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Life in Freshwater

Abiotic Factors affecting streams and rivers

Although many of the factors are shared with all freshwater habitats moving water has the speed of flow as a major addition.

Abiotic Factors in Lotic Habitats

The Current

The speed of water flow will determine the substrate at the bottom of the stream or river. Fast flow will remove all but the heaviest material and send this down stream. If the velocity is extreme then only bedrock will exist. A river in spate occurs at the time of high rainfall. The amount of water will have increased but the channel remains the same: hence it flows faster. As the channel widens there may be sections that have a slower speed and here deposition of material will occur. By the time a river reaches the lowland areas the velocity will have slackened so much that it can become pond-like and here silt and sand are deposited on the bottom allowing rooted plants to become established.

River showing differing speeds of flow
River showing differing speeds of flow

When liquids flow they can do so in three different ways:

Laminar Flow - regular and smooth with little mixing

Turbulent Flow - irregular flow with maximum mixing

Transitional Flow - somewhere between the two

 

Laminar flow is almost impossible to find as any river or stream will have at least some mixing. Even as water flows over smooth sand there will be some disturbance. Water in contact with a non-eroding surface has zero velocity and so across an area of river to the sides or bottom there will be a gradient of current speed such that as it almost touches the substrate the speed is negligible. This is called the boundary effect or boundary layer. If an organism is very small, like a singled celled alga, it will have very little drag upon it being within the boundary layer.

It is generally believed that the current velocity will determine the communities present. Certainly plants will not be able to take root until the substrate is soft sediment and even then there is always a possibility of being uprooted later. In animals the shape of the body profile tends to fit with the current that they will be able to live with. A low, flattened shape, i.e. a streamlined body, is better suited to fast flow. In the very small this may be flattened to make as much use as possible of the boundary layer. Much is down to hydrodynamic forces but the life cycles of the animals and exactly where they are found within the river needs to be looked at. The behaviour of the different species need a close analysis, for example the ability of a net to be spun across stones by caseless caddisfly larvae in similar current speeds.

Substrate

As said above the size of the substrate is determined by the current. The substrate is made up of both inorganic matter (silt, sand, pebbles and rock) and organic matter (coarse or fine particulate organic matter). When sand is deposited by a sluggish current there will be particulate organic material as well. Organic matter however can exist from a dead leaf or a complete, dead organism like a tree fallen into the river through to a dead sheep. All will be important substrates suitable for colonization and may change the structure of the animal community. There will also be a correlation of sluggish rivers with high organic matter and low oxygen. This is due to low dissolved oxygen through minimum turbulence followed by maximum bacterial action decaying the organic matter and using the oxygen up in respiration. Conversely, fast flowing water has greater turbulence that mixes with air to oxygenate it but little organic matter will be present as it gets swept away. There is therefore negligible bacterial action using up the oxygen.

The effect of substrate on the communities present does not have a straightforward, linear control as do current or temperature. Within a short stretch of a river there can be any amount of different inorganic matter. Also the importance to organism may not be the surface of the stones but the size of the spaces between them. It is also here that the organic material becomes trapped. Even sand grains vary. Under a microscope the surface can be rounded or pitted in different ways. Pits will allow bacteria and algal cells to adhere. Sand is not seen as a particularly good substrate for large invertebrates. Gills can become clogged whilst the substrate itself is unstable. Animals that live in spaces less than 0.5mm, between sand grains, is called meiofauna and can be abundant. The geology of the rock will affect the river, especially if it is alkaline like chalk or limestone. This will release large amounts of calcium suitable particularly for molluscan growth.

With the fact that the substrate is so complex and with many variants it is not surprising that this complexity is found in the fauna living within the river.

Temperature

This will vary not only along the length of the river but also through seasonal and even diurnal periods. The altitude, local climate and the extent of the vegetation at the side of the river will influence temperature. Exceptions do occur and rivers with chalk at the source will have surprisingly constant temperature. It is this constancy that encourage commercial water cress growers in southern England.

Temperature influences metabolism and therefore the ability of cold-blooded creatures to function. This varies enormously between species, especially their threshold of survival ability. For example, Trout will not reproduce below 3 degrees and ideally between 5 and 16 degrees. Carp, by contrast, reproduce best above 20 degrees. Temperature influences growth and development. Some insect eggs when laid in autumn will not hatch until the spring if the temperature is too low. Warmer temperatures increase the rate at which the larvae feed and grow. There is always an upper limit above which all metabolism will slow down. 25 degrees C is the upper limit for many midge larvae although black fly can go substantially above this.

Graph showing the relationship between the hatching of eggs laid by Ephemerella (mayfly)
Graph showing the relationship between the hatching of eggs laid by Ephemerella (mayfly)

Oxygen

 

If water is unpolluted and flowing the saturation by oxygen should be just about at its maximum. As a consequence oxygen would not be a major contributory factor in the distribution of organisms in the river. With a theoretical constant oxygen supply the invertebrates in rivers tend to be respiratory conformers. That is they do not actively maintain respiratory currents over the body as in most pond dwellers (called respiratory regulators). However, depending on substrate and current oxygen does vary. Temperature would change the level of oxygen but as this is more stable than in ponds is less of an issue.

 

 


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