Abstract The global ocean plays an important role in the overall budgets of nitrous oxide (N 2 O) and methane (CH 4 ), especially in continental estuaries and shelf areas. Four cruises were conducted between 2021 and 2022, covering the spring, summer, and fall seasons, to study the spatial and seasonal characteristics of N 2 O and CH 4 distributions and emissions in the Taiwan Strait (TWS). The surface N 2 O and CH 4 concentrations gradually decreased from the coast to the open sea, with maximum values (14.3 and 15.6 nmol L −1 ) occurring near the Jiulong and Minjiang estuaries, respectively. The mean surface N 2 O concentration (8.2 nmol L −1 ) was highest in the spring and approximately the same in the summer as in the fall. The mean surface concentrations of CH 4 (8.8 nmol L −1 ) were greater in summer than in spring and fall, probably because of the high freshwater input in summer. Except for several stations in fall, surface waters were oversaturated with N 2 O and CH 4 relative to the atmosphere in other seasons, and the TWS was a net source of atmospheric N 2 O and CH 4 in the spring, summer, and fall. In situ production is the main source of N 2 O and CH 4 in the TWS, with nitrification being the dominant mechanism of N 2 O production in the TWS. In contrast, physical influences (riverine inputs and water mass mixing) reshape the distributions of N 2 O and CH 4 . The annual emissions of N 2 O and CH 4 from the TWS were estimated to be 0.9 × 10 −3 ± 2.9 × 10 −3 Tg yr −1 and 1.6 × 10 −3 ± 3.0 × 10 −3 Tg yr −1 , respectively. Taken together, the TWS accounts for 0.025% of the surface area of the world's oceans and 0.022% ± 0.07% and 0.017% ± 0.033% of global oceanic N 2 O and CH 4 emissions, respectively.
Abstract Rising temperatures in the Arctic Ocean can cause considerable changes, such as decreased ice cover and increased water inflow from the Pacific/Atlantic sector, which may alter dissolved methane (CH 4 ) cycles over the Arctic Ocean. However, the fate of dissolved CH 4 in the Arctic remains uncertain. Here, we show that CH 4 in the Chukchi Sea is enhanced in the shelf/slope areas, stored in the Upper Halocline (UHC), and transported to the central Arctic, contributing to the CH 4 excess (ΔCH 4 ) in the basins. The concentration of ΔCH 4 in the UHC was increasing (0.1 nM per year) and the ΔCH 4 has been distributed deeper and farther in the last decade than in the 1990s because of the intensification of Pacific water inflow due to oceanographic (currents) and atmospheric forcings (winds). We found heterogeneous CH 4 (208.4% ± 131.7%) in the Polar Mixed Layer and CH 4 supersaturation (1,100.9%–1,245.4%) in the below‐ice seawater in the basins, which may indicate the effect of sea ice cycles with the support of sediment‐origin CH 4 . We estimate the sea‐to‐air flux to be 1.1–2.4 μmol CH 4 m −2 day −1 during the ice‐free period in the Chukchi Sea, which suggests that the Chukchi Sea is currently a minor source (0.003 Tg in summer) of atmospheric CH 4 . Taken together, we propose a bottom‐up mechanism for CH 4 transport and emission and are concerned that the increases in the concentration of ΔCH 4 and the transport distance/rate of ΔCH 4 plume are occurring, with the potential to affect CH 4 emissions in the Pacific sector of the Arctic Ocean.
Rapid warming and loss of sea ice in the Arctic Ocean could play an important role in the dissolution and emission of greenhouse gas nitrous oxide (N2O). We investigated dissolved N2O in spatiotemporal distribution on the northeastern Bering Sea shelf (NEBS) in the summer of 2012. The results showed that N2O concentrations were higher in the Chirikov Basin (mean ± SD, 14.8 ± 2.4 nmol/L) than in the south of St. Lawrence Island (mean ± SD, 17.7 ± 2.3 nmol/L). In the Chirikov Basin, N2O displayed a decreasing distribution pattern from west (~20.4 nmol/L) to east (~12.9 nmol/L). In the area south of St. Lawrence Island, N2O almost presented a two-layer structure, although it showed a vertically homogeneous distribution in the inner shelf. In the cold bottom water, the N2O was affected mainly by in situ production or sediment emission. Longer resident time may cause N2O accumulation in the cold bottom water. The calculated sea–air flux (−1.6~36.2 μmol/(m2·d)) indicates that the NEBS is an important potential source of atmospheric N2O and could play an important role in global oceanic N2O emission with intensifying global issues.
