape properties (lake area, perimeter-to-area ratio, catchment size, catchment area-to-lake area ratio, and types of catchment land use). We used multiple regressions and structural equation modeling (SEM) to determine the direct and indirect effects of the main in-lake variables and landscape properties on the water pCO₂ variance. We found that these systems were mostly supersaturated with CO₂ (92% spatially and 72% seasonally) regardless of their trophic status and landscape properties. The pCO₂ values (9-40,020 μatm) were within the range found in tropical ecosystems, and higher (p < 0.005) than pCO₂ values recorded from high-latitude ecosystems. Water volume had a negative effect on the trophic state (r = -0.63), which mediated a positive indirect effect on pCO₂ (r = 0.4), representing an important negative feedback in the context of climate change-driven reduction in precipitation. Our results demonstrated that precipitation drives the pCO₂ seasonal variability, with significantly higher pCO₂ during the rainy season (F = 16.67; p < 0.001), due to two potential main mechanisms: (1) phytoplankton dilution and (2) increasing inputs of terrestrial CO₂ from the catchment. We conclude that at low latitudes, precipitation is a major climatic driver of pCO₂ variability by influencing volume variations and linking lentic ecosystems to their catchments.

Global patterns and drivers of ecosystem functioning in rivers and riparian zones

River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth's biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented "next-generation biomonitoring" by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale.

Global patterns in the metacommunity structuring of lake macrophytes: regional variations and driving factors

We studied community-environment relationships of lake macrophytes at two metacommunity scales using data from 16 regions across the world. More specifically, we examined (a) whether the lake macrophyte communities respond similar to key local environmental factors, major climate variables and lake spatial locations in each of the regions (i.e., within-region approach) and (b) how well can explained variability in the community-environment relationships across multiple lake macrophyte metacommunities be accounted for by elevation range, spatial extent, latitude, longitude, and age of the oldest lake within each metacommunity (i.e., across-region approach). In the within-region approach, we employed partial redundancy analyses together with variation partitioning to investigate the relative importance of local variables, climate variables, and spatial location on lake macrophytes among the study regions. In the across-region approach, we used adjusted R2 values of the variation partitioning to model the community-environment relationships across multiple metacommunities using linear regression and commonality analysis. We found that niche filtering related to local lake-level environmental conditions was the dominant force structuring macrophytes within metacommunities. However, our results also revealed that elevation range associated with climate (increasing temperature amplitude affecting macrophytes) and spatial location (likely due to dispersal limitation) was important for macrophytes based on the findings of the across-metacommunities analysis. These findings suggest that different determinants influence macrophyte metacommunities within different regions, thus showing context dependency. Moreover, our study emphasized that the use of a single metacommunity scale gives incomplete information on the environmental features explaining variation in macrophyte communities.

Warming enhances sedimentation and decomposition of organic carbon in shallow macrophyte-dominated systems with zero net effect on carbon burial

Temperatures have been rising throughout recent decades and are predicted to rise further in the coming century. Global warming affects carbon cycling in freshwater ecosystems, which both emit and bury substantial amounts of carbon on a global scale. Currently, most studies focus on the effect of warming on overall carbon emissions from freshwater ecosystems, while net effects on carbon budgets may strongly depend on burial in sediments. Here, we tested whether year-round warming increases the production, sedimentation, or decomposition of particulate organic carbon and eventually alters the carbon burial in a typical shallow freshwater system. We performed an indoor experiment in eight mesocosms dominated by the common submerged aquatic plant Myriophyllum spicatum testing two temperature treatments: a temperate seasonal temperature control and a warmed (+4°C) treatment (n = 4). During a full experimental year, the carbon stock in plant biomass, dissolved organic carbon in the water column, sedimented organic matter, and decomposition of plant detritus were measured. Our results showed that year-round warming nearly doubled the final carbon stock in plant biomass from 6.9 ± 1.1 g C in the control treatment to 12.8 ± 0.6 g C (mean ± SE), mainly due to a prolonged growing season in autumn. DOC concentrations did not differ between the treatments, but organic carbon sedimentation increased by 60% from 96 ± 9.6 to 152 ± 16 g C m^-2  yaer^-1 (mean ± SE) from control to warm treatments. Enhanced decomposition of plant detritus in the warm treatment, however, compensated for the increased sedimentation. As a result, net carbon burial was 40 ± 5.7 g C m^-2  year^-1 in both temperature treatments when fluxes were combined into a carbon budget model. These results indicate that warming can increase the turnover of organic carbon in shallow macrophyte-dominated systems, while not necessarily affecting net carbon burial on a system scale.

