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Life in Freshwater

Communities Down the River

In both the sections on Stages in River and the River Continuum Concept we see that there is a difference as you move from river source to estuary. There can be many factors (see abiotic factors) that influence the change. Current is particularly significant as it is not constant on the way down to the sea. Invariably there will be a lowland stretch where the water can be sluggish and may even become pond like.

The source (low order numbers) will have less volume and that does influence diversity. As we have seen elsewhere food resources have to be the main determining factor as to what animals exist in a particular section of the river. If the river passes through an extensive region of forest so that the banks have trees depositing large quantities of leaves into the water then lower down the communities will comprise of the animals that will shred this organic matter into finer particles which in turn encourages the filterers and suspension feeders.

So distinct is the change from the source down a river that early classification of flowing water was made according to the fish species present. This was put forward by Dr Kathleen Carpenter in 1928 (see also Stream Order):

1. The Headstream: small, often torrential and without fish

2. The Troutbeck: larger and more constant, often solid rock or boulders. Trout will be the only permanent fish of open water but the miller's thumb will be present in the sheltered areas.

3. The Minnow Reach: still swift flowing but with patches of silt where water crowfoot grows. This is the zone where minnows are often found in the UK.

4. The Lowland Reach: usually slow flowing, meandering and muddy with vegetation. Coarse fish thrive. This is also known as the bream zone.

Prof Tansley was a botanist and in the 1930s revised the classification based more on vegetation.

Zone 1: Mosses and liverworts dominant with minimum macrophyte (This was an amalgam of 1 and 2 of the Carpenter scale)

Zone 2: Water Crowfoot dominant

Zone 3: the moderate current deposits coarse materials and so the range of macrophyte increases: still crowfoot but bur-reeds and Potamogetons grow as well as canadian pond-weed

Zones 4 and 5: water crowfoot has disappeared and a lengthy list of macrophytes are found in the sluggish water.

No really clear system has been found for classification but these older ideas help to emphasise the changes that are likely to be seen longitudinally on an entire river.

None of the above includes the point where the freshwater becomes affected by the presence of salt. Human activities have often placed a barrier to the sea in the form of weirs and sluice gates so that the rivers passes through but the seawater on high spring tides cannot get far up stream. However, there will be a mix in the communities from fresh to brackish water. Some saltwater fish like the flatfish flounder can live in virtually pure water whilst the freshwater fish, the three spined stickleback can live (with adaptations to the scales) in seawater. Freshwater floats on top of saltwater but eventually they will mix in the estuary and when the tide drops completely then only freshwater will flow. In these situations the freshwater community must be able to tolerate the salt or avoid it altogether and will become most active at lowtide. Below are a few examples of estuarine species:

Limnephilus affinis is the only caddis known in the UK to be able to complete its full life history in concentrations of 17 parts per thousand of salt. It actually regulates the salt in the blood.

Several chironomids live in estuaries as well as some mosquitoes. Diving beetles form a significant group that can live in brackish water and will survive in pure salinities for short times. Colymbetes fuscus and Hydrophilus piceus are two such species. Sigara stagnalis is a lesser water boatman bug that is common in pools on salt marshes. Pond skaters will be unaffected by the salt and can be found in the quieter backwaters of estuaries.

 


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