Abstract The oceans are natural sources of atmospheric methane (CH 4 ), but the origin of excess CH 4 at the surface remains enigmatic. Incubation experiments were conducted in the western North Pacific (WNP) and its marginal seas (i.e., Yellow Sea and South China Sea [SCS]) to identify the degradation of methylphosphonate (MPn) to CH 4 in the oceans and the microbes associated with MPn‐driven CH 4 production. In the coastal seawater of the Yellow Sea, CH 4 was observed to accumulate after MPn enrichment with a high MPn to CH 4 conversion efficiency (approximately 60%). Dissolved inorganic phosphorus (Pi) did not effectively restrict the microbial utilization of MPn in the eutrophic coastal waters. The results of 16S rRNA gene sequencing showed that Vibrio spp. were the dominant bacteria in the MPn‐amended treatments. Moreover, several Vibrio isolates isolated from the coastal waters were found to produce CH 4 while growing in culture using MPn as the sole P source, thereby indicating that Vibrio spp. might be the major contributors to MPn‐dependent CH 4 production. In oligotrophic areas, such as the SCS and WNP, CH 4 production from MPn metabolism was also observed in the surface seawater. In contrast to coastal waters, this pathway in oligotrophic areas is regulated by dissolved Pi availability. This work confirms that aerobic CH 4 formation from MPn degradation can occur both in eutrophic coastal waters and oligotrophic oceans driven by MPn‐utilizing microorganisms (especially heterotrophic bacteria), which may have a significant impact on our understanding of the CH 4 and P cycles in global oceans.
Nitrous oxide (N 2 O) is one of the most important greenhouse gases and contributes to the depletion of ozone in the stratosphere. Estuaries are areas of intensive biological production and associated N 2 O emissions through both denitrification and nitrification processes. The spatial and temporal variations of N 2 O in the Jiulong River Estuary, a subtropical estuary, were explored to evaluate sources and sinks of N 2 O in this area. The estuary was found to be a strong source of N 2 O, its saturation in the surface water ranged from 113 to 2926% relative to the ambient atmospheric concentrations, showing great temporal and spatial variations and was influenced by multiple factors such as the concentration of dissolved inorganic nitrogen (DIN, i.e., NO3− , NH4+ , and NO2− ), salinity and dissolved oxygen. N 2 O concentrations were at a high level in upper estuary but reduced to the lower parts of the estuary. Groundwater input could be another contributor to N 2 O in the estuary. Almost all N 2 O within the estuary was released into the atmosphere rather than being transported to the bay. The N 2 O flux in the estuary (mean 597 μmol/m 2 /d) was at the higher end of the range observed in estuaries worldwide due to the very high DIN loads in the Jiulong River Estuary. Our data indicate that the N 2 O saturation in the estuary continues to increase, although the DIN inputs began to decline in 2011, which might be relate to the improved environmental conditions with increased oxygen concentrations. N 2 O production pathways have changed from predominantly denitrification in the past toward significant production from nitrification in the present. Further investigation is needed to better understand the behavior of N 2 O in the Jiulong River Estuary.
N 2 O is one of the most important greenhouse gases and ozone depletor, which was a matter of more and more concerned. The Southern Ocean was considered as one of the most important N 2 O source and was believed to account for ~1/4 of oceanic budget. However, there is uncertainty about this budget due to limited data availability. In this study, field and lab works were conducted for better understanding of N 2 O dynamics during sea ice melting and sea ice formation. In the field study, taking advantage of the Chinese Antarctic cruise, a 10 days’ time series study was carried out at a station in the Prydz Bay, Antarctica, where, surface water N 2 O was observed continuously, and the adjacent ice cores were taken for N 2 O analysis. In the lab, an ice growing simulation system was constructed to study the N 2 O dynamics during the sea ice formation. The result of endmember mixing models and calculation of N 2 O partition in three phases during sea ice formation provide important information about the dynamics of N 2 O during ice melting and sea ice formation processes, that is, the sea ice melting regulated N 2 O concentration and saturation status, which can be an explanation for reported N 2 O undersaturation observed in polar oceans, whereas during the sea ice formation, most of the N 2 O will be expelled to the deeper water while a small amount of retain the sea ice and less amount of N 2 O release to the atmosphere.
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