Marine Communities


Marine communities are collections of organisms within an area of the ocean that interact with one another more than with other such collections. These organisms may span from microbes to seaweeds to large animals, but their composition changes drastically with habitat. Intertidal environments along the shoreline support low diversity but often high abundances of organisms adapted to highly variable environmental conditions. In contrast, the deep ocean, which is devoid of sunlight and where temperature and salt content vary little, supports low abundances of a high diversity of life. Sediments cover much of the seabed, creating the largest habitat on Earth in area, whereas the thousands of metres of water above the bottom create the largest habitat by volume. Coral reefs, which occur only in shallow tropical waters, support the most species dense habitat in the ocean and possibly on the planet. Highly productive transition zones between land and sea such as salt marshes, mangroves and seagrasses often provide nursery areas for adjacent coastal species. Nonetheless, many coastal habitats face multiple pressures from human activities and already show rapid change.

Key Concepts

  • Marine communities differ among habitats spanning from low‐diversity intertidal communities adapted to highly variable environments to species‐rich deep‐sea communities that live in far less dynamic environments.

  • Intertidal communities are the best‐studied marine community because of their ease of access and relative simplicity.

  • Estuarine habitats, where freshwater and seawater mix, often support high productivity and complex habitats such as mangroves and salt marshes that are important nurseries for a community of juvenile fishes and invertebrates.

  • Coral reefs are highly diverse and productive habitats but human activities threaten reef communities in many parts of the world where coral reefs are in sharp decline.

  • Scientists have sampled only a tiny fraction of the deep ocean and the potential for new discoveries of species and processes remains very high.

Keywords: benthos; plankton; pelagic; neritic; coastal; deep sea; diversity

Figure 1. Schematic representation of the ocean floor where scales are greatly exaggerated and not drawn to proportion (particularly the horizontal). See text for explanation of terms.
Figure 2. Some of the organisms commonly found in marine systems. (a) and (b) are planktonic, whereas (c–f) are benthic. (a) is a copepod, a type of crustacean that is often very abundant in surface waters; (b) is a small jellyfish, which can also be very numerous in surface waters; (c) is a cumacean; (d) is a polychaete; (e) is a sipunculid and (f) is a nematode. All of these benthic taxa live in bottom sediments. For scale, most of these organisms are several millimetres in length. Photographs by Patricia Ramey.
Figure 3. Global distribution of corals, mangroves and salt marshes superimposed over map of seafloor bathymetry. Red areas are continental shelf, above which are found neritic communities, whereas blue areas are open ocean where oceanic communities occur. Green areas show plate spreading regions, along which vents and seeps occur. Bathymetry image from Walter Smith at National Oceanic and Atmospheric Administration, Boulder, Colorado.


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Snelgrove, Paul(Jan 2015) Marine Communities. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0003175.pub2]