Coastal Ocean and Continental Shelves

The research community has made tremendous progress in improving understanding and constraining rates of carbon cycling in coastal waters since SOCCR1 (CCSP 2007), primarily because of a greatly expanded suite of observations, process studies, and models. However, quantification of many coastal carbon fluxes remains a significant challenge. Carbon is constantly exchanged across the air-sea interface as well as the interfaces between land and coastal ocean, coastal and open-ocean waters, and water and sediment. Net exchange fluxes and trends are relatively small signals masked by a large and fluctuating background. At present, most of these fluxes are not quantified well enough to derive well-constrained carbon budgets for North American coastal waters or to project how those fluxes will change in the future due to various drivers.

This chapter focused primarily on the role of ocean margins in sequestering atmospheric CO₂ and coastal ocean acidification. In the coastal ocean, a net removal of carbon from direct interaction with the atmospheric reservoir can occur by export of dissolved or particulate carbon to the deep ocean or by permanent burial in sediments. Neither of these is easily observed or well quantified. The best-observed flux is gas exchange across the air-sea interface, although extracting the small net flux and its trend from a variable background remains a challenge. Ultimately, the removal of anthropogenic carbon is the relevant quantity for assessing the contribution of ocean margins to the uptake of anthropogenic carbon; however, the separation of anthropogenic fluxes from the natural background is thus far elusive for coastal waters.

Estimates of air-sea CO₂ fluxes currently provide the best evidence for the contribution of coastal waters to overall carbon uptake by the ocean. In the broad shelf system of the North American Atlantic Coast, shelf water is separated from the adjacent open ocean by persistent shelf break currents and density fronts. Available estimates suggest that the overall North American Atlantic Coast is a weak sink, with some subregions acting as sources (e.g., nearshore regions of the SAB), while others are either neutral (Scotian Shelf and GOM) or act as weak sinks (MAB and outer SAB). Large sections of the narrow shelf of the North American Pacific Coast are dominated by upwelling circulation, which leads to strong CO₂ outgassing near the coast. However, compensating for this outgassing is biologically driven uptake from upwelled nutrients further offshore. Recent estimates are consistent in suggesting that the region is a weak to moderate sink of atmospheric CO₂. The relatively wide shelves in the GMx are considered a weak net sink, with the West Florida Shelf and the western Gulf shelf acting as sources; the Mexico shelf being neutral; and only the northern shelf a clear sink that is driven largely by anthropogenic nutrient inputs from the Mississippi River. The wide, seasonally ice-covered shelves in the North American Arctic consistently are acting as a sink for atmospheric CO₂. The low ­surface-water pCO₂ in this region primarily results from low water temperatures and the decreased uptake of atmospheric CO₂ during a significant fraction of the year because of seasonal ice cover. Overall, North American coastal waters act as a sink, but regional variations and uncertainties are large.

Several drivers influence secular trends in coastal carbon fluxes and will continue to do so in the future. These drivers include rising atmospheric CO₂ levels, changes in atmosphere-ocean interactions (e.g., wind forcing and heat fluxes), changes in the hydrological cycle, and anthropogenic perturbations of global nutrient cycling (particularly, the nitrogen cycle). Coastal surface pCO₂ clearly does not closely track atmospheric pCO₂. Although there are a number of plausible mechanisms for potential future changes in coastal carbon uptake, the total effect cannot be predicted with any confidence. Regional model studies are beginning to address these challenges.

A major concern is coastal acidification, which can affect the growth, metabolism, and life cycles of many marine organisms, specifically calcifiers, and can trigger cascading ecosystem-scale effects. Most vulnerable are those organisms that precipitate aragonite, one of the more soluble forms of biogenic CaCO₃ in the ocean. Aragonite saturation states are routinely below saturation (i.e., favoring dissolution) in North American Arctic coastal waters. In the North American Pacific Coast region, atmospheric CO₂ uptake in combination with intensified upwelling that brings low-pH, low-oxygen water onto the shelves leads to aragonite levels below the saturation threshold in large portions of the subsurface waters. In the northern GMx, aragonite saturation states are well above the dissolution threshold. Although eutrophication-induced acidification occurs in bottom waters influenced by Mississippi River inputs of nutrients and freshwater, saturation levels remain well above the dissolution threshold.

Given the importance of coastal margins, both in contributing to carbon budgets and in the societal benefits they provide, further efforts to improve assessments of the carbon cycle in these regions are paramount. Critical needs are maintaining and expanding existing coastal observing programs, continuing national and international coordination and integration of observations, increasing development of modeling capabilities, and addressing stakeholder needs.