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In the current era of rapid climate change, accurate characterization of climate-relevant gas dynamics-namely production, consumption, and net emissions-is required for all biomes, especially those ecosystems most susceptible to the impact of change. Marine environments include regions that act as net sources or sinks for numerous climateactive trace gases including methane (CH4) and nitrous oxide (N2O). The temporal and spatial distributions of CH4 and N2O are controlled by the interaction of complex biogeochemical and physical processes. To evaluate and quantify how these mechanisms affect marine CH4 and N2O cycling requires a combination of traditional scientific disciplines including oceanography, microbiology, and numerical modeling. Fundamental to these efforts is ensuring that the datasets produced by independent scientists are comparable and interoperable. Equally critical is transparent communication within the research community about the technical improvements required to increase our collective understanding of marine CH4 and N2O. A workshop sponsored by Ocean Carbon and Biogeochemistry (OCB) was organized to enhance dialogue and collaborations pertaining to marine CH4 and N2O. Here, we summarize the outcomes from the workshop to describe the challenges and opportunities for near-future CH4 and N2O research in the marine environment.

We discuss concentrations of dissolved CH2, N2O, O2, NO-3 and NH+4, and emission fluxes of CH4 and N2O for river sites in the western Congo Basin, Republic of Congo (ROC). Savannah, swamp forest and tropical forest samples were collected from the Congo main stem and seven of its tributaries during November 2010 (41 samples; "wet season") and August 2011 (25 samples; "dry season"; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: NH+4 C NO-3 ; wet season) was dominated by NO-3 (63-19% of DIN). Total DIN concentrations (1.5-45.3 "mol L-1) were consistent with the near absence of agricultural, domestic and industrial sources for all three land types. Dissolved O2 (wet season) was mostly undersaturated in swamp forest (36-29 %) and tropical forest (77-36 %) rivers but predominantly supersaturated in savannah rivers (100-17%). The dissolved concentrations of CH4 and N2O were within the range of values reported earlier for sub-Saharan African rivers. Dissolved CH4 was found to be supersaturated (11.2-9553 nmol L-1; 440-354 444 %), whereas N2O ranged from strong undersaturation to supersaturation (3.2-20.6 nmol L-1; 47-205 %). Evidently, rivers of the ROC are persistent local sources of CH4 and can be minor sources or sinks for N2O. During the dry season the mean and range of CH4 and N2O concentrations were quite similar for the three land types. Wet and dry season mean concentrations and ranges were not significant for N2O for any land type or for CH4 in savannah rivers. The latter observation is consistent with seasonal buffering of river discharge by an underlying sandstone aquifer. Significantly higher wet season CH4 concentrations in swamp and forest rivers suggest that CH4 can be derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers also exhibited both low (47 %) and high (205 %) N2O saturation but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always supersaturated (103-266 %) and for which the overall means and ranges of N2O were not significantly different. In swamp and forest rivers O2 saturation co-varied inversely with CH4 saturation (log %) and positively with % N2O. A significant positive correlation between N2O and O2 saturation in swamp rivers was coincident with strong N2O and O2 undersaturation, indicating N2O consumption during denitrification in the sediments. In savannah rivers persistent N2O supersaturation and a negative correlation between N2O and O2 suggest N2O production mainly by nitrification. This is consistent with a stronger correlation between N2O and NH+4 than between N2O and NO-3. Our ranges of values for CH4 and N2O emission fluxes (33-48 70 mol CH4 m-2-1; 1-67 µmol N2Om-2d-1) are within the ranges previously estimated for sub-Saharan African rivers but they include uncertainties deriving from our use of "basin-wide" values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which is quite likely for CH4 (at least 20 %), we consider our emission fluxes for CH4 to be conservative. [ABSTRACT FROM AUTHOR]

We report concentrations of dissolved CH4, N2O, O2, NO3- and NH4+, and corresponding CH4 and N2O emissions for river sites in savanna, swamp forest and tropical forest, along the Congo main stem and in several of its tributary systems of the Western Congo Basin, Republic of Congo, during November 2010 (41 samples; ''wet season'') and August 2011 (25 samples; ''dry season''; CH4 and N2O only). Dissolved inorganic nitrogen (DIN: wet season; NH4+ + NO3-) was dominated by NO3- (63 ± 19 % of DIN), total DIN concentrations (1.5-45.3 nmol L-1) being consistent with small agricultural, domestic and industrial sources. Dissolved O2 (wet season) was mostly under-saturated in swamp forest (36 ± 29 %) and tropical forest (77 ± 36 %) rivers but predominantly super-saturated in savannah rivers (100 ± 17 %). Dissolved CH4 and N2O were within previously reported ranges for sub-Saharan African rivers. While CH4 was always super-saturated (11.2-9553 nmol L-1; 440-354 400 %), N2O ranged from strong under-saturation to strong super-saturation (3.2-20.6 nmol L-1; 47-205 %). Evidently, rivers of the ROC are persistent local sources of tropospheric CH4 but can be small sources or sinks for N2O. Dry season concentration means and ranges of CH4 and N2O were indistinguishable for all three land types and seasonal differences in means and ranges were not significant for N2O for any land type or for CH4 in savannah rivers; the latter is consistent with seasonal buffering of river discharge by an underlying sandy-sandstone aquifer. By contrast, swamp and forest river CH4 was significantly higher in the wet season, possibly reflecting CH4 derived from floating macrophytes during flooding and/or enhanced methanogenesis in adjacent flooded soils. Swamp rivers exhibited both low (47 %) and high (205 %) N2O saturations but wet season values were overall significantly lower than in either tropical forest or savannah rivers, which were always super-saturated (103-266 %) and for which the overall means and ranges of N2O were not significantly different. In swamp and forest rivers % O2 co-varied negatively with log % CH4 and positively with % N2O. The strong positive N2O-O2 correlation in swamp rivers was coincident with strong N2O and O2 under-saturation, indicating N2O consumption by sediment denitrification. In savannah rivers persistent N2O super-saturation and a negative N2O-O2 correlation may indicate N2O production mainly by nitrification, consistent with a stronger correlation between N2O and NH4+ than between N2O and NO3-. Our range in CH4 and N2O emissions fluxes (33-48 705 μmol CH4 m-2 d-1; 1-67 μmol N2O m-2 d-1), is wider than previously estimated for sub-Saharan African rivers but it includes uncertainties deriving from our use of ''basin-wide'' values for CH4 and N2O gas transfer velocities. Even so, because we did not account for any contribution from ebullition, which for CH4 is likely to be at least 20 %, our emissions estimates for CH4 are probably conservative. [ABSTRACT FROM AUTHOR]