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Continental shelf

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This article describes the habitat of the continental shelf. It is one of the sub-categories within the section dealing with biodiversity of marine habitats and ecosystems. It gives an overview about the characteristics, processes such as sedimentation and biota. A short section about legal aspects is also added.


Introduction

The continental shelf is a shallow, near horizontal seafloor extension from the shoreline to the upper continental slope. This shelf forms the shallow margin of each deep-ocean basin. At the ocean side it is terminated by a pronounced change in bottom gradient (degree of slope). This is called the shelf break. The continental shelf is characterized by a very gentle slope less than 1 degree. The average depth is about 150 m and it has an average width of 70 km. But local variations are common, ranging from more than 1000 km in the Arctic Ocean to a few kilometers along the Pacific coast of North and South America. The water above the continental shelf is called neritic water. Below the shelf break is the continental slope. This zone is much steeper than the continental shelf. At the base of this steep slope is the continental rise which finally merges into the deep ocean floor, the abyssal plain. The continental shelf, slope and rise are part of the continental margin. This is the transition zone between the continental and the oceanic crust.

Basic composition of the continental margins with sediment (light brown), rocks (middle brown) and the mantle of the earth (dark brown) [1]


Generally it is one of the most productive parts of the ocean. Many benthic, coastal animals have evolved larval stages which swim for a time in the water. These larvae are also abundant in the neritic water. Although the continental shelf zones comprise only 7.6% of the surface area of the world oceans, they provide 15-30% of the oceanic primary production. [2] A special type of ecosystem that can be found on the continental shelves is the kelp forest. This is one of the more productive ecosystems on the continental shelf.


Examples of these shelf seas are the Baltic an North Sea, Yellow and East China, Hudson Bay, Bering Sea,…

The global continental shelf (marked in turquoise) [3]


Shelf sedimentation

Energy for eroding and transporting sediment grains is provided by the tides and wind-generated waves and currents. In general, waves seem to be the dominant process affecting the sea bottom. Because the continental shelf is shallow, the waves have a large impact on the bottom in comparison to the open ocean. Water becomes increasingly calmer with depth, so the deeper you go, the more your bottom is unaffected by waves. Breaking waves affect the shoreline and remove and suspend all the fine sediment into the water. Only medium and coarse sand and gravel can be deposited on the beach and in the nearshore zone. More seaward the bottom energy induced by waves decreases with depth. This causes a decreasing grain size with distance offshore. Sedimentation under different depositional conditions in the past indicate the past sea level changes and are known as relict sediments. This shows the importance of sea level fluctuations for the sediment composition.


The distribution of sediment types of the continental shelf show a regular pattern that vary with latitude and that depend on climate. At the equator, a broad band of biogenic sediment extends into the subtropics. These deposits include coral reefs and accumulations of grain fragments, mainly composed of calcium carbonate (CaCO_3) derived from the hard parts of organisms. This band of deposition material is broader along the western edges of the oceans. The reason for this is that warm, westerly flowing equatorial currents diverge from the equator and flow to the poles. At the eastern edges, cold currents flow from the poles to the equator. At the temperate latitudes, the continental shelves are covered with terrigenous deposits transported by river outflow. This is especially composed of quartz and feldspar derived from weathering of granite on land. Poorly sorted glacial deposits are dumped at the poles by glaciers and ice-rafted debris (IRD).


Biota

The neritic waters contain a rich community of organisms. The number and types of organisms that can live in and on the continental shelf are mainly determined by the types and characteristics of the sediments. Sediments contain nutrients such as nitrogen, phosphorus, silica, calcium that are essential for the organisms. Nutrients also reach the coastal seas by upwelling. This, together with the large amount of sunlight, makes the continental shelves a productive area.


