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Life in Freshwater

Primary Production in Flowing Water

Unlike still waters primary production can be remarkably low in rivers. Ponds may have a mass of vegetation growing down through the swamp into quite deep water but by the very nature of flowing water vegetation may not be able to get a grip, particularly where the water flows so fast that the substrate is rocky. In addition, phytoplankton that can form extensive blooms in static water would be carried away by the current and taken down stream. In lowland areas the speed may slow but the water turbidity may be high with material that the moving water has brought up from the bottom, reducing light.

However, there will be diatoms here, called potamoplankton. These drift with slowish water and can replicate quickly.

The macrophytes are the large plants capable of photosynthesising and fixing the carbon and energy into the ecosystem. Rivers show scant examples of these until the flow diminishes sufficiently to allow some sediment to collect between the rocks and stones. Young plants can then get their roots established.

Upland River Middle Reaches
Example of a river when the speed slows down and macrophytes can finally get rooted in the river rather than just being able to grow at the side. Much of this will be water crowfoot species.

A calcareous variety of water crowfoot grows very well in chalk rivers. These rivers tend to be the exception: clear slow flowing water allows for good production with a range of species able to grow in the high nutrient water.

Single celled algae will grow well in streams and rivers, attaching to the stones and rocks of the river bed. The tiny nature of them may prevent their removal by the current. They do not contribute a high level of production like ponds but are still important. This growth is called the periphyton. The diagram below is based on Steinmand (1992, from Oecologia vol. 91) after Gregory (1980, PhD Oregon State University) and is a good summary of the various growth forms on stones.

The three zones of Periphyton on stones
The three zones of Periphyton on stones. Zones relate to the ability of the animals to consume the material

Riparian vegetation is the name given to the terrestrial plants at the edge of rivers. These plants have two important influences on the communities living in the water.

1. It may shade the river so that light does not reach the producers in the water and so primary production is reduced, sometimes severely.

Riparian Vegetation
Example of riparian vegetation that will reduce the light so that primary production will be very low.

2. The riparian vegetation contributes a high level of organic matter. In the photo above it is obvious that when the leaves are shed from the trees they will end up in the water.

Rivers and streams passing through woodland will therefore have a much greater emphasis on the invertebrates that are able to cope with large particles of organic matter - the shredders (see Feeding). It is an environment very much based on detritus and decay. Away from dominant riparian vegetation the emphasis will change to grazers and scrapers removing periphyton from stones and rocks.

Other feeding groups will be encouraged by the high level of organic matter - the collectors of drifting material like the gatherers (e.g. net spinning caseless caddisfly larvae) and filterers (e.g. pearl mussel, see below).

Pearl Mussel
This is a freshwater pearl mussel. They are not a common species as they have been hunted for their tiny pearls. Living in fast flowing water they can be found in upland areas, especially Scotland and Ireland. One of the best places to see them are just down stream of a wooded riparian vegetation where they wedge themselves under large rocks within the fast sections of the flow. They draw water in through one of the siphons, through the gills which remove oxygen and detritus and then pass the filtered water out with waste matter through the second one.

A sudden reduction in riparian vegetation, for example the felling of trees near the river, will have a marked affect on life in the river especially if the clearance is to make way for agricultural land. The loss of tree roots and the production of crops will increase the likelihood of high nutrient run-off that will pollute the water. This will encourage plants to grow and the entire community structure will change.

The origins of detritus can be important. Most plant detritus is low quality for invertebrates. For example the carbon is present in cellulose and this is indigestible by animals. Hyphomycete fungi are central to the breakdown as they can digest cellulose. So the quality of the detritus depends on the level of fungal activity and input of material by plants that have low lignin levels. Alder is a tree often found along the bank that has low levels and is ideal detrital material. Beech, however, is high in lignin and so will be slow to degrade. A further important factor will be the invertebrates capable of shredding the material to increase the surface area that can be colonised by the fungi.

See also Primary Production in Ponds

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