Advances in understanding of air-sea exchange and cycling of greenhouse gases in the upper ocean

\ua9 2024 University of California Press. All rights reserved. The air–sea exchange and oceanic cycling of greenhouse gases (GHG), including carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), carbon monoxide (CO), and nitrogen oxides (NOx \ubc NO \ufe NO2), are fundamental in controlling the evolution of the Earth’s atmospheric chemistry and climate. Significant advances have been made over the last 10 years in understanding, instrumentation and methods, as well as deciphering the production and consumption pathways of GHG in the upper ocean (including the surface and subsurface ocean down to approximately 1000 m). The global ocean under current conditions is now well established as a major sink for CO2, a major source for N2O and a minor source for both CH4 and CO. The importance of the ocean as a sink or source of NOx is largely unknown so far. There are still considerable uncertainties about the processes and their major drivers controlling the distributions of N2O, CH4, CO, and NOx in the upper ocean. Without having a fundamental understanding of oceanic GHG production and consumption pathways, our knowledge about the effects of ongoing major oceanic changes—warming, acidification, deoxygenation, and eutrophication—on the oceanic cycling and air–sea exchange of GHG remains rudimentary at best. We suggest that only through a comprehensive, coordinated, and interdisciplinary approach that includes data collection by global observation networks as well as joint process studies can the necessary data be generated to (1) identify the relevant microbial and phytoplankton communities, (2) quantify the rates of ocean GHG production and consumption pathways, (3) comprehend their major drivers, and (4) decipher economic and cultural implications of mitigation solutions

Aquatic Ecosystems are the Largest Source of Methane on Earth

Methane concentrations in the atmosphere have almost tripled since the industrial revolution, contributing 16% of the additional radiative forcing by anthropogenic greenhouse gas emissions. Aquatic ecosystems are an important but poorly constrained source of methane (CH4) to the atmosphere. Here, we present the first global methane emission assessment from all major natural, impacted and human-made aquatic ecosystems including streams and rivers, freshwater lakes and reservoirs, aquaculture ponds, estuaries, coastal vegetated wetlands (mangroves, salt-marshes, seagrasses), tidal flats, continental shelves and the open ocean, in comparison to recent estimates from natural wetlands and rice paddies. We find that aquatic systems are the largest source of methane globally with contributions from small lakes and coastal ocean ecosystems higher than previously estimated. We suggest that increased biogenic methane from aquatic ecosystems due to a combined effect of climate-feedbacks and human disturbance, may contribute more than expected to rising methane concentrations in the atmosphere

Aquatic Ecosystems are the Most Uncertain but Potentially Largest Source of Methane on Earth

Atmospheric methane is a potent greenhouse gas that has tripled in concentration since pre-industrial times. The causes of rising methane concentrations are poorly understood given its multiple sources and complex biogeochemistry. Natural and human-made aquatic ecosystems, including wetlands, are potentially the largest single source of methane, but their total emissions relative to other sources have not been assessed. Based on a new synthesis of inventory, remote sensing and modeling efforts, we present a bottom-up estimate of methane emissions from streams and rivers, freshwater lakes and reservoirs, estuaries, coastal wetlands (mangroves, seagrasses, salt-marshes), intertidal flats, aquaculture ponds, continental shelves, along with recently published estimates of global methane emissions from freshwater wetlands, rice paddies, the continental slope and open ocean. Our findings emphasize the high variability of aquatic methane fluxes and a possibly skewed distribution of currently available data, making global estimates sensitive to statistical assumptions. Mean emissions make aquatic ecosystems the largest source of methane globally (53% of total global methane emissions). Median emissions are 42% of the total global methane emissions. We argue that these emissions will likely increase due to urbanization, eutrophication and climate change

Advances in understanding of air–sea exchange and cycling of greenhouse gases in the upper ocean

