Continental shelf habitat
This article describes the principal environmental gradients, biological communities and ecological connections that distinguish continental-shelf habitats, while referring to separate articles for the underlying physical, sedimentary and biogeochemical processes and for particular shelf ecosystems and human uses.
Contents
Introduction
The continental shelf is the gently sloping submerged margin of a continent, extending from the coast to the shelf break, where the seabed descends more steeply along the continental slope. At the ocean side it is terminated by a pronounced change in bottom slope, called shelf break. The average slope of the continental shelf is generally very gentle, less than 1 degree. The average depth is about 150 m and it has an average width of 70 km. Individual shelves have widely varying widths ranging from more than 1000 km in the Arctic Ocean to a few kilometers at some places along the Pacific coast of North and South America. The broadest shelves are found where continental margins are stable and/or where large rivers discharge. Examples include the North Sea, Yellow Sea, Sunda shelf, Bering Sea and Arctic shelf; much narrower shelves occur along tectonically active continental margins.
The shallow marine waters overlying the continental shelf are commonly called shelf seas or neritic waters. Their ecological boundaries do not always coincide exactly with the geophysical shelf break and rarely with jurisdictional boundaries. Beyond the shelf break is the much steeper continental slope. At the base of this 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.
The continental shelf is one of the most productive parts of the ocean. Continental shelves occupy only a small fraction of the global ocean but contribute disproportionately to marine primary production and fisheries (Gattuso et al., 2001[2]). Their productivity varies strongly among regions and seasons and depends on nutrient supply, light availability, vertical mixing, upwelling, stratification and recycling at the seabed.
Hydro-sedimentary shelf processes
Waves, tides, wind-driven currents and larger-scale circulation continually redistribute water, sediment, nutrients and organisms across continental shelves. Their relative importance differs greatly among shelves. On microtidal shelves, wave and wind forcing can dominate well beyond 20 m during storms. On macrotidal shelves, tidal currents can dominate even in shallow water. They interact with seabed topography and wind-driven circulations. Internal tides and bottom currents can be important near the shelf break. Storm waves commonly disturb shallow and intermediate-depth seabeds, whereas tidal and wind-driven currents can shape both shallow and deeper shelf environments. Near the shelf break, fronts, internal tides and exchanges with the open ocean may also be important. More detailed descriptions are given in the articles Shelf sea exchange with the ocean, Ocean and shelf tides, Tidal motion in shelf seas, Sand ridges in shelf seas.
The wave-dominated processes in the nearshore zone, in particular on the shoreface, play an important role for accretion or erosion of the coast (see e.g. Shoreface profile). In this zone, the seabed usually consists of coarser sediment (sand or gravel) because fine sediment is stirred up by wave action and transported to deeper water. Exceptions are the mud coasts where abundant sediment supply mainly consists of very fine sediment. However, a transition from coarser to finer bottom sediments from the coast to deeper waters is the more usual situation, as described in Characteristics of sedimentary shores and Coastal and marine sediments. In deeper water, shelf seabeds may be characterized by spatial mosaics of rock, gravel, sand and mud. Their distribution largely reflects inherited geological deposits. Some twenty thousand years ago, large parts of the continental shelves were land, that since has been subject to progressive marine transgression. Modern hydrodynamic sorting has created additional bedforms and contributed to further habitat heterogeneity.
Biota
The neritic waters contain a rich community of organisms. Nutrients reach shelf waters from rivers, the atmosphere and the adjacent ocean and are repeatedly regenerated in the water column and seabed. Upwelling and tidal or wind-driven mixing can supply nutrients from deeper water. The relative importance of these sources differs among shelf regions. Because shelf waters are comparatively shallow, flocs (aggregates of sedimentary material that contain organic matter produced in the water column) can rapidly reach the seabed. Benthic organisms and microorganisms consume and transform the organic material, while regenerated nutrients return to the water column and support further primary production. This close benthic–pelagic coupling and rapid nutrient recycling contribute to the high productivity of many continental shelves (see the article Nutrient conversion in the marine environment). The high primary productivity of many continental shelves supports abundant organisms at higher trophic levels, including commercially important fish stocks.
The organic material deposited on the seabed that escapes mineralization is buried in the seabed, especially in depositional areas where it is stored for long periods. Continental shelves therefore constitute an important marine carbon store and contribute to long-term carbon sequestration[3][4], see Blue carbon sequestration.
