A substantial amount of the atmospheric carbon taken up on land through photosynthesis and chemical weathering is transported laterally along the aquatic continuum from upland terrestrial ecosystems to the ocean. So far, global carbon budget estimates have implicitly assumed that the transformation and lateral transport of carbon along this aquatic continuum has remained unchanged since pre-industrial times. A synthesis of published work reveals the magnitude of present-day lateral carbon fluxes from land to ocean, and the extent to which human activities have altered these fluxes. We show that anthropogenic perturbation may have increased the flux of carbon to inland waters by as much as 1.0 Pg C yr(-1) since pre-industrial times, mainly owing to enhanced carbon export from soils. Most of this additional carbon input to upstream rivers is either emitted back to the atmosphere as carbon dioxide (similar to 0.4 Pg C yr(-1)) or sequestered in sediments (similar to 0.5 Pg C yr(-1)) along the continuum of freshwater bodies, estuaries and coastal waters, leaving only a perturbation carbon input of similar to 0.1 Pg C yr(-1) to the open ocean. According to our analysis, terrestrial ecosystems store similar to 0.9 Pg C yr(-1) at present, which is in agreement with results from forest inventories but significantly differs from the figure of 1.5 Pg C yr(-1) previously estimated when ignoring changes in lateral carbon fluxes. We suggest that carbon fluxes along the land-ocean aquatic continuum need to be included in global carbon dioxide budgets

[1] Mangrove forests are highly productive but globally threatened coastal ecosystems, whose role in the carbon budget of the coastal zone has long been debated. Here we provide a comprehensive synthesis of the available data on carbon fluxes in mangrove ecosystems. A reassessment of global mangrove primary production from the literature results in a conservative estimate of $218 ± 72 Tg C a À1 . When using the best available estimates of various carbon sinks (organic carbon export, sediment burial, and mineralization), it appears that >50% of the carbon fixed by mangrove vegetation is unaccounted for. This unaccounted carbon sink is conservatively estimated at $112 ± 85 Tg C a À1 , equivalent in magnitude to $30-40% of the global riverine organic carbon input to the coastal zone. Our analysis suggests that mineralization is severely underestimated, and that the majority of carbon export from mangroves to adjacent waters occurs as dissolved inorganic carbon (DIC). CO 2 efflux from sediments and creek waters and tidal export of DIC appear to be the major sinks. These processes are quantitatively comparable in magnitude to the unaccounted carbon sink in current budgets, but are not yet adequately constrained with the limited published data available so far.

[1] Air-water CO 2 fluxes were up-scaled to take into account the latitudinal and ecosystem diversity of the coastal ocean, based on an exhaustive literature survey. Marginal seas at high and temperate latitudes act as sinks of CO 2 from the atmosphere, in contrast to subtropical and tropical marginal seas that act as sources of CO 2 to the atmosphere. Overall, marginal seas act as a strong sink of CO 2 of about À0.45 Pg C yr À1. This sink could be almost fully compensated by the emission of CO 2 from the ensemble of near-shore coastal ecosystems of about 0.40 Pg C yr À1. Although this value is subject to large uncertainty, it stresses the importance of the diversity of ecosystems, in particular near-shore systems, when integrating CO 2 fluxes at global scale in the coastal ocean.