This is the final version. Available on open access from University of California Press via the DOI in this recordThe air–sea exchange and oceanic cycling of greenhouse gases (GHG), including carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), carbon monoxide (CO), and nitrogen oxides (NOx = NO + NO2), are fundamental in controlling the evolution of the Earth’s atmospheric chemistry and climate. Significant advances have been made over the last 10 years in understanding, instrumentation and methods, as well as deciphering the production and consumption pathways of GHG in the upper ocean (including the surface and subsurface ocean down to approximately 1000 m). The global ocean under current conditions is now well established as a major sink for CO2, a major source for N2O and a minor source for both CH4 and CO. The importance of the ocean as a sink or source of NOx is largely unknown so far. There are still considerable uncertainties about the processes and their major drivers controlling the distributions of N2O, CH4, CO, and NOx in the upper ocean. Without having a fundamental understanding of oceanic GHG production and consumption pathways, our knowledge about the effects of ongoing major oceanic changes—warming, acidification, deoxygenation, and eutrophication—on the oceanic cycling and air–sea exchange of GHG remains rudimentary at best. We suggest that only through a comprehensive, coordinated, and interdisciplinary approach that includes data collection by global observation networks as well as joint process studies can the necessary data be generated to (1) identify the relevant microbial and phytoplankton communities, (2) quantify the rates of ocean GHG production and consumption pathways, (3) comprehend their major drivers, and (4) decipher economic and cultural implications of mitigation solutions.European Space AgencyConvex Seascape SurveyEuropean Union Horizon 2020U.S. National Science Foundatio

A global database of dissolved organic matter (DOM) concentration measurements in coastal waters (CoastDOM v1)

Measurements of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) con-centrations are used to characterize the dissolved organic matter (DOM) pool and are important components ofbiogeochemical cycling in the coastal ocean. Here, we present the first edition of a global database (CoastDOMv1; available at https://doi.org/10.1594/PANGAEA.964012, L\uf8nborg et al., 2023) compiling previously pub-lished and unpublished measurements of DOC, DON, and DOP in coastal waters. These data are complementedby hydrographic data such as temperature and salinity and, to the extent possible, other biogeochemical variables(e.g. chlorophyll a, inorganic nutrients) and the inorganic carbon system (e.g. dissolved inorganic carbon andtotal alkalinity). Overall, CoastDOM v1 includes observations of concentrations from all continents. However,most data were collected in the Northern Hemisphere, with a clear gap in DOM measurements from the SouthernHemisphere. The data included were collected from 1978 to 2022 and consist of 62 338 data points for DOC,20 356 for DON, and 13 533 for DOP. The number of measurements decreases progressively in the sequenceDOC > DON > DOP, reflecting both differences in the maturity of the analytical methods and the greater focuson carbon cycling by the aquatic science community. The global database shows that the average DOC concen-tration in coastal waters (average \ub1 standard deviation (SD): 182 \ub1 314 μmol C L−1; median: 103 μmol C L−1) is13-fold higher than the average coastal DON concentration (13.6 \ub1 30.4 μmol N L−1; median: 8.0 μmol N L−1),which is itself 39-fold higher than the average coastal DOP concentration (0.34 \ub1 1.11 μmol P L−1; median:0.18 μmol P L−1). This dataset will be useful for identifying global spatial and temporal patterns in DOM and willhelp facilitate the reuse of DOC, DON, and DOP data in studies aimed at better characterizing local biogeochem-ical processes; closing nutrient budgets; estimating carbon, nitrogen, and phosphorous pools; and establishing abaseline for modelling future changes in coastal waters

Divergent gas transfer velocities of CO₂, CH₄, and N₂O over spatial and temporal gradients in a subtropical estuary

High global uncertainties remain in water-air CO₂, CH₄, and N₂O fluxes from estuaries due to spatial and temporal variability and the poor predictability of the gas transfer velocity (k₆₀₀). This is the first study that directly compares k₆₀₀ of CO₂, CH₄, and N₂O in an estuary with the aim to evaluate the accuracy of using a uniform k₆₀₀ value for estimating water-air fluxes. We calculated 155 k₆₀₀ values from CO₂, CH₄, and N₂O fluxes over spatial (across, along) and temporal (tidal cycle) surveys in the subtropical Maroochy estuary using the floating chamber method. Combined k₆₀₀ values showed a large range over the entire estuary (0.1–198.6 cm h−1) with slightly lower k₆₀₀ in the lower compared to the upper estuary. Overall, temporal variability was greater than spatial variability of k₆₀₀. We found the highest variability of k₆₀₀ between gas species in the lower estuary, whereas the variability was less distinct in the upper estuary. In the Maroochy estuary, k₆₀₀CO₂ (mean 26.4 ± 37.3 cm h−1) was mostly higher than k₆₀₀ CH₄ (mean 10.9 ± 10.6 cm h−1) and k₆₀₀N₂O (mean 9.9 ± 12.3 cm h−1), likely due to chemical and enzymatic enhancements and/or microbial activity in the surface microlayer. We demonstrate that empirical k₆₀₀ models intended for CO₂ may not accurately predict CH₄ and N₂O fluxes in estuaries. Our tested k₆₀₀ models predicted the measured fluxes within an uncertainty range of 5%–40% (over or underestimation), but precise flux estimates should be based on in situ k₆₀₀ of all three gases

