The main abiotic factors in freshwater are:
- water temperature
- water velocity
- water pH
- oxygen concentration
Temperature will affect the ability of an organism to carry out metabolism; the warmer the conditions the better able enzymes can operate. You might expect lowland water to be warmer than that in upland. However, water does provide a stable temperature over short periods of time
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. The reason that organisms are affected by temperature is largely because they are cold blooded or poikilothermic. The external environment will determine their internal temperature and therefore metabolic activity. This varies enormously between species, especially their threshold of survival ability. For example, some insect eggs when laid in autumn will not hatch until the spring if the temperature is too low. Also 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. \(25 ^\circ C\) is the upper limit for many midge larvae although black fly can survive substantially above this.
Moving water has speed of flow as a major additional consideration. 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 invertebrates this may be flattened to make as much use as possible of the boundary layer. The behaviour of the different species is sometimes influenced by current, for example, the ability of a net to be spun across stones by caseless caddisfly larvae in is inhibited by fast flowing water.
pH can vary enormously between ponds and lakes, due to bedrock, topography and even the species of plant present. For example on heathlands where ponds form, there are likely to be Sphagnum or bog mosses present that increase acidity. Some animals are specific to calcium-rich, alkaline water like the crayfish. In addition, snails need calcium for their shells and so will be limited to waters rich in this mineral. The most significant aspect to pH is the amount of carbonic acid present. Carbon dioxide dissolves in water to produce carbonic acid so it is a measurement of the level of \(CO_2\) available for photosynthesis. The pH is often a limiting factor of enzyme-controlled reactions.
If water is unpolluted and flowing the saturation by oxygen should be just about at its maximum. Therefore, 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.
The dissolved minerals present will depend on the geology of the land. In uplands of granite and other igneous rock, nutrients may be quite low. Calcium ions are essential for maximising population densities, as it is needed for skeletal tissue, cell walls and shells in molluscs. This is the reason that chalk rivers are more diverse than upland ones. Nitrogen enters in the form of ammonia or nitrates. Often due to run-off from agricultural land and will encourage eutrophication. Phosphate occurs naturally in small amounts and combines with iron to form ferric phosphate, precipitating to the benthic region. However, phosphate is now a significant pollutant of water entering from farmland and will result in eutrophication. Organic matter is an important source of nutrients.
For much of freshwater the primary source of energy is light. However, the levels of light under water will vary according to many factors; the time of year, time of day, cloud cover. The two most important factors affecting the amount of light in a freshwater environment are the cleanliness and the depth of the water.
Any plant living within the water, e.g. water crowfoot may be just under the surface but the surface itself will reflect a considerable amount of light away. By being in water there is less light for photosynthesis. As if this is not enough, the deeper you go the intensity continues to diminish and the wavelengths of light able to penetrate change, e.g. red light passes through the least, green the most.
The substrate consist of both inorganic matter (silt, sand, pebbles and rock) and organic matter (coarse or fine particulate organic matter). Both organic and inorganic substrates are important for colonisation by plant species and may change the structure of the animal community. There will be a correlation between slow velocity and high organic matter in the substrate and between 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.