Atmospheric methane is a potent greenhouse gas that plays a major role in controlling the Earth's climate. The causes of the renewed increase of methane concentration since 2007 are uncertain given the multiple sources and complex biogeochemistry. Here, we present a meta-data analysis of methane fluxes from all major natural, impacted and human-made aquatic ecosystems. Our revised bottom-up global aquatic methane emissions combine diffusive, ebullitive and plant-mediated and/or fluxes from several sediment-water-air interfaces. We emphasize the high variability of methane fluxes within and between aquatic ecosystems and a positively skewed distribution of empirical data, making global estimates sensitive to statistical assumptions and sampling design. We find aquatic ecosystems contribute (median) 41% or (mean) 53% of total global methane emissions from anthropogenic and natural sources. We show that methane emissions increase from natural to impacted aquatic ecosystems, and from coastal to freshwater ecosystems. We argue that aquatic emissions will likely increase due to urbanization, eutrophication and positive climate-feedbacks, and suggest changes in land-use management as potential mitigation strategies to reduce aquatic methane emissions. Main text:Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2), accounting for 16 to 25% of atmospheric warming to date 1,2 . Atmospheric methane nearly tripled since pre-industrial times with a steady rise between 1984 and 2000 (8.4 ± 0.6 ppb yr -1 ) 3 , little or no growth between 2000 and 2006 (0.5 ± 0.5 ppb yr -1 ) 3 , and a renewed growth to present day (2007 to 2020: 7.3 ± 0.6 ppb yr -1 ) 3-6 . Whether the renewed increase is caused by emissions from anthropogenic or natural sources, or by a decline in the oxidative capacity of the atmosphere, or a combination of all three factors remains unresolved [7][8][9] . Depending on the approach used, total Rivers (ice-corrected) 5.8 (1.8-21.0) 30.5 ± 17.1 This study Lakes (ice-cover, ice-melt corrected) < 0.001 km 2 21.2 (9.1-53.5) 54.5 ± 48.5 This study 0.001 -0.01 km 2 13.2 (5.6-33.1) 31.1 ± 23.7 This study 0.01 -0.1 km 2 4.4 (1.4-16.7) 22.4 ± 18.4 This study 0.1 -1 km 2 3.0 (1.1-8.0) 9.9 ± 7.0 This study > 1 km 2 14.0 (6.0-31.0) 33.0 ± 45.0 This study All lakes 55.8 (23.3-142.3) 150.9 ± 73.0 This study Reservoirs (ice-cover, ice-melt corrected) < 1 km 2 0.4 (0.1-1.3) 2.4 ± 4.7 This study > 1 km 2 14.7 (8.7-27.1) 22.0 ± 6.4 This study All reservoirs 15.1 (8.8-28.4) 24.3 ± 8.0 This study Freshwater wetlands 150.1 (138.3-164.6) 148.6 ± 15.2 Saunois et al. 11 (A) Freshwater aquaculture ponds 4.4 (0.4-7.9) 14.0 ± 18.8 This study Rice cultivation 29.9 (24.9-32.1) 29.8 ± 6.7 Saunois et al. 11 (B) Total inland waters 261.0 (197.5-396.2) 398.1 ± 79.4 This study Estuaries 0.23 (0.02-0.91) 0.90 ± 0.29 This study Coastal wetlands Saltmarshes 0.18 (0.02-0.89) 2.00 ± 1.51 This study Mangroves 0.21 (0.06-0.77) 1.46 ± 0.91 This study Seagrasses 0.13 (0.07-0.21) 0.18 ± 0.19 This study Tidal flats 0.17 (0.04...

a b s t r a c tDespite their moderately sized surface area, continental marginal seas play a significant role in the biogeochemical cycles of carbon, as they receive huge amounts of upwelled and riverine inputs of carbon and nutrients, sustaining a disproportionate large biological activity compared to their relative surface area. A synthesis of worldwide measurements of the partial pressure of CO 2 (pCO 2 ) indicates that most open shelves in the temperate and high-latitude regions are under-saturated with respect to atmospheric CO 2 during all seasons, although the low-latitude shelves seem to be over-saturated. Most inner estuaries and near-shore coastal areas on the other hand are over-saturated with respect to atmospheric CO 2 . The scaling of air-sea CO 2 fluxes based on pCO 2 measurements and carbon massbalance calculations indicate that the continental shelves absorb atmospheric CO 2 ranging between 0.33 and 0.36 Pg C yr À1 that corresponds to an additional sink of 27% to $30% of the CO 2 uptake by the open oceans based on the most recent pCO