The continental shelf, continental margin, coastal ocean and coastal zone are fuzzy concepts for which various definitions have been proposed.
The continental shelf is the area extending from the coast to the shelf break, which is usually defined by the 200 meter depth isobath. The continental margin is the transition zone between the continental crust and the oceanic crust, including the coastal plain, continental shelf, slope and rise. The coastal ocean is the portion of the global ocean where physical, biological and biogeochemical processes are directly affected by land. It is either defined as the part of the global ocean covering the continental shelf or the continental margin. The coastal zone usually includes the coastal ocean as well as the portion of the land adjacent to the coast that influences coastal waters. It can readily be appreciated that none of these concepts has a clear operational definition.
The coastal ocean is a shallow (<200 m) area, covering approximately 7% (26x106 km2) of the surface of the global ocean, where land, ocean and atmosphere interact; in oceanography this region is termed the Epipelagic Zone. The depth of the coastal zone has been established through an interesting historical accident—essentially based on the shallowest isobath that was originally used in the early 20th century hypsographic analysis. If the coastal zone is delineated on the basis of the shelf break, it has been established that the average depth of the shelf-slope break is closer to 125 m. The choice makes little difference in terms of the area. The world coastline extends over 350,000-1,000,000 km, depending upon how finely the "length" is resolved. Within its extent, the coastal ocean and the immediately landward region of the coastal zone displays a wide diversity of geomorphological types and ecosystems.
Despite its relatively modest surface area, the coastal zone plays a considerable role in the biogeochemical cycles, because virtually all land-derived materials (water, sediments, dissolved and particulate nutrients, etc.) enter this region in surface runoff or groundwater flow. These terrestrial inputs are changing, largely as a consequence of human influence; about 40% of the world population lives within 100 km of the coastline, and this proportion is increasing. In addition, the coastal ocean exchanges large amounts of matter and energy with the open ocean. As a consequence of these external influences the coastal ocean constitutes one of the most geochemically and biologically active areas of the biosphere. For example, it accounts for at least 15% of oceanic primary production, 80% of organic matter burial, 90% of sedimentary mineralization, 75-90% of the oceanic sink of suspended river load and ca. 50% of the deposition of calcium carbonate. Additionally, it represents 90% of the world fish catch and its overall economic value has been recently estimated as at least 40% of the value of the world's ecosystem services and natural capital.
Despite its potential importance, the coastal ocean has been relatively neglected until recently, probably because of its intrinsic complexity. It is the focus of several national and international ongoing research programs. The Land-Ocean Interactions in the Coastal Zone (LOICZ) program was established as part of the IGBP Global Change Programme (IGBP) in 1993. It is now also a core project of the International Human Dimensions Programme on Global Environmental Change (IHDP). The European Union has launched a coastal core project (European Land-Ocean Interaction Studies, ELOISE).
Major coastal ecosystems
|Table 1: Surface area of the
main coastal ecosystems.
(106 km2 )
The main ecosystems of the coastal ocean are: estuaries, macrophyte communities, mangroves, coral reefs, salt marshes and the remaining continental shelves (Table 1).
These areas are only approximately known, and there is some double-counting among the ecosystem types. For example, coral reefs, estuaries and the "remaining shelf" all include macrophyte-dominated communities.
Whereas the metabolism of the open ocean is by far dominated by phytoplanktonic primary production, the coastal ocean exhibits a great diversity of primary producers, often inhabiting the same area, which makes it difficult to subdivide this region into specific subdomains.
Compilation of metabolic data indicates that the ratio gross primary production versus respiration (GPP:R) exhibit a large variability and are in most cases not statistically different from 1. The open shelf, the various macrophyte systems, and coral reefs are typically net autotrophs as shown by GPP:R > 1 and net ecosystem production (NEP) > 0). Many estuaries exhibit a negative net ecosystem production (-10 Tmol C yr-1) and are net heterotrophs. There remains a debate as to whether the coastal ocean taken as a whole is net autotrophic or heterotrophic. Up-scaling the system-level data to the whole coastal zone, provides a GPP value of 789 Tmol C yr-1; or 23% of the global marine gross primary production. This is of the same order of magnitude as the previous estimate of 500 Tmol C yr-1.
