Background
Ecological Aspects of Stream Recovery after Bushfire
The effects of fire on streams are seen mainly over short time-frames, reducing as the catchment's burnt vegetation and biodiversity gradually recover. Recovery generally begins in the first weeks after the fires. (Australian native vegetation is largely resilient to fire, but introduced species such as pine trees can be killed outright.) Gentle rain can speed recovery, but heavy rain, particularly in the first weeks after fire can exacerbate the fires' effects.
Short-term effects (see Burning of Riverbank Vegetation Affects Aquatic Habitat for more detail)
- Following burning of riparian vegetation:
- more light reaches the stream, affecting in-stream biota (fish, insects, plants, algae).
- greater temperature range and organic matter in stream water, affecting dissolved oxygen, and development of in-stream biota.
- reduced filtering of runoff, affecting the influx of sediment and organic debris into the stream (with potentially large effects on riverbed structure and water quality).
- stream banks that are unprotected by vegetation and roots are more likely to erode, affecting the influx of sediment and organic debris into the stream (with potentially large effects on riverbed structure and water quality).
- overhanging vegetation no longer offers cover against predators of fish and aquatic insects or larvae, nor supplies terrestrial insects and organic matter to the stream.
Monitoring of these effects could continue until the riparian vegetation recovers.
- Immediately following burning of catchment vegetation:
- ash and burnt organic matter fall into the water, raising nutrient concentrations in water (such as nitrogen compounds, phosphorus) and using-up dissolved oxygen particularly as the material decays (fish kills are possible).
- water is blackened and turbid and scummy from the ash, affecting visibility for fish and possibly clogging gills (crayfish have been observed walking out of the river in these conditions); probably affects any reptiles, frogs and river mammals that have survived the fire.
- fire suppressant chemicals that have entered the stream may cause fish kills or affect nutrient concentrations.
Monitoring of these effects could be complete within a few days or weeks of the fire, depending on inflows and flushing.
- From several days to years after burning of catchment vegetation:
- loss of groundcover in the catchment exposes the soil to wash from heavy rain (if the tree canopies are not burnt) or moderate and heavy rain (if canopies have been destroyed) and erosion is probable, particularly in granitic and sedimentary catchments, depending on slope, rainfall and fire intensity and the depth of soil sterilisation; a large influx of eroded sediment can entirely fill a small stream and it will certainly erase riverbed niches and habitats, fill small pools, etc. The effects will be dependent on continued sources of sediment and other materials and the rates of flushing from the stream.
- sediment and organic matter washed into the stream raise the nutrient concentrations of the water and use up dissolved oxygen as the organic matter decays, affecting fish, insect larvae, river mammals and waterbirds and algal growth and drinking-water quality.
- concentrations of nutrients such as nitrogen and phosphorus may remain higher than pre-fire for several weeks.
- sediment washed into streams that feed reservoirs settles in the reservoirs, affecting storage capacity, and water quality for human uses.
- sediment moving downstream scours algae off river rocks and logs and changes the topography of the riverbed, affecting fish, fish spawning, insect larvae and river mammals.
Monitoring of the various effects of fire on streams could continue for months or years.
- Contamination by fire retardants
Fire suppressants can act on aquatic biota, particularly in small headwater streams, either via ammonium toxicity from the fertilisers (ammonium phosphate or ammonium sulphate) contained in one broad type of suppressant, or via the effects of wetting agents (surfactants) in the other broad type of suppressant used.
Algae, macroinvertebrates, plankton and fish can all be affected to some degree, depending on the type of suppressant, and the dilution factor. There is at least one report of almost complete death of trout after fire-retardants were dropped into a river (in USA) during a major fire. See also www.npwrc.usgs.gov/resource/habitat/fireweb/publicat.htm.
In Australia, soon after two small streams were contaminated by fire-suppressants during a natural wildfire, a team measured more phosphorus in the water in one almost dry contaminated stream than in a nearby uncontaminated stream. The difference disappeared after flushing rains. The macroinvertebrates in the contaminated streams did not appear to be affected, except that there were more species than in the other stream.
Raised nutrient concentrations from fire-retardants would be likely to stimulate algal growth especially in streams whose overhanging vegetation has been burnt.
Longer-term effects
- After severe fires, where dead trees fall into the streams, they have the long-term effect of forming debris dams in small streams, or altering water flow patterns in larger streams, often resulting in changes to the stream channel and improving the in-stream habitat structure.
- Nutrient concentrations can (but may not) remain high for a number of months, possibly as a side-effect of changed nutrient cycling in a severely burnt catchment, or continuing decomposition of woody debris in-stream.
- Where a stream is dammed in a burnt catchment, recovery of the ecology downstream of the dam can be slower than in an undammed stream, because the dam modifies the flow downstream. Comparing two adjacent catchments burnt in January 2003, one with a dammed river and one with no dams, the aquatic insect larvae and crustaceans (macroinvertebrates) were reduced in number of species and number of individuals the following spring (about 8 months post-fire). A further 12 months later (about 20 months post-fire), though, the macroinvertebrates in the undammed river had returned to prefire numbers, whereas the macroinvertebrate populations in the dammed river were still reduced in numbers.
- Sediment movement can take years to recover to pre-fire conditions (see 'Sediment movement'). Sediment flushing after fires is more likely to occur in undammed rivers.
- Other long-term effects include effects on water-yield; see relevant pages on this web site.
