Sharley, D.; Amos, C.; Pettigrove, V