Planktonic and benthic incubations (bare and Posidonia oceanica vegetated sediments) were performed at monthly intervals from March 2001 to October 2002 in a seagrass vegetated area of the Bay of Palma (Mallorca, Spain). Results showed a contrast between the planktonic compartment, which was on average near metabolic balance (−4.6±5.9 mmol O₂ m^-2 d^-1) and the benthic compartment, which was autotrophic (17.6±8.5 mmol O₂ m^-2 d^-1). During two cruises in March and June 2002, planktonic and benthic incubations were performed at several stations in the bay to estimate the whole-system metabolism and to examine its relationship with partial pressure of CO₂ (pCO₂) and apparent oxygen utilisation (AOU) spatial patterns. Moreover, during the second cruise, when the residence time of water was long enough, net ecosystem production (NEP) estimates based on incubations were compared, over the Posidonia oceanica meadow, to rates derived from dissolved inorganic carbon (DIC) and oxygen (O₂) mass balance budgets. These budgets provided NEP estimates in fair agreement with those derived from direct metabolic estimates based on incubated samples over the Posidonia oceanica meadow. Whereas the seagrass community was autotrophic, the excess organic carbon production therein could only balance the planktonic heterotrophy in shallow waters relative to the maximum depth of the bay (55 m). This generated a horizontal gradient from autotrophic or balanced communities in the shallow seagrass-covered areas, to strongly heterotrophic communities in deeper areas of the bay. It seems therefore that, on an annual scale in the whole bay, the organic matter production by the Posidonia oceanica may not be sufficient to fully compensate the heterotrophy of the planktonic compartment, which may require external organic carbon inputs, most likely from land.

The air‐sea CO₂ gradient at the subtropical NE Atlantic was strongly dependent on the metabolism of the planktonic community within the top cms, but independent of that of the communities deeper in the water column. Gross primary production (GPP) and community respiration (R) of the planktonic community within the top cms exceeded those of the communities deeper in the water column by >10‐fold and >7 fold, respectively. Net autotrophic metabolism (GPP > R) at the top cms of the water column in some stations drove CO₂ uptake by creating a CO₂ deficit at the ocean surface, while net heterotrophic metabolism (GPP < R) at the top cms of the water column in other stations resulted in strong CO₂ supersaturation, driving CO₂ emissions. These results suggest a strong control of the air‐sea pCO₂ anomaly by intense biological processes.

Coastal wetlands are dynamic ecosystems that exist at the interface between land and sea. They represent environments with a great diversity of habitats and communities, high carbon sequestration capacity and a wide range of ecosystem services. In the Mediterranean, the largest coastal wetlands are found in deltaic areas like that of the Ebro River (Spain), which has a coastline length of approximately 50 km, occupying a total area of 325 km². The Ebro Delta is included in different national and international frameworks for environmental conservation, despite which there are several risks that threaten it. The lack of sedimentary contributions due to the regulation of the Ebro riverbed (irrigation, reservoirs, and hydroelectric power generation) has caused erosion and the retreat of certain sections of its coastline. To this situation of sediment deficit must be added the threat posed by the effects of global change, such as the rise in sea level, the increase in temperature and in the frequency and intensity of storms. This study analyses the particularities of the coastal wetland of the Ebro Delta, identifying the main threats it faces, as well as possible adaptation and mitigation strategies to these changes.