Tipping points, at which complex systems can shift abruptly from one state to another, are notoriously difficult to predict. Theory proposes that early warning signals may be based on the phenomenon ...that recovery rates from small perturbations should tend to zero when approaching a tipping point; however, evidence that this happens in living systems is lacking. Here we test such 'critical slowing down' using a microcosm in which photo-inhibition drives a cyanobacterial population to a classical tipping point when a critical light level is exceeded. We show that over a large range of conditions, recovery from small perturbations becomes slower as the system comes closer to the critical point. In addition, autocorrelation in the subtle fluctuations of the system's state rose towards the tipping point, supporting the idea that this metric can be used as an indirect indicator of slowing down. Although stochasticity prohibits prediction of the timing of critical transitions, our results suggest that indicators of slowing down may be used to rank complex systems on a broad scale from resilient to fragile.
Shallow lakes, the most prevalent type of freshwater ecosystems, can shift between clear states with macrophyte dominance and turbid, phytoplankton-dominated states. Such transformations, commonly ...termed regime shifts, have gained increasing attention in recent decades. Of 1084 studies documenting regime shifts, only 28% investigated the consequences for ecosystem functions and services such as habitat (13%), carbon processing (4%), or nutrient retention (4%). Although there is general consensus that a clear macrophyte state supports a higher diversity of aquatic organisms than a turbid one, the effects of shifts on primary production, carbon burial, greenhouse-gas emissions, and nutrient retention remain ambiguous. Shifts between the two states also affect drinking-water quality and the recreational value of lakes, leading to conflicting management measures and potentially deteriorating natural functions. We call for more comprehensive studies on the effects of regime shifts on ecosystem functions in shallow lakes to guide their sustainable management.
Freshwater denitrification removes a considerable amount of nitrogen from inland waters, which are under pressure from eutrophication and warming. However, incomplete denitrification can lead to the ...formation of N2O, a potent greenhouse gas, which can amplify climatic warming. Although temperature effects on denitrification are well studied in individual habitats and experiments, global patterns in temperature‐responses of denitrification and N2O emissions remain to be elucidated. Here, we investigated the temperature sensitivity (Q10) of denitrification and N2O emissions in freshwater ecosystems worldwide, using a meta‐analytic approach. To this end, Q10 values from in‐situ and temperature manipulation studies were related to environmental nutrient conditions, O2, pH, sediment organic matter (SOM), and geographic location. Temperature sensitivity of denitrification displayed a strong positive correlation with environmental nitrogen concentrations, pH and O2. Significant correlations with SOM and SOM:N ratios were observed as well, but the direction of the effect differed between in‐situ and temperature manipulation studies. Surprisingly, temperature sensitivity of N2O emissions did not correlate with pH, SOM, nutrient or O2 conditions. Temperature sensitivity of the ratio between N2O emission and NO3− concentration (adapted EF5 values) was 6.6 times higher in Australia and New Zealand compared to other geographic regions. As global temperatures and nitrogen deposition in freshwater ecosystems are expected to increase over the coming decades, our results suggest enhanced future denitrification, which may present a natural way to balance eutrophication. The observed temperature sensitivity of N2O emission factors, however, may indicate enhanced denitrification‐derived N2O emissions from freshwater ecosystems in a future warmer world.
Key Points
One degree warming enhances denitrification and N2O emissions 8%–14 %. In‐situ Q10N2O is 2 times lower than temperature manipulation Q10N2O
Q10denitrification increases with nitrogen and O2 availability. SOM correlations differ between in‐situ and temperature manipulation studies
Adapted Q10EF5 is 6.6 times higher in the Australia and New Zealand region compared to regions on the northern hemisphere
Global warming and the alteration of the global nitrogen cycle are major anthropogenic threats to the environment. Denitrification, the biological conversion of nitrate to gaseous nitrogen, removes a ...substantial fraction of the nitrogen from aquatic ecosystems, and can therefore help to reduce eutrophication effects. However, potential responses of denitrification to warming are poorly understood. Although several studies have reported increased denitrification rates with rising temperature, the impact of temperature on denitrification seems to vary widely between systems.
We explored the effects of warming on denitrification rates using microcosm experiments, field measurements and a simple model approach. Our results suggest that a three degree temperature rise will double denitrification rates. By performing experiments at fixed oxygen concentrations as well as with oxygen concentrations varying freely with temperature, we demonstrate that this strong temperature dependence of denitrification can be explained by a systematic decrease of oxygen concentrations with rising temperature. Warming decreases oxygen concentrations due to reduced solubility, and more importantly, because respiration rates rise more steeply with temperature than photosynthesis.
Our results show that denitrification rates in aquatic ecosystems are strongly temperature dependent, and that this is amplified by the temperature dependencies of photosynthesis and respiration. Our results illustrate the broader phenomenon that coupling of temperature dependent reactions may in some situations strongly alter overall effects of temperature on ecological processes.
Microbial interaction is an integral component of microbial ecology studies, yet the role, extent, and relevance of microbial interaction in community functioning remains unclear, particularly in the ...context of global biogeochemical cycles. While many studies have shed light on the physico-chemical cues affecting specific processes, (micro)biotic controls and interactions potentially steering microbial communities leading to altered functioning are less known. Yet, recent accumulating evidence suggests that the concerted actions of a community can be significantly different from the combined effects of individual microorganisms, giving rise to emergent properties. Here, we exemplify the importance of microbial interaction for ecosystem processes by analysis of a reasonably well-understood microbial guild, namely, aerobic methane-oxidizing bacteria (MOB). We reviewed the literature which provided compelling evidence for the relevance of microbial interaction in modulating methane oxidation. Support for microbial associations within methane-fed communities is sought by a re-analysis of literature data derived from stable isotope probing studies of various complex environmental settings. Putative positive interactions between active MOB and other microbes were assessed by a correlation network-based analysis with datasets covering diverse environments where closely interacting members of a consortium can potentially alter the methane oxidation activity. Although, methanotrophy is used as a model system, the fundamentals of our postulations may be applicable to other microbial guilds mediating other biogeochemical processes.
