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Life in Freshwater

Changes across the Stream and River Channel

In any one section of the stream or river there will be any number of micro-habitats and micro-climates.

In the photo below both riffles and deeper water is visible. There are stones on the left which will have spaces between them where animals can shelter temporarily, e.g. spin webs to trap material. On the right there are larger stones in the stream that will be more stable and provide a greater area for periphyton to attach to. With the erosion of the right-hand bank sediment will be generate that will be deposited in the quieter sections.

Stream under cutting the bank
A section of a channel showing a range of micro-habitats.

This range of micro-habitats is largely due to the current velocity. Plant material from the bank will add to the organic content.

Not surprisingly, one would expect to see a relationship between the invertebrates found in these different micro-habitats and the speed of flow and substrate. In the pebbles one would expect to find the net-spinning caddis larvae. As the water flows between the spaces of the stones so fine particulate organic matter (FPOM) will become entrapped and consumed by the larvae. In the areas of faster flow flattened mayfly larvae will be lying flat to the stones where the current keeps them on the stone rather than wash them away to enter the invertebrate drift. At the bottom of the deep pools the water will move more slowly but will still be well oxygenated. Fish, like the bull-head or miller's thumb will be resting here. Grazing on the periphyton on the underside of the larger stones might be stone-cased caddis larvae and other mayfly larvae. Predators in the form of large stonefly larvae are likely to be hunting amongst the stones.

Current and substrate will be major factors in determining the distribution but it will be influenced by what is happening up-stream (e.g. the amount of riparian vegetation). Other things to consider could be the rock type that will have particular influence on the molluscs. The tiny river limpet can exist when calcium is relatively low and would be a grazer on the larger stones.

As well as measuring density and distribution of certain species in the cross section of the river you could try looking at the diversity. This can be due to the amount of area for a particular habitat. But in all cases of understanding community structure look at the strength and frequency of biotic interactions - interspecific competition. There is plenty of evidence for resource and habitat partitions in flowing water. Competition is a common feature at certain seasons when food may be in short supply or when conditions are favourable and densities reach high levels. Invertebrate drift and dispersal will greatly change the density of the population, far more so than say predation could. Predation will be greatest where shelter is least, e.g. on a riffle rather than a deep pool. In short, density, distribution and diversity can all vary across, as well as, along a river. They are affected by the size or area of the microhabitats and also the strength and frequency of biotic interactions.

The number of species that you will find when sampling these communities will depend very much on sampling effort. This is one environment where it has been shown that the more you look the more you find. If you double the samples taken it is likely that you will increase the diversity index for that community.



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