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Ecosystem Processes - Food Webs

food web diagram
Diagram of benthic foodweb click to enlarge. Diagram: David Gillett. Animal symbols courtesy of the Integration and Application Network

Simple food chains are common in the open ocean where, for example, phytoplankton are eaten by krill, which in turn are eaten by whales.  Such simple food chains are rare in estuaries.  This is due, in part, to the abundance of different types of primary producers and to importance of detritus in estuarine food webs.

Many benthic invertebrates, called deposit feeders, feed on specific sizes of particles, rather than on specific types.  As a result, they may ingest a variety of food types, including benthic microalgae, plant fragments, detritus, bacteria and smaller invertebrates, in addition to inorganic sediment.  In addition, larger consumers of estuarine and shallow coastal ecosystems, such as fish and crabs, are typically generalist feeders.  This means that they tend to feed on whatever is most abundant.  Diagrams of estuarine food webs typically have many boxes and arrows showing these complex feeding relationships.

Estuarine organisms may play different roles in the food web at different life history stages. Adult blue crabs eat a diet partially composed of benthic infauna, such as clams and polychaetes, while juvenile crabs are themselves the prey of larger crabs, as well as of demersal fishes.

Habitat degradation by human-induced eutrophication or other types of pollution can affect the structure of benthic food webs.  Microbes often shift in relative abundance, and less desirable forms may become dominant.  Some larger organisms are intolerant to these changes, and thus do not persist in the ecosystem. Others are more tolerant and may increase in abundance, up to a point.  Generally, food webs in degraded habitats are simplified.  Larger bivalves, polychaetes, and crustaceans, which are easily processed by predators, may be replaced by fewer and smaller species, which are less available to predators. Changes in the composition of food webs can also affect nutrient cycling.

fish feeding

For further information about food webs in estuarine and coastal marine habitats, refer to the following:

Mann, K. H. 2000. Ecology of coastal waters, with implications for management.  Blackwell Publishing; Chapter 8 – Estuarine benthic systems.

Valiela, I. 1995. Marine Ecological Processes (2nd Edition). Springer; Chapter 8 – Trophic structure 1: Controls in benthic food webs.




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