Background
Burning of Riparian Vegetation Affects Aquatic Habitat
Unburnt riparian vegetation provides shade, filters runoff, is a source of insects and small creatures as fish food, and contributes plant organic matter to the stream. Its dappling effect on the water and its leaves and roots in contact with the water provide cover where biota can hide from predators. It is a landing for birds that feed on stream biota, and for insects maturing from aquatic life stages.
After riparian vegetation has been burnt, streams are more open to light and offer less cover against predators, and less edgewater habitat (i.e. at the edges of the stream).
The extra light on the water stimulates algal growth (and plant growth) in-stream, not only upsetting the plant-insect-fish foodchain if the resident biota are not adapted to eat algae, but also possibly reducing drinking-water quality, and possibly boosting aquatic weed growth. The extra nutrients generally washing into the stream in runoff after fires would exacerbate this effect.
The exposed stream is not protected from heating and cooling, and so often has a larger daily temperature range, with temperature increases of up to 10°C, though this depends on stream size and flow, influx of groundwater, altitude, season, etc.
At higher temperatures, water releases dissolved oxygen, reducing the amount available in the water. As well, dissolved oxygen is used up by the decomposition of organic matter supplied by dead leaves immediately after the fire, and algal and other plant growth in the following months. In some cases low dissolved oxygen levels may become limiting to fish and other aquatic animals.
If temperatures are altered for a long time, the development of insect larvae and fish fry can be affected.
Loss of riparian habitat for terrestrial biota reduces that source of food for in-stream biota.
Loss of the riparian zone also reduces the filtering effect of the riparian vegetation, so more sediment and organic debris are liable to wash into the water after moderate to heavy rain, potentially with great effect on water quality.
