Evaluation of vegetation communities, water table, and peat composition as drivers of greenhouse gas emissions in lowland tropical peatlands

© 2019 Elsevier B.V. Tropical peatlands are globally important source of greenhouse gases to the atmosphere, but data on carbon fluxes from these ecosystems is limited due to the logistical challenges of measuring gas fluxes in these ecosystems. Proposals to overcome the difficulties of measuring gas carbon fluxes in the tropics include remote sensing (top-down) approaches. However, these require information on the effect of vegetation communities on carbon dioxide (CO2) and methane (CH4) fluxes from the peat surface (bottom-up). Such information will help reducing the uncertainty in current carbon budgets and resolve inconsistencies between the top-down and bottom-up estimates of gas fluxes from tropical peatlands. We investigated temporal and spatial variability of CO2 and CH4 fluxes from tropical peatlands inhabited by two contrasting vegetation communities (i.e., mixed forest and palm swamp) in Panama. In addition, we explored the influence of peat chemistry and nutrient status (i.e., factorial nitrogen (N) and phosphorus (P) addition) on greenhouse gas fluxes from the peat surface. We found that: i) CO2 and CH4 fluxes were not significantly different between the two vegetation communities, but did vary temporally across an annual cycle; ii) precipitation rates and peat temperature were poor predictors of CO2 and CH4 fluxes; iii) nitrogen addition increased CH4 fluxes at the mixed forests when the water table was above the peat surface, but neither nitrogen nor phosphorus affected gas fluxes elsewhere; iv) gas fluxes varied significantly with the water table level, with CO2 flux being 80% greater at low water table, and CH4 fluxes being 81% higher with the water table above the surface. Taken together, our data suggested that water table is the most important control of greenhouse gas emissions from the peat surface in forested lowland tropical peatlands, and that neither the presence of distinct vegetation communities nor the addition of nutrients outweigh such control

Anaerobic oxidation of methane and associated microbiome in anoxic water of Northwestern Siberian lakes

Arctic lakes emit methane (CH4) to the atmosphere. The magnitude of this flux could increase with permafrost thaw but might also be mitigated by microbial CH4 oxidation. Methane oxidation in oxic water has been extensively studied, while the contribution of anaerobic oxidation of methane (AOM) to CH4 mitigation is not fully understood. We have investigated four Northern Siberian stratified lakes in an area of discontinuous permafrost nearby Igarka, Russia. Analyses of CH4 concentrations in the water column demonstrated that 60 to 100% of upward diffusing CH4 was oxidized in the anoxic layers of the four lakes. A combination of pmoA and mcrA gene qPCR and 16S rRNA gene metabarcoding showed that the same taxa, all within Methylomonadaceae and including the predominant genus Methylobacter as well as Crenothrix, could be the major methane-oxidizing bacteria (MOB) in the anoxic water of the four lakes. Correlation between Methylomonadaceae and OTUs within Methylotenera, Geothrix and Geobacter genera indicated that AOM might occur in an interaction between MOB, denitrifiers and iron-cycling partners. We conclude that MOB within Methylomonadaceae could have a crucial impact on CH4 cycling in these Siberian Arctic lakes by mitigating the majority of produced CH4 before it leaves the anoxic zone. This finding emphasizes the importance of AOM by Methylomonadaceae and extends our knowledge about CH4 cycle in lakes, a crucial component of the global CH4 cycle

The effects of hypoxia on zooplankton population estimates and migration in lakes

Many zooplankton species typically exhibit diel vertical migration (DVM), where zooplankton migrate from the hypolimnion to the epilimnion of lakes at night. Zooplankton exhibit this behavior to avoid visual predators and UV radiation by remaining in the bottom waters during the day and ascending to the surface waters to feed on phytoplankton at night. However, hypoxic conditions in the hypolimnion of lakes mayinterfere with DVM and force zooplankton to increase diel horizontal migration (DHM) to find predation refuge in littoral zones. Climate change and eutrophication are expected to increase the prevalence and severity of hypoxic conditions worldwide and thereby possibly alter zooplankton migration patterns. We hypothesize that hypoxia will force zooplankton to shift their migration patterns from predominantly DVM to DHM to avoid oxygen-depleted bottom waters. To test our hypothesis, we are conducting a standardized global sampling program to test whether pelagic, full water column estimates of zooplankton are greater at night versus the day under hypolimnetic hypoxic versus oxic conditions. Participants are aiming to sample at least one lake with an oxic hypolimnion and one lake with a hypoxic hypolimnion during the thermally-stratified period at midday and midnight. With our global dataset (currently expecting about 60 lakes in 22 countries), our goal is to improve our understanding of how global change may alter zooplankton migration behavior and patterns in lakes.info:eu-repo/semantics/publishedVersio