The types and characteristics of the sediments strongly influence the abundance and types of organisms that can live on the continental shelf. Two types of benthic communities can be distinguished, related to the characteristics of the substrate: the soft-bottom communities and the hard-bottom communities
- Hard-bottom communities occur on exposed bedrock, boulders, stones, gravel and biogenic structures. Stable hard substrates are generally unsuitable for burrowing and interstitial organisms but provide attachment surfaces for sessile organisms. Mobile gravel and coarse sand support different communities adapted to frequent seabed disturbance. Food carried along by the flow makes it a suitable area for sedentary or sessile filter-feeders or suspension-feeders. Sponges, anemones and colonial cnidarians (Hydrozoa) can attach to coarse sediments, boulders and rocky outcrops. Due to the uneven surfaces of the substrate, a large number of niches are created that allow the growth of a rich fauna. In sufficiently shallow and clear water, hard substrate can also support extensive macroalgal vegetation, including kelp forests.
 Sea anemone Actinia equina[5]. |
 Sponge Polymastia boletiformis[6]. |
 Hydrozoan Tubularia indivisa[7]. |
- Soft-bottom communities are generally best developed in areas with limited hydrodynamic disturbance, on unconsolidated sediments ranging from mud to sand and mixed gravelly deposits. Their communities vary with grain size, organic content, oxygen conditions and the frequency of seabed disturbance. This is a suitable habitat for burrowing organisms such as polychaete worms, amphipods and bivalves. Soft-bottom communities include both deposit feeders and suspension feeders. Deposit feeders are often prominent in organically enriched muddy sediments, whereas suspension feeders can be abundant where water movement supplies suitable suspended food.
 Tubeworm Lanice conchilega [8]. |
 Bivalves [9]. |
 Amphipod Onisimus edwardsi[10] |
Sediments are usually not homogeneously distributed over the continental shelf. This causes an unequal distribution of benthic organisms called patchiness.
The main food source 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 organisms of the shelf-sea water column include plankton and actively swimming animals. Plankton form the base of most shelf-sea food webs. They obtain energy through photosynthesis and need nutrients and sunlight to produce oxygen and organic matter; this so-called primary production also depends on temperature. In mid- and high-latitude zones, primary production is strongly influenced by seasonal variations in temperature and solar irradiance. Reduced light penetration can be limiting in turbid river plumes and shallow waters where waves and currents keep fine sediment in suspension, see the article Which resource limits coastal phytoplankton growth/ abundance: underwater light or nutrients? Seasonal changes in water temperature, salinity and nutrient input create a succession of phytoplankton species in the temperate and polar seas. Plankton is unevenly distributed and often occurs in patches. Further details on primary production and biota can be found in the articles Marine Plankton and Plankton bloom.
Human uses
Many continental shelves are heavily exploited for various human uses, such as fisheries, mariculture, aggregate mining, shipping and wind farms. These uses often have a strong impact on the ecosystem, by modifying the natural seabed habitat and by other disturbances such as pollution, noise and spreading of non-native species. This is described in the articles Effects of fisheries on marine biodiversity, Coastal pollution and impacts, Threats to the coastal zone and Non-native species invasions.
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 natural resources at its part of the continental shelf. These natural resources are minerals and other non-living resources of the seabed and subsoil. Sedentary species are also included in the natural resources. The rights of the State are not applicable to the water and the air above it. [11]
The EEZ extends up to 200 nautical miles from the baselines and often overlaps a large part of the legal continental shelf. These continental-shelf rights do not alter the legal status of the superjacent waters or the airspace above them.
A more complete overview is given in Legislation for the sea.
Related articles
- Shelf sea exchange with the ocean
- Coriolis and tidal motion in shelf seas
- Ocean and shelf tides
- Coastal and marine sediments
- Nutrient conversion in the marine environment
- Marine Plankton
- Plankton bloom
- Deep sea habitat
References
- ↑ http://en.wikipedia.org/wiki/Continental_shelf
- ↑ Gattuso, J.-P., Frankignoulle, M. and Wollast, R. 1998. Carbon and carbonate metabolism in coastal aquatic ecosystems. Annual Review of Ecology and Systematics 29: 405–434
- ↑ Atwood, T. B., Witt, A., Mayorga, J., Hammill, E. and Sala, E. 2020. Global Patterns in Marine Sediment Carbon Stocks. Front. Mar. Sci. 7, 165
- ↑ Chatting, M., Diesing, M., Hunter, W. R., Grey, A., Kelleher, B. P. and Coughlan, M. 2025. Improving marine sediment carbon stock estimates: the role of dry bulk density and predictor adjustments. Biogeosciences 22: 5975–5990
- ↑ http://www.marbef.org - Decleer M.
- ↑ http://www.marbef.org - Emblow C.S.
- ↑ http://www.marbef.org - Norro A.
- ↑ http://www.marbef.org - Decleer M.
- ↑ http://www.marbef.org - Nuyttens F.
- ↑ http://www.marbef.org – Legezynska.
- ↑ http://www.un.org/Depts/los/convention_agreements/texts/unclos/closindx.htm
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