Based on the characteristics of the substrate, two benthic communities are determined: the soft-bottom communities and the hard-bottom communities

  • The hard-bottom communities are these which occur in areas with strong current flows. Because of these strong flows, the bottom is composed of coarse sediments like gravel, rocks and sand. This is not a suitable habitat for burrowing and interstitial organisms because of the frequently shifting bottom. The flows carry a large amount of food. This makes it a suitable area for sedentary or sessile filter-feeders or suspension-feeders. The coarse sediments allow them to attach themselves. Common organisms in coarse sediments are sponges, anemones and colonial cnidarians (Hydrozoa). Due to the uneven surfaces of the substrate, a large number of niches are created. This, together with the bulky grow of seaweeds, makes the growth of a rich benthic fauna possible.



  • The soft-bottom communities are these which occur in areas with weak current flows. The bottom is composed of fine sediments like sand and silt. This is a suitable habitat for burrowing organisms like polychaete worms, amphipods and bivalves. Most of these organisms are deposit-feeders, feeding on particles of organic matter in the sediment. Filter-feeders are not abundant because there is less suspended matter in the water and the fine sediments will obstruct the filtering structures.



The distribution of the sediment types is not equally along the continental shelf. This causes an unequal distribution of benthic organisms called patchiness. The food for the benthic community is detritus, originating from the water above. It consists of fecal pellets, dead organisms and organic debris. There are several ways to capture detritus such as tentacles, filter apparatus, scraping and cilia.

The water column or the neritic zone is dominated by plankton. This contains of plants, animals, bacteria, viruses, eggs and larvae. The most abundant types of phytoplankton are diatoms and dinoflagellates. They obtain energy through photosynthesis and need sunlight to produce oxygen and carbohydrates. This is not a problem in the neritic zone, because it is a shallow zone and light penetrates deep enough for the phytoplankton to use it. Seasonal changes in water temperature, salinity and nutrient input creates a regular succession of phytoplankton species in the temperate and polar seas. The zooplankton is dominated by copepods, larvae, protozoa, crustaceans and jellyfishes and feeds on the phytoplankton by grazing. The plankton is not evenly distributed in the water and forms patches or aggregates in dense clusters.


The nekton in the water are the group of the active swimmers. They can avoid unfavorable conditions, search actively for food and breeding grounds. Like the plankton, the nekton is aggregated in schools or clusters. They act like one unit and all the members are of the same size. The benefit for the species is that they are protected against predators and reducing the chance they will be captured.[10] [11]


Legal aspect

In the United Nations Convention on the Law of the Sea (UNCLOS 1982), the definition of the continental shelf is: ‘The continental shelf of a coastal State comprises the seabed and subsoil of the submarine areas that extend beyond its territorial sea throughout the natural prolongation of its land territory to the outer edge of the continental margin or to the distance of 200 nautical miles from the baselines from which the breadth of its territorial sea is measured where the outer edge of the continental margin does not extend up to that distance.’


The coastal State has sovereign rights for exploring and exploiting its natural resources. These natural resources are minerals and other non-living resources of the seabed and subsoil. The sedentary organisms are also included in the natural resources. The rights of the State are not applicable to the water and the air above it. The State can allow and regulate drilling on the continental shelf. [12]


References

  1. http://www.onr.navy.mil/Focus/ocean/regions/oceanfloor2.htm
  2. Yool A. Fashman M.J.R. 2001. An examination of the ‘continental shelf pump’ in an open ocean general circulation model. Global Biogeochemical Cycles 15(4):831-844
  3. http://en.wikipedia.org/wiki/Continental_shelf
  4. http://www.marbef.org - Decleer M.
  5. http://www.marbef.org - Emblow C.S.
  6. http://www.marbef.org - Norro A.
  7. http://www.marbef.org - Decleer M.
  8. http://www.marbef.org - Nuyttens F.
  9. http://www.marbef.org - Legezynska J.
  10. Karleskint G. 1998. Introduction to marine biology. Harcourt Brace College Publishers. p.378
  11. Pinet P.R. 1998.Invitation to Oceanography. Jones and Barlett Publishers. p. 508
  12. http://www.un.org/Depts/los/convention_agreements/texts/unclos/closindx.htm


The main author of this article is TÖPKE, Katrien
Please note that others may also have edited the contents of this article.