Synthesizing redox biogeochemistry at aquatic interfaces

The exchange of matter and energy between confined components of aquatic ecosystems requires the passage through their interfaces. This passage is characterized by rapid changes in physical, chemical and biological conditions and often triggers chemical transformations that involve the exchange of electrons: redox reactions. Over the last decades, research in aquatic biogeochemistry has resulted in many new but conceptually isolated findings that, together, frame an emergent view on the overarching principles of aquatic redox processes. A thermodynamic assessment may reveal the maximum available energy from such redox reactions. However, this energy can rarely be released due to various morphological, ecological and kinetic constrains on the turnover reactions. As these constrains set the boundary conditions for aquatic ecosystem functioning, they deserve particular attention in freshwater research. Here, we illustrate how physical and structural traits shape a complex redox environment and how this environment ultimately exercises control on the inhabiting microbial community and its metabolism. This aquatic microbiome is the key entity of material turnover. At the same time, the biome possesses the capability to feed back on its environment by shaping the local redox conditions and sustain niche existences. We discuss current and emerging ideas of how microorganisms engineer their environment, affecting aquatic redox reactions. In total, we examine this feed-back cycling between the physical environment and its colonizing biome to encourage the reader to take on the redox perspective when analyzing processes at aquatic interfaces. Understanding electron fluxes on both temporal and spatial scales is essential for the overall comprehension of matter and energy fluxes through freshwater environments. We pinpoint the methodological frontiers that will need to be challenged in future studies of aquatic redox processes. An improved mechanistic understanding will be instrumental in estimating sink and source properties of aquatic ecosystems

A combined microbial and biogeochemical dataset from high-latitude ecosystems with respect to methane cycle

High latitudes are experiencing intense ecosystem changes with climate warming. The underlying methane (CH4) cycling dynamics remain unresolved, despite its crucial climatic feedback. Atmospheric CH4 emissions are heterogeneous, resulting from local geochemical drivers, global climatic factors, and microbial production/consumption balance. Holistic studies are mandatory to capture CH4 cycling complexity. Here, we report a large set of integrated microbial and biogeochemical data from 387 samples, using a concerted sampling strategy and experimental protocols. The study followed international standards to ensure inter-comparisons of data amongst three high-latitude regions: Alaska, Siberia, and Patagonia. The dataset encompasses different representative environmental features (e.g. lake, wetland, tundra, forest soil) of these high-latitude sites and their respective heterogeneity (e.g. characteristic microtopographic patterns). The data included physicochemical parameters, greenhouse gas concentrations and emissions, organic matter characterization, trace elements and nutrients, isotopes, microbial quantification and composition. This dataset addresses the need for a robust physicochemical framework to conduct and contextualize future research on the interactions between climate change, biogeochemical cycles and microbial communities at high-latitudes.publishe

Biogeochemical Distinctiveness of Peatland Ponds, Thermokarst Waterbodies, and Lakes

Small lentic freshwater ecosystems play a disproportionate role in global biogeochemical cycles by processing large amounts of carbon (C), nitrogen (N), and phosphorus (P), but it is unlikely that they behave as one homogenous group for the purpose of extrapolation. Here, we synthesize biogeochemical data from >12,000 geographically distinct freshwater systems: lakes, peatland ponds, and thermokarst waterbodies. We show that peatland ponds are biogeochemically distinct from the more widely studied lake systems, while thermokarst waterbodies share characteristics with peatland ponds, lakes, or both. For any given size or depth, peatland ponds tend to have dissolved organic carbon concentrations several-fold higher and are 100-fold more acidic than lakes because of the organic matter-rich settings in which they develop. The biogeochemical distinctiveness of freshwater ecosystems highlights the need to account for the fundamental differences in sources and processing of organic matter to understand and predict their role in global biogeochemical cycles

