Macrobenthos

What are benthic macrofauna?

Benthic animals are those associated with the bottom of seas, rivers, lakes, etc. Epibenthos lives on the surface and infauna buried within the sediment. Macrofauna is larger and meiofauna smaller. The benthos is divided into three size groups. Microbenthos are the single celled animals, macrobenthos are animals large enough to be retained on a screen with mesh size of 1 or 0.5 mm and meiobenthos, an ill defined group of a size between these two. Worms, shells, shrimps and other crustaceans are the most common animals in the macrobenthos.

The macrobenthos is an important component of the estuarine environment.A large proportion of the estuarine habitat's biodiversity is found in the benthic community. Many of the worms, shrimps, snails and bivalves are important food sources for fish and birds. Without the benthos these animals can not survive. For these reasons alone the benthos deserves to be studied. However, there are other advantages for studying the benthos over the fish, bird or plankton communities.

Amphipods are one of the most common animals in estuarine macrobenthos.

A large proportion of the estuarine habitat's biodiversity is found in the benthic community. Many of the worms, shrimps, snails and bivalves are important food sources for fish and birds. Without the benthos these animals can not survive. For these reasons alone the benthos deserves to be studied. However, there are other advantages for studying the benthos over the fish, bird or plankton communities.

Unlike fish and plankton which can move up and down in the water column the benthos lives in what is essentially a two dimensional environment. This makes sampling a lot easier. Also aiding sampling is the fact that many of the benthic animals have restricted mobility. Concentrations of fish and plankton will form and disperse in response to tides and weather. Because of their reduced mobility benthic communities will not change greatly with the tide or weather. Also, where there is intermittent pollution or the concentration changes with the tides, fish and plankton will rapidly recolonise an area. Many benthic animals can only recolonise an area by larvae settling thus can not recolonise till the next breeding season. Also many of the mobile animals will only move slowly into the area. Thus short term pollution events will be detectable in the benthic community for a considerable time. The macrobenthos also live in an environment where concentration of pollutants is likely to occur.

"Foam" on estuarine waters caused by cooling water discharge from a power station.

Chemical changes associated with the change from freshwater to saltwater result in the flocculation of dissolved materials that have been transported in the water down the river into the estuary. Because the flow of water in estuaries is slower than in the river the flocculant material and suspended particles can settle out of the water into the sediments. These processes allow pollutants to reach greater concentrations in the sediments than in the water. Because of their close association with the sediment benthic organisms will show a response to these pollutants before the animals in the water column. Thus the benthic community may be the first component of the estuarine fauna to show weakening environmental health.