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Life in Freshwater

Community Structure

Usually we look at communities as a sequence of energy (trophic) levels, i.e.

Producers - the Primary Production of energy, usually where green plants fix carbon using sunlight

Herbivores - grazing of the plants by animals

Carnivores - initial predation of the grazers which may in turn be predated upon by higher/larger carnivores

In freshwater it is not quite so straightforward. Check out Primary Production in both static and flowing water. To start with there may be large plants (the macrophytes) and microscopic ones (microphytes - see periphyton). The latter can be growing over the macrophyte and many grazers feed only on the microphyes, scraping them off the surface of the large plants. Green plants can often be lacking and the microscopic plants dominant. The imput of organic matter, either as CPOM (coarse particulate OM) or FPOM (fine particulate OM) is then one of the most important food sources. You may want to look at the diagram in the River Continuum Concept and see where these inputs occur but with green plants, periphyton, large pieces of organic matter from leaves, microbes like bacteria and fungi decomposing the organic matter and increasingly finer particulate matter present the idea of a straightforward grazing system is lost.

The main animal feeding roles present in the community will be from the following:


Food Resource

Feeding Mechanism



Periphyton. Mainly diatoms


Scraping, rasping and browsing adaptations


Some families of mayfly, and caddis; some diptera, beetles and moth

Hydroptilid caddis larvae


Non-woody CPOM (leaves) with fungi

Chewing and mining

Some families of caddis, stonefly and crustacea; some diptera & snails

Shredder Gouger

Woody CPOM with fungi; mainly superficial layers eaten

Chewing and mining

Some Diptera, beetles and caddis



FPOM with bacteria

Collect material from surface and may burrow

Many mayflies and the Chironomids


FPOM with bacteria; periphyton shed from stones into water

Collect particles with hairs, nets, secretions or special adaptations

Net-spinning caddisfly larvae, black fly larva and other Diptera; some mayflies



Biting and piercing

Dragonfly, damselfly, some stonefly, Diptera and beetles


A community in flowing water will therefore be a mix of these feeding roles. But to say that there is structure suggests that there is a particular composition present and that similar environments will have a similar structure. It may be better to say that this community structure is dynamic and is very dependent upon the abiotic and biotic factors at that time. With drift and dispersal taking place the chances are the individuals are on the move and as fast as they move on replacements occur. The community is very dependent on what is happening in surrounding ecosystems.

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