While research on aquatic plants used in treatment wetlands is abundant, little is known about the use of plants in hydroponic ecological wastewater treatment, and its simultaneous effect on ...greenhouse gas (GHG) emissions. Here, we assess the effectiveness of floating and submerged plants in removing nutrients and preventing GHG emissions from wastewater effluent. We grew two species of floating plants, Azolla filiculoides and Lemna minor, and two species of submerged plants, Ceratophyllum demersum and Callitriche platycarpa, on a batch of domestic wastewater effluent without any solid substrate. In these systems, we monitored nitrogen and phosphorus removal and fluxes of CO
, CH
and N
O, for 2 weeks. In general, floating plants produced the most biomass, whereas submerged plants were rapidly overgrown by filamentous algae. Floating plants removed nutrients most efficiently; both floating species removed 100% of the phosphate while Lemna also removed 97-100% of the inorganic nitrogen, as opposed to a removal of 81-88% in submerged plants with algae treatments. Moreover, aquaria covered by floating plants had roughly three times higher GHG uptake than the treatments with submerged plants or controls without plants. Thus, effluent polishing by floating plants can be a promising avenue for climate-smart wastewater polishing.
Wastewater treatment plants (WWTPs) are a point source of nutrients, emit greenhouse gases (GHGs), and produce large volumes of excess sludge. The use of aquatic organisms may be an alternative to ...the technical post-treatment of WWTP effluent, as they play an important role in nutrient dynamics and carbon balance in natural ecosystems. The aim of this study was therefore to assess the performance of an experimental wastewater-treatment cascade of bioturbating macroinvertebrates and floating plants in terms of sludge degradation, nutrient removal and lowering GHG emission. To this end, a full-factorial experiment was designed, using a recirculating cascade with a WWTP sludge compartment with or without bioturbating Chironomus riparius larvae, and an effluent container with or without the floating plant Azolla filiculoides, resulting in four treatments. To calculate the nitrogen (N), phosphorus (P) and carbon (C) mass balance of this system, the N, P and C concentrations in the effluent, biomass production, and sludge degradation, as well as the N, P and C content of all compartments in the cascade were measured during the 26-day experiment. The presence of Chironomus led to an increased sludge degradation of 44% compared to 25% in the control, a 1.4 times decreased transport of P from the sludge and a 2.4 times increased transport of N out of the sludge, either into Chironomus biomass or into the water column. Furthermore, Chironomus activity decreased methane emissions by 92%. The presence of Azolla resulted in a 15% lower P concentration in the effluent than in the control treatment, and a CO2 uptake of 1.13 kg ha-1 day-1. These additive effects of Chironomus and Azolla resulted in an almost two times higher sludge degradation, and an almost two times lower P concentration in the effluent. This is the first study that shows that a bio-based cascade can strongly reduce GHG and P emissions simultaneously during the combined polishing of wastewater sludge and effluent, benefitting from the additive effects of the presence of both macrophytes and invertebrates. In addition to the microbial based treatment steps already employed on WWTPs, the integration of higher organisms in the treatment process expands the WWTP based ecosystem and allows for the inclusion of macroinvertebrate and macrophyte mediated processes. Applying macroinvertebrate-plant cascades may therefore be a promising tool to tackle the present and future challenges of WWTPs.
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.
Eutrophication has been identified as the primary cause of water quality deterioration in inland waters worldwide, often associated with algal blooms or fish kills. Eutrophication can be controlled ...through watershed management and in-lake measures. An extreme heatwave event, through its impact on mineralization rates and internal nutrient loading (phosphorus—P, and nitrogen—N), could counteract eutrophication control measures. We investigated how the effectiveness of a nutrient abatement technique is impacted by an extreme heatwave, and to what extent biogeochemical processes are modulated by exposure to heatwaves. To this end, we carried out a sediment-incubation experiment, testing the effectiveness of lanthanum-modified bentonite (LMB) in reducing nutrients and greenhouse gas emissions from eutrophic sediments, with and without exposure to an extreme heatwave. Our results indicate that the effectiveness of LMB may be compromised upon exposure to an extreme heatwave event. This was evidenced by an increase in concentration of 0.08 ± 0.03 mg P/L with an overlying water volume of 863 ± 21 mL, equalling an 11% increase, with effects lasting to the end of the experiment. LMB application generally showed no effect on nitrogen species, while the heatwave stimulated nitrification, resulting in ammonium loss and accumulation of dissolved oxidized nitrogen species as well as increased dissolved nitrous oxide concentrations. In addition, carbon dioxide (CO
2
)-equivalent was more than doubled during the heatwave relative to the reference temperature, and LMB application had no effect on mitigating them. Our sediment incubation experiment indicates that the rates of biogeochemical processes can be significantly accelerated upon heatwave exposure, resulting in a change in fluxes of nutrient and greenhouse gas between sediment and water. The current efforts in eutrophication control will face more challenges under future climate scenarios with more frequent and intense extreme events as predicted by the IPCC.
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