Aquatic food webs

~ Organic matter from aquatic and terrestrial sources provides the carbon energy that 'drives' aquatic food webs. Most streams and rivers are heterotrophic — that is, more carbon is consumed (e.g. by animals and bacteria) than is produced within the system by aquatic plants. However, despite the presence of vast amounts of carbon in streams and rivers, only a small proportion of the total is truly available for consumption by aquatic animals. ~ A large proportion of the total carbon pool in many streams and rivers is in the form of wood, which provides an important substrate for algal colonisation, especially in lowland rivers. ~ In temperate forest streams, coarse-particulate organic matter, fine-particulate organic matter and dissolved organic matter derived largely from the riparian zone are important sources of carbon for aquatic food webs. ~ Food webs in tropical, subtropical and arid zone streams show a greater dependence on algal carbon, as do those in most lowland rivers. Macrophytes in larger rivers and wetlands appear to contribute very little directly to aquatic food webs, though they are clearly an important food source for some species of water-birds. ~ Riparian fruits and arthropods may also be an important food source for fish and other vertebrates in forest streams. ~ Riparian vegetation regulates in-stream primary production in small streams and supplies energy and nutrients; consequently, its removal can radically change the quality and quantity of carbon in food webs and the function of aquatic ecosystems.