Abstract. The coastal ocean is a crucial link between land, the open ocean and the atmosphere. The shallowness of the water column permits close interactions between the sedimentary, aquatic and atmospheric compartments, which otherwise are decoupled at long time scales ( ∼ =1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO 2 and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates total alkalinity (A T ) and increases the CO 2 buffer capacity of seawater. At both the basin wide and annual scales anaerobic A T generation in the North Sea's tidal mud flat area irreversibly facilitates 7-10%, or taking into consideration benthic denitrification in the North Sea, 20-25% of the North Sea's overall CO 2 uptake. At the global scale, anaerobic A T generation could be accountable for as much as 60% of the uptake of CO 2 in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO 2 conditions oceanic CO 2 storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity.

We report partial pressure of CO 2 (pCO 2 ) and ancillary data in three rivers (Bia, Tanoé, and Comoé) and five lagoons (Tendo, Aby, Ebrié, Potou, and Grand-Lahou) in Ivory Coast (West Africa), during four cruises covering the main climatic seasons. The three rivers were oversaturated in CO 2 with respect to atmospheric equilibrium, and the seasonal variability of pCO 2 was due to dilution during the flooding period. Surface waters of the Potou, Ebrié, and Grand-Lahou lagoons were oversaturated in CO 2 during all seasons. These lagoons behaved similarly to the oligohaline regions of macrotidal estuaries that are CO 2 sources to the atmosphere due to net ecosystem heterotrophy and inputs of riverine CO 2 rich waters. The Aby and Tendo lagoons were undersaturated in CO 2 with respect to the atmosphere because of their permanent haline stratification (unlike the other lagoons) that seemed to lead to higher phytoplankton production and export of organic carbon below the pycnocline.

We report a data-set of dissolved methane (CH 4 ) in three rivers (Comoé, Bia and Tanoé) and five lagoons (Grand-Lahou, Ebrié, Potou, Aby and Tendo) of Ivory Coast (West Africa), during the four main climatic seasons (high dry season, high rainy season, low dry season and low rainy season). The surface waters of the three rivers were over-saturated in CH 4 with respect to atmospheric equilibrium (2221-38719%), and the seasonal variability of CH 4 seemed to be largely controlled by dilution during the flooding period. The strong correlation of CH 4 concentrations with the partial pressure of CO 2 (pCO 2 ) and dissolved silicate (DSi) confirm the dominance of a continental sources (from soils) for both CO 2 and CH 4 in these rivers. Diffusive air-water CH 4 fluxes ranged between 25 and 1187 lmol m -2 day -1 , and annual integrated values were 288 ± 107, 155 ± 38, and 241 ± 91 lmol m -2 day -1 in the Comoé, Bia and Tanoé rivers, respectively. In the five lagoons, surface waters were also over-saturated in CH 4 (ranging from 1496 to 51843%). Diffusive air-water CH 4 fluxes ranged between 20 and 2403 lmol m -2 day -1 , and annual integrated values were 78 ± 34, 338 ± 217, 227 ± 79, 330 ± 153 and 326 ± 181 lmol m -2 day -1 in the Grand-Lahou, Ebrié, Potou, Aby and Tendo lagoons, respectively. The largest CH 4 oversaturations were observed in the Tendo and Aby lagoons that are permanently stratified systems (unlike the other three lagoons), leading to anoxic bottom waters favorable for a large CH 4 production. In addition, these two stratified lagoons showed low pCO 2 values due to high primary production, which suggests an efficient transfer of organic matter across the pycnocline. As a result, the stratified Tendo and Aby lagoons were respectively, a low source of CO 2 to the atmosphere and a sink of atmospheric CO 2 while the other three well-mixed lagoons were strong sources of CO 2 to the atmosphere but less oversaturated in CH 4 .