Estuaries as sources and sinks of N₂O across a land use gradient in subtropical Australia

Intensifying agricultural production and coastal urbanization are increasing nitrogen (N) loads to estuaries, potentially increasing emissions of the greenhouse gas nitrous oxide (N₂O). Here we present a first assessment of how changes in land use intensity affect estuarine N₂O fluxes. We measured N₂O concentrations over marine-freshwater transects in the wet and dry seasons in eight subtropical estuaries selected for differences in land use intensity. Daily estuary N loads ranged from 0.5 ± 0.4 kg N km¯² d¯¹ (minimally impacted) to 51 ± 30 kg N km¯² d¯¹ (highly impacted), corresponding to higher concentrations of all inorganic N species (nitrate, ammonium, and N₂O) in the highly impacted estuaries. Net N₂O fluxes from the eight estuaries ranged from −20 μg N₂O-N km¯² d¯¹ (sink) to +300 μg N₂O-N km¯² d¯¹(source). However, neither N concentrations nor N loads explained the variations in N₂O fluxes. Instead, seasonal differences in freshwater flushing times increased either N₂O uptake (minimally impacted systems) or N₂O efflux (moderately impacted systems) relative to N load. The lack of relationship between freshwater flushing times (kinetics) and N₂O fluxes from the highly impacted estuaries, combined with evidence for both low carbon quality and phosphorous limitation in those systems, suggests that N₂O emissions from highly impacted estuaries were controlled by stoichiometry rather than kinetics. This study shows that estuaries can shift from net sinks to sources of N₂O as land use intensity increases but that the magnitude of this switch cannot be predicted based on N loads alone

River ecosystem metabolism and carbon biogeochemistry in a changing world

River networks represent the largest biogeochemical nexus between the continents, ocean and atmosphere. Our current understanding of the role of rivers in the global carbon cycle remains limited, which makes it difficult to predict how global change may alter the timing and spatial distribution of riverine carbon sequestration and greenhouse gas emissions. Here we review the state of river ecosystem metabolism research and synthesize the current best available estimates of river ecosystem metabolism. We quantify the organic and inorganic carbon flux from land to global rivers and show that their net ecosystem production and carbon dioxide emissions shift the organic to inorganic carbon balance en route from land to the coastal ocean. Furthermore, we discuss how global change may affect river ecosystem metabolism and related carbon fluxes and identify research directions that can help to develop better predictions of the effects of global change on riverine ecosystem processes. We argue that a global river observing system will play a key role in understanding river networks and their future evolution in the context of the global carbon budget

Methane Emissions across Aquatic Ecosystems - From Headwater Streams to the Open Ocean

Aquatic systems are an important but poorly constrained source of methane (CH4) to the atmosphere. The coastal ocean in particular has been insufficiently represented in global methane budgets and assessments like the IPCC 5th report. Here, we present a combination of revised and new global methane emissions from freshwater systems including rivers and streams, lakes and reservoirs, freshwater aquaculture ponds; brackish systems including inner estuaries, coastal vegetated wetlands (mangroves, salt-marshes, seagrasses), coastal aquaculture ponds; and marine systems including continental shelves, in comparison to previous estimates of methane emissions from the open ocean, freshwater wetlands, and rice paddies. We find that human impacted sites have higher emissions than more natural ones. We also assess the main factors controlling methane emissions in different aquatic systems, as well as identifying drivers that may become increasingly important under global change

Coastal vegetation and estuaries are collectively a greenhouse gas sink

Coastal ecosystems release or absorb carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O), but the net effects of these ecosystems on the radiative balance remain unknown. We compiled a dataset of observations from 738 sites from studies published between 1975 and 2020 to quantify CO2, CH4 and N2O fluxes in estuaries and coastal vegetation in ten global regions. We show that the CO2-equivalent (CO2e) uptake by coastal vegetation is decreased by 23–27% due to estuarine CO2e outgassing, resulting in a global median net sink of 391 or 444 TgCO2e yr−1 using the 20- or 100-year global warming potentials, respectively. Globally, total coastal CH4 and N2O emissions decrease the coastal CO2 sink by 9–20%. Southeast Asia, North America and Africa are critical regional hotspots of GHG sinks. Understanding these hotspots can guide our efforts to strengthen coastal CO2 uptake while effectively reducing CH4 and N2O emissions