Though small in area, the continental margins are the focal point of land, sea, and atmosphere interactions. They have special significance, not only for biogeochemical cycling and processes, but also increasingly for human habitation. In comparison with the relatively uniform environment of the open oceans, or the rapidly mixing atmosphere, the spatial and temporal heterogeneity of the world's coastal zone is considerable. In addition, human impacts on these relatively small areas are disproportionately large (Pacyna et al. 2000).
Several recent studies show that the continental margins currently function as substantial carbon sinks. Ver et al. (1999a, b) and Mackenzie et al. (2000) estimated a source of 0.2 PgC y-1 in the preindustrial era. They argue that the flux has decreased since 1800, a consequence of an ever-increasing nutrient discharge. Recently Mackenzie and coworkers (Rabouille et al. 2001) concluded that the continental margins were net sinks of 0.24 PgC y-1 in the preanthropogenic state (Figure 18.2). Assuming that the estimates of Ver et al. (1999a, b) and Mackenzie et al. (2000) were consistently low by 0.44 PgC y-1, the continental margins in 2000 would have been sinks of 0.34 PgC y-1. Though this adjustment may be an oversimplification, it reconciles the air-sea flux ofVer et al. (1999a, b) and Mackenzie et al. (2000) (Figure 18.3) with my result of 0.36 PgC y-1 (Figure 18.1). This is a significant fraction (16 percent) of the "global" air-to-sea flux of 2.2 PgC y-1 (Takahashi et al. 2002).
The coastal oceans are also strongly influenced by river inputs, which are sensitive to numerous factors, including regional weather and climate, as well as dams and diversions. Rivers are the major conduits for the transfer of water, nutrients, organic material, and particulate matter from land to sea. Inputs of nutrients and organic matter provide critical support for fish breeding in the estuaries. Dams block the downstream transport of particulate matter, an important source of nutrients and food for aquatic biota. A large dam, such as the Nile River's Aswan Dam in Upper Egypt, can have a dramatic impact on fish stocks in connected estuaries. The major source of shelf nutrients, however, is onshore advection of subsurface waters (Chen 2000). Decreasing river out-
1800 1850 1900 1950 2000 2050
Figure 18.3. Organic carbon balance (dashed line) and net exchange flux of CO2 across the air-seawater interface (solid line) for the coastal margin system, in units of 1012 moles C y-1 and Pg y-1. Positive values indicate the CO2 flux is directed toward the surface waters (modified from Ver et al. 1999a by adding 36.7 X 1012 mol y-1 or 0.44 Pg y-1 to their results).
flow will reduce the cross-shelf water exchange through a reduced buoyancy effect, and it will also diminish the onshore nutrient supply. Decreased primary production and fish catch on the shelf typically follow dam construction.
Globally, approximately 40 percent of the freshwater and particulate matter entering the oceans is transported by the 10 largest rivers. Buoyant plumes move much of the water and nutrients to the open shelves. Hence, these shelves also experience a diminished biological pump and fish production when damming reduces freshwater outflow. The fraction of riverine particulate carbon that is deposited in the deltas or beaches or converted to the ever-increasing DOC pool is not well known. On the other hand, the coastal oceans may be heterotrophic but nevertheless absorb CO2.
The first LOICZ report (Kempe 1995) concluded that coastal seas could be net sinks or sources of CO2 for the atmosphere, with slim prospects for a quick resolution. Now, mass balance calculations, as well as direct pCO2 measurements, indicate a consistent pattern. Taken together, continental shelves are significant sinks for atmospheric CO2, absorbing 0.36 PgC a year. This flux is a composite over many estuaries, coastal waters, and intensive upwelling areas, which are typically supersaturated with respect to CO2, and most open shelf areas, which are probably undersaturated. This "continental shelf pump" is primarily fueled by the cross-shelf transport of nutrients from nutrient-rich subsurface waters offshore. Though they are sinks for CO2, the shelves release 0.1 X 1012 mol y-1 CH4 and 0.07 X 1012 mol y-1 DMS into the atmosphere.
New production supported by the external sources of nutrients represents about 13 percent of primary production. The other 87 percent is respired and recycled on the shelf. Some of the organic material that is not recycled accumulates in the sediments, but most of the detrital organic matter, mainly in its dissolved form, is exported to the slopes and open oceans.
I am indebted to the National Science Council of the Republic of China, which provided partial financial assistance (NSC 92-2611-M-110-003). N. Gruber made detailed and constructive criticisms, which strengthened the manuscript.
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