Methane and carbon dioxide cycles in lakes of the King George Island, maritime Antarctica

International audienceFreshwater ecosystems are important contributors to the global greenhouse gas budget and a comprehensive assessment of their role in the context of global warming is essential. Despite many reports on freshwater ecosystems, relatively little attention has been given so far to those located in the southern hemisphere and our current knowledge is particularly poor regarding the methane cycle in non-perennially glaciated lakes of the maritime Antarctica. We conducted a high-resolution study of the methane and carbon dioxide cycle in a lake of the Fildes Peninsula, King George Island (Lat. 62°S), and a succinct characterization of 10 additional lakes and ponds of the region. The study, done during the ice-free and the ice-seasons, included methane and carbon dioxide exchanges with the atmosphere (both from water and surrounding soils) and the dissolved concentration of these two gases throughout the water column. This characterization was complemented with an ex-situ analysis of the microbial activities involved in the methane cycle, including methanotrophic and methanogenic activities as well as the methane-related marker gene abundance, in water, sediments and surrounding soils. The results showed that, over an annual cycle, the freshwater ecosystems of the region are dominantly autotrophic and that, despite low but omnipresent atmospheric methane emissions, they act as greenhouse gas sinks

Sub-oxycline methane oxidation can fully uptake CH4 produced in sediments: case study of a lake in Siberia

International audienceIt is commonly assumed that methane (CH4) released by lakes into the atmosphere is mainly produced in anoxic sediment and transported by diffusion or ebullition through the water column to the surface of the lake. In contrast to that prevailing idea, it has been gradually established that the epilimnetic CH4 does not originate exclusively from sediments but is also locally produced or laterally transported from the littoral zone. Therefore, CH4 cycling in the epilimnion and the hypolimnion might not be as closely linked as previously thought. We utilized a high-resolution method used to determine dissolved CH4 concentration to analyze a Siberian lake in which epilimnetic and hypolimnetic CH4 cycles were fully segregated by a section of the water column where CH4 was not detected. This layer, with no detected CH4, was well below the oxycline and the photic zone and thus assumed to be anaerobic. However, on the basis of a diffusion-reaction model, molecular biology, and stable isotope analyses, we determined that this layer takes up all the CH4 produced in the sediments and the deepest section of the hypolimnion. We concluded that there was no CH4 exchange between the hypolimnion (dominated by methanotrophy and methanogenesis) and the epilimnion (dominated by methane lateral transport and/or oxic production), resulting in a vertically segregated lake internal CH4 cycle

Temperature differently affected methanogenic pathways and microbial communities in sub-Antarctic freshwater ecosystems

International audienc

In Situ Measurement of Dissolved Methane and Carbon Dioxide in Freshwater Ecosystems by Off-Axis Integrated Cavity Output Spectroscopy

A novel low-cost method for the combined, real-time, and in situ determination of dissolved methane and carbon dioxide concentrations in freshwater ecosystems was designed and developed. This method is based on the continuous sampling of water from a freshwater ecosystem to a gas/liquid exchange membrane. Dissolved gas is transferred through the membrane to a continuous flow of high purity nitrogen, which is then measured by an off-axis integrated cavity output spectrometer (OA-ICOS). This method, called M-ICOS, was carefully tested in a laboratory and was subsequently applied to four lakes in Mexico and Alaska with contrasting climates, ecologies, and morphologies. The M-ICOS method allowed for the determination of dissolved methane and carbon dioxide concentrations with a frequency of 1 Hz and with a method detection limit of 2.76 × 10^–10 mol L^–1 for methane and 1.5 × 10^–7 mol L^–1 for carbon dioxide. These detection limits are below saturated concentrations with respect to the atmosphere and significantly lower than the minimum concentrations previously reported in lakes. The method is easily operable by a single person from a small boat, and the small size of the suction probe allows the determination of dissolved gases with a minimized impact on shallow freshwater ecosystems