Carbon and carbonate metabolism in coastal aquatic ecosystems

The coastal zone is where land, ocean, and atmosphere interact. It exhibits a wide diversity of geomorphological types and ecosystems, each one displaying great variability in terms of physical and biogeochemical forcings. Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles because it receives massive inputs of terrestrial organic matter and nutrients, is among the most geochemically and biologically active areas of the biosphere, and exchanges large amounts of matter and energy with the open ocean. Coastal ecosystems have therefore attracted much attention recently and are the focus of several current national and international research programs (e.g. LOICZ, ELOISE). The primary production, respiration, calcification, carbon burial and exchange with adjacent systems, including the atmosphere, are reviewed for the major coastal ecosystems (estuaries, macrophyte communities, mangroves, coral reefs, and the remaining continental shelf). All ecosystems examined, except estuaries, are net autotrophic. The contribution of the coastal zone to the global carbon cycle both during pristine times and at present is difficult to assess due to the limited metabolic data available as well as because of major uncertainties concerning the magnitude of processes such as respiration, exchanges at the open ocean boundary, and air-sea fluxes of biogases.