Coastal-zone biogeochemical dynamics under global warming

The coastal zone, consisting of the continental shelves to a depth of 200 meters, including bays, lagoons, estuaries, and near-shore banks, is an environment that is strongly affected by its biogeochemical and physical interactions with reservoirs in the adjacent domains of land, atmosphere, open ocean, and marine sediments. Because the coastal zone is smaller in volume and areal coverage relative to the open ocean, it traditionally has been studied as an integral part of the global oceans. In this paper, we show by numerical modeling that it is important to consider the coastal zone as an entity separate from the open ocean in any assessment of future Earth-system response under human perturbation. Model analyses for the early part of the 21st century suggest that the coastal zone plays a significant modifying role in the biogeochemical dynamics of the carbon cycle and the nutrient cycles coupled to it. This role is manifested in changes in primary production, storage, and/or export of organic matter, its remineralization, and calcium carbonate precipitation- all of which determine the state of the coastal zone with respect to exchange of CO₂ with the atmosphere. Under a scenario of future reduced or complete cessation of the thermohaline circulation (THC) of the global oceans, coastal waters become an important sink for atmospheric CO₂, as opposed to the conditions in the past and present, when coastal waters are believed to be a source of CO₂, to the atmosphere. Profound changes in coastal-zone primary productivity underscore the important role of phosphorus as a limiting nutrient. In addition, our calculations indicate that the saturation state of coastal waters with respect to carbonate minerals will decline by ∼15% by the year 2030. Any future slowdown in the THC of the oceans will increase slightly the rate of decline in saturation state.