Coastal Ocean

Although the coastal zones, consisting of the continental shelves with depths less than 200 m including bays and estuaries, occupy only 10 percent of the total ocean area, they play a crucial role in the global carbon cycle. Carbon is transported to the coastal zone by riverine inputs and transport of inorganic carbon from the open ocean. Estuaries and proximal coastal seas are believed to be sources of CO2 because of the decay of terrestrial organic carbon. Since the riverine flux of nutrients has risen continually over the past few decades, however, these areas may now have enhanced biological productivity and hence may be releasing less CO2. At present a large fraction (~80 percent) of the land-derived organic and inorganic materials that are transported to the ocean is trapped on the proximal continental shelves (Mackenzie and Ver 2001). The much wider open shelves, on the other hand, probably serve as sinks for atmospheric carbon. A recent overview suggests that the global coastal waters and marginal seas (extending to a water depth of200 m) are now absorbing about 0.36 PgC y-1 from the atmosphere (Chen, Chapter 18, this volume).

Across most of the coastal seas and continental margins, surface waters are transported offshore because of fresh water inputs from land. This surface transport draws nutrient-rich subsurface waters from the open ocean onto the shelves. Such external sources of nutrients support high primary productivity. Most of the organic material produced is respired and recycled on the shelves. The organic matter that is not recycled either accumulates in the sediments or is exported to the slopes and open oceans. The coastal zone may account for 30—50 percent of the total calcium carbonate accumulation and up to 80 percent of the organic carbon accumulation in ocean sediments (Mackenzie and Ver 2001). Globally, the shelf seas are estimated to transport 0.6 PgC y-1 of DOC, 0.5 PgC y-1 of POC, and 0.2 PgC y-1 of PIC to the open oceans (Chen, Chapter 18, this volume). Although these transports have large uncertainties because of high variability and inadequate data coverage, they represent an important and often neglected link in the global carbon cycle. Shelves and estuaries are also important sources of other greenhouse or reactive gases, such as methane and dimethyl sulfide.

Finally, although humans appear to have had only a small direct impact on the open ocean, they have had a profound and poorly understood impact on the coastal oceans. Furthermore, direct and indirect human perturbations vis-à-vis the continental margins (e.g., pollution, eutrophication) are likely to have large and dire consequences on marine ecosystems in the future. As much as 40—60 percent of the world population lives in coastal areas, depending on the definitions and methodologies applied. The coastal regions have the most rapidly growing populations, because of migration from rural areas. Most megacities in this century will develop in coastal zones, approximately half of them in Asia (IHDP 2000). Such patterns of urbanization have affected and will continue to affect coastal and marine systems, through processes such as land use change, pressure on infrastructure (water, sewage, and transportation), coastal resource depletion and degradation, eutrophication, and other carbon-relevant impacts. Coastal zones support more than 60 percent of the global commercial fish production (World Resources Institute 1996). It is not clear how these fisheries will be affected by the increasing human pressures on the coastal zone. Additional studies of the coastal zone and the interaction between humans and the coastal biogeochemical systems are needed.

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