Microorganisms oxidize organic nitrogen to nitrate in a series of steps. Nitrite, an intermediate product, accumulates at the base of the sunlit layer in the subtropical ocean, forming a primary nitrite maximum, but can accumulate throughout the sunlit layer at higher latitudes. We model nitrifying chemoautotrophs in a marine ecosystem and demonstrate that microbial community interactions can explain the nitrite distributions. Our theoretical framework proposes that nitrite can accumulate to a higher concentration than ammonium because of differences in underlying redox chemistry and cell size between ammonia- and nitrite-oxidizing chemoautotrophs. Using ocean circulation models, we demonstrate that nitrifying microorganisms are excluded in the sunlit layer when phytoplankton are nitrogen-limited, but thrive at depth when phytoplankton become light-limited, resulting in nitrite accumulation there. However, nitrifying microorganisms may coexist in the sunlit layer when phytoplankton are iron- or light-limited (often in higher latitudes). These results improve understanding of the controls on nitrification, and provide a framework for representing chemoautotrophs and their biogeochemical effects in ocean models.

Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cycling, storing 10× more C than temperate forests. Methane (CH 4 ), a potent greenhouse gas, can form in the sediments of these ecosystems. Currently, CH 4 emissions are a missing component of VCE C budgets. This review summarizes 97 studies describing CH 4 fluxes from mangrove, salt marsh, and seagrass ecosystems and discusses factors controlling CH 4 flux in these systems. CH 4 fluxes from these ecosystems were highly variable yet they all act as net methane sources (median, range; mangrove: 279.17, −67.33 to 72,867.83; salt marsh: 224.44, −92.60 to 94,129.68; seagrass: 64.80, 1.25-401.50 µmol CH 4 m −2 day −1 ). Together CH 4 emissions from mangrove, salt marsh, and seagrass ecosystems are about 0.33-0.39 Tmol CH 4 -C/year-an addition that increases the current global marine CH 4 budget by more than 60%. The majority (~45%) of this increase is driven by mangrove CH 4 fluxes. While organic matter content and quality were commonly reported in individual studies as the most important environmental factors driving CH 4 flux, they were not significant predictors of CH 4 flux when data were combined across studies. Salinity was negatively correlated with CH 4 emissions from salt marshes, but not seagrasses and mangroves. Thus the available data suggest that other environmental drivers are important for predicting CH 4 emissions in vegetated coastal systems. Finally, we examine stressor effects on CH 4 emissions from VCEs and we hypothesize that future changes in temperature and other anthropogenic activites (e.g., nitrogen loading) will likely increase CH 4 emissions from these ecosystems. Overall, this review highlights the current and growing importance of VCEs in the global marine CH 4 budget. K E Y W O R D S global carbon budget, mangrove, methanogenesis, methanotrophy, salt marsh, seagrass, synthesis | 2989 AL-HAJ And FULWEILER About two-thirds of the total global CH 4 emissions can be attributed to human activity (e.g., agriculture, sewage and landfill waste, fossil fuel consumption; Saunois et al., 2016). Vegetated coastal ecosystems (VCEs; i.e., mangroves, salt marshes, and seagrasses) play a critical role in global carbon (C) cy-S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Al-Haj AN, Fulweiler RW. A synthesis of methane emissions from shallow vegetated coastal ecosystems. Glob Change Biol. 2020;26:2988-3005. https://

The role of coastal wetlands as natural ‘blue carbon’ sinks may be counter‐acted by emissions of methane (CH4) and nitrous oxide (N2O). Site‐specific fluxes of these two potent greenhouse gases from coastal wetlands show high spatial and temporal variability , making global estimates sensitive to statistical assumptions and uncertainties. Here, we review the magnitude of CH4 and N2O fluxes from mangroves, saltmarshes and seagrasses and identify direct and indirect drivers that can control production and consumption pathways. Significant research is required to better quantify fluxes and to understand factors causing flux variability from different transport pathways before restoration of coastal blue carbon ecosystems can be reliably used for climate mitigation.

Production of animal protein is associated with high greenhouse gas (GHG) emissions. Globally, oyster aquaculture is increasing as a way to meet growing demands for protein, yet its associated GHG-emissions are largely unknown. We quantified oyster aquaculture GHG-emissions from the three main constituents of GHG-release associated with terrestrial livestock production: fermentation in the animal gut, manure management, and fodder production. We found that oysters release no methane (CH4) and only negligible amounts of nitrous oxide (0.00012 ± 0.00004 μmol N2O gDW–1 hr–1) and carbon dioxide (3.556 ± 0.471 μmol CO2 gDW–1 hr–1). Further, sediment fluxes of N2O and CH4 were unchanged in the presence of oyster aquaculture, regardless of the length of time it had been in place. Sediment CO2 release was slightly stimulated between 4 and 6 years of aquaculture presence and then returned to baseline levels but was not significantly different between aquaculture and a control site when all ages of culture were pooled. There is no GHG-release from oyster fodder production. Considering the main drivers of GHG-release in terrestrial livestock systems, oyster aquaculture has less than 0.5% of the GHG-cost of beef, small ruminants, pork, and poultry in terms of CO2-equivalents per kg protein, suggesting that shellfish aquaculture may provide a a low GHG alternative for future animal protein production compared to land based sources. We estimate that if 10% of the protein from beef consumption in the United States was replaced with protein from oysters, the GHG savings would be equivalent to 10.8 million fewer cars on the road.

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