Estimating the likelihood and impact of 'Blackwater' events after a flooding - M/BUS/35
MDFRC Technical Report
1 of 2 reports associated with project.
Background: Blackwater events are a natural part of the ecology of lowland river systems. During a flood, carbon compounds are leached from leaf litter laying on the floodplain in much the same way that tea is leached from tea leaves. The amount of carbon leached will depend on a number of factors such as the type and age of leaf litter, the amount of litter that has accumulated on the floodplain and whether or not the litter has been flooded before. If floods occur more frequently, then less carbon is leached from the litter. For a typical flood in a River Red Gum forest about 350 kg of carbon can leach from each hectare of floodplain. The carbon and nutrients leached from litter on the floodplain are believed to play an important role in the functioning of river systems. Overseas studies have shown that, in some instances, much of the production that occurs in the river channel is based on carbon that has come from the river's floodplain. Therefore, it is important that there is the opportunity for flooding to occur to allow the movement of carbon from the floodplain to the river channel. However, blackwater events can markedly change water quality. Microbes can immediately use about one-third of the carbon leached from the leaf litter. As the microorganisms consume the dissolved carbon they use up oxygen in the water - often at a rate faster than the oxygen can be replenished. Therefore, blackwater plumes often have very low levels of dissolved oxygen. The lack of dissolved oxygen can cause the death of fish and other aquatic animals in the plume. Native fish and other large aquatic organisms require at least 2 mg/L of oxygen in the water to survive, but may begin to suffer at levels below 4-5 mg O2/L. Once all the oxygen has been used, the dissolved carbon in the blackwater can be broken down by microorganisms that use nitrogen, iron and sulfur compounds instead of oxygen for respiration. Some of the by-products of this anaerobic respiration (e.g. rotten egg gas - hydrogen sulfide) can also be toxic to aquatic organisms. Not all of the carbon leached from the litter can be used immediately by microorganisms. For example, about 70% of the carbon leached from River Red Gum leaf litter needs to be broken down before it can be used by aquatic microbes. Sunlight plays an important role in the breakdown of these more recalcitrant carbon fractions. The energy from sunlight, particularly ultraviolet light, breaks down the dissolved carbon compounds into smaller, more easily utilised compounds. If blackwater from Red Gum litter is exposed to sunlight for long enough, almost all of the carbon will be converted to CO2 - even in the absence of bacteria. The degradation of blackwater by sunlight also uses up oxygen, further depleting the dissolved oxygen in the blackwater plume. Because blackwater adsorbs sunlight, it heats up more quickly than less coloured water which can lead to heat shock in susceptible species. Also, because the sunlight light can not penetrate as far into blackwater, photosynthesis by algae and submerged water plants can be greatly reduced. The chemicals leached from litter can also affect the pH of the floodwater. The exact effect will depend on the type of litter leached. If the plant material is high in free hydroxyl groups, the pH will become more alkaline; if the plant material contains organic acids, the water will become more acidic. Leachate from River Red Gum litter often is acidic. Therefore, it is not uncommon for blackwater from flooded Red Gum forests to have a pH of 5 or even lower. Some of the compounds leached from litter can be toxic to aquatic organisms. Like pH, the toxicity will depend on the source of the leachate. However, as blackwater events occur naturally in our river systems, you would expect native species to be better adapted to cope with these events than introduced species.