Cross continental increase in methane ebullition under climate change

Methane (CH4) strongly contributes to observed global warming. As natural CH4 emissions mainly originate from wet ecosystems, it is important to unravel how climate change may affect these emissions. This is especially true for ebullition (bubble flux from sediments), a pathway that has long been underestimated but generally dominates emissions. Here we show a remarkably strong relationship between CH4 ebullition and temperature across a wide range of freshwater ecosystems on different continents using multi-seasonal CH4 ebullition data from the literature. As these temperature-ebullition relationships may have been affected by seasonal variation in organic matter availability, we also conducted a controlled year-round mesocosm experiment. Here 4 °C warming led to 51% higher total annual CH4 ebullition, while diffusion was not affected. Our combined findings suggest that global warming will strongly enhance freshwater CH4 emissions through a disproportional increase in ebullition (6-20% per 1 °C increase), contributing to global warming.

Fate of methane in aquatic systems dominated by free-floating plants

Worldwide the area of free-floating plants is increasing, which can be expected to alter methane (CH₄) emissions from aquatic systems in several ways. A large proportion of the CH₄ produced may become oxidized below the plants due to the accumulation of CH₄ as a result of a decrease in the diffusive water-atmosphere flux and the entrapment of part of the ebullitive CH₄, in combination with suitable conditions for methane oxidizing (MOX) bacteria in the aerobic rhizosphere. We used a set of essays to test this hypothesis and to explore the effect of different densities for three widespread free-floating species: Azolla filiculoides, Salvinia natans, and Eichhornia crassipes. The gas exchange velocity, proportion of CH₄ bubbles trapped by the plants, occurrence of radial oxygen loss from roots, and MOX rates on the roots were assessed. We subsequently used the outcome of these experiments to parameterize a simple model. With this model we estimated the proportion of the produced CH₄ that is oxidized, for different plant species and different densities. We found that in a shallow (1 m) system up to 70% of the CH₄ produced may become oxidized as a result of a strong decrease in gas exchange combined with high MOX activity of the rhizosphere microbiome. As floating plants also are likely to increase CH₄ production by organic matter production, especially when their presence induces anaerobic conditions, the overall effect on CH₄ emission will strongly depend on local conditions. This explains the contrasting effects of floating plants on CH₄ emissions in literature as reviewed here. As the effect of floating plants on CH₄ emissions, including the high MOX rates we show here, can be substantial, there is an urgent need to consider this impact when assessing greenhouse gas budgets.

High Primary Production Contrasts with Intense Carbon Emission in a Eutrophic Tropical Reservoir

Recent studies from temperate lakes indicate that eutrophic systems tend to emit less carbon dioxide (CO₂) and bury more organic carbon (OC) than oligotrophic ones, rendering them CO₂ sinks in some cases. However, the scarcity of data from tropical systems is critical for a complete understanding of the interplay between eutrophication and aquatic carbon (C) fluxes in warm waters. We test the hypothesis that a warm eutrophic system is a source of both CO₂ and CH₄ to the atmosphere, and that atmospheric emissions are larger than the burial of OC in sediments. This hypothesis was based on the following assumptions: (i) OC mineralization rates are high in warm water systems, so that water column CO₂ production overrides the high C uptake by primary producers, and (ii) increasing trophic status creates favorable conditions for CH₄ production. We measured water-air and sediment-water CO₂ fluxes, CH₄ diffusion, ebullition and oxidation, net ecosystem production (NEP) and sediment OC burial during the dry season in a eutrophic reservoir in the semiarid northeastern Brazil. The reservoir was stratified during daytime and mixed during nighttime. In spite of the high rates of primary production (4858 ± 934 mg C m^-2 d^-1), net heterotrophy was prevalent due to high ecosystem respiration (5209 ± 992 mg C m^-2 d^-1). Consequently, the reservoir was a source of atmospheric CO₂ (518 ± 182 mg C m^-2 d^-1). In addition, the reservoir was a source of ebullitive (17 ± 10 mg C m^-2 d^-1) and diffusive CH₄ (11 ± 6 mg C m^-2 d^-1). OC sedimentation was high (1162 mg C m^-2 d^-1), but our results suggest that the majority of it is mineralized to CO₂ (722 ± 182 mg C m^-2 d^-1) rather than buried as OC (440 mg C m^-2 d^-1). Although temporally resolved data would render our findings more conclusive, our results suggest that despite being a primary production and OC burial hotspot, the tropical eutrophic system studied here was a stronger CO₂ and CH₄ source than a C sink, mainly because of high rates of OC mineralization in the water column and sediments.

Warming accelerates termination of a phytoplankton spring bloom by fungal parasites

Climate change is expected to favour infectious diseases across ecosystems worldwide. In freshwater and marine environments, parasites play a crucial role in controlling plankton population dynamics. Infection of phytoplankton populations will cause a transfer of carbon and nutrients into parasites, which may change the type of food available for higher trophic levels. Some phytoplankton species are inedible to zooplankton, and the termination of their population by parasites may liberate otherwise unavailable carbon and nutrients. Phytoplankton spring blooms often consist of large diatoms inedible for zooplankton, but the zoospores of their fungal parasites may serve as a food source for this higher trophic level. Here, we investigated the impact of warming on the fungal infection of a natural phytoplankton spring bloom and followed the response of a zooplankton community. Experiments were performed in ca. 1000 L indoor mesocosms exposed to a controlled seasonal temperature cycle and a warm (+4 °C) treatment in the period from March to June 2014. The spring bloom was dominated by the diatom Synedra. At the peak of infection over 40% of the Synedra population was infected by a fungal parasite (i.e. a chytrid) in both treatments. Warming did not affect the onset of the Synedra bloom, but accelerated its termination. Peak population density of Synedra tended to be lower in the warm treatments. Furthermore, Synedra carbon: phosphorus stoichiometry increased during the bloom, particularly in the control treatments. This indicates enhanced phosphorus limitation in the control treatments, which may have constrained chytrid development. Timing of the rotifer Keratella advanced in the warm treatments and closely followed chytrid infections. The chytrids' zoospores may thus have served as an alternative food source to Keratella. Our study thus emphasizes the importance of incorporating not only nutrient limitation and grazing, but also parasitism in understanding the response of plankton communities towards global warming.

Lake and watershed characteristics rather than climate influence nutrient limitation in shallow lakes

Both nitrogen (N) and phosphorus (P) can limit primary production in shallow lakes, but it is still debated how the importance of N and P varies in time and space. We sampled 83 shallow lakes along a latitudinal gradient (5 degrees 55 degrees S) in South America and assessed the potential nutrient limitation using different methods including nutrient ratios in sediment, water, and seston, dissolved nutrient concentrations, and occurrence of N-fixing cyanobacteria. We found that local characteristics such as soil type and associated land use in the catchment, hydrology, and also the presence of abundant submerged macrophyte growth influenced N and P limitation. We found neither a consistent variation in nutrient limitation nor indications for a steady change in denitrification along the latitudinal gradient. Contrary to findings in other regions, we did not find a relationship between the occurrence of (N-fixing and non-N-fixing) cyanobacteria and the TN:TP ratio. We found N-fixing cyanobacteria (those with heterocysts) exclusively in lakes with dissolved inorganic nitrogen (DIN) concentrations of < 100 microg/L, but notably they were also often absent in lakes with low DIN concentrations. We argue that local factors such as land use and hydrology have a stronger influence on which nutrient is limiting than climate. Furthermore, our data show that in a wide range of climates N limitation does not necessarily lead to cyanobacterial dominance.