The ecosystem approach provides an NEP estimate of 235 Tmol C yr-1, a value in good agreement with that provided by Wollast (200 Tmol C year-1) but much higher than the figure of -7 Tmol C yr-1 proposed by Smith & Hollibaugh or the value of 12 Tmol C yr-1 given by Rabouille. The continental shelf (excluding specific ecosystems) is the major contributor to the NEP of the costal zone (75%) followed by macrophyte-dominated ecosystems (16%), mangroves (7%), marshes (3%) and coral reefs (2.6%).
The contribution of the coastal ocean to marine calcium carbonate production is greater than 40% (23 Tmol CaCO3 yr-1 out of 53 Tmol yr-1). The highest deposition occurs in coral reef habitats (9 Tmol yr-1), followed by banks and embayments (4 Tmol yr-1), carbonate shelves (6 Tmol yr-1) and non-carbonate shelves (4 Tmol year-1).
Coastal zone carbon budgets (LOICZ program)
Until recently, there were few carbon budgets available for coastal ecosystems but many were constructed by the LOICZ program. Approximately 200 budgets demonstrate the extreme heterogeneity of the coastal zone, with most of the variability towards both autotrophy and heterotrophy being in smaller systems. To date, a conclusion from the LOICZ studies is that evaluating the metabolic status of the coastal ocean by a summation of small-system budgets is unlikely to resolve the net metabolism. The heterogeneity about a net metabolism near zero is a more striking characteristic than a strong trend towards net autotrophy versus heterotrophy.
Air-sea CO2 fluxes
Borges used another approach based on a compilation of literature data on annually integrated air-water carbon dioxide (CO2) flux data from 44 coastal environments. He concluded that the coastal ocean behaves as a source of CO2 for the atmosphere (0.38 mol C m-2 yr-1). More recently, Cai et al. applied a similar approach for seven classes of open shelves. They concluded that shelves are a net sink of approximately 0.8 mol C m-2 yr-1. However, those authors observed that it was not possible to determine how much of that CO2 flux into shelf waters is a result of elevated atmospheric CO2, and how much represents shelf autotrophy.
Data presently available indicate that the coastal ocean is metabolically very active and very heterogeneous in comparison to the remainder of the ocean. It also is clearly being impacted by human modifications of fluxes of water, nutrients, and organic matter, particularly in inshore waters. Because this region is of great socio-economic value, improved ecolgical management is imperative.
- Alongi D. M., 1998. Coastal ecosystem processes. 419 p. Boca Raton: CRC Press.
- Cai, W-J., Dai, M., and Wang, Y., 2006. Air-sea exchanges of carbon dioxide in ocean margins: a province-based approach. Geophysical Research Letters 33(12): L12603 10.1029/2006GL026219.
- Borges A. V., 2005. Do we have enough pieces of the jigsaw to integrate CO2 fluxes in the coastal ocean? Estuaries 28(1):3-27.
- Crossland, C. J., et al. 2005. Coastal fluxes in the Anthropocene. 231 p. Springer, Berlin.
- Gattuso J.-P., Frankignoulle M. & Wollast R., 1998. Carbon and carbonate metabolism in coastal aquatic ecosystems. Annual Review of Ecology and Systematics 29:405-434.
- Rabouille C., Mackenzie F. T. & Ver L. M., 2001. Influence of the human perturbation on carbon, nitrogen, and oxygen biogeochemical cycles in the global coastal ocean. Geochimica et Cosmochimica Acta 65(21):3615-3641.
- Smith S. V. & Hollibaugh J. T., 1993. Coastal metabolism and the oceanic organic carbon balance. Reviews of Geophysics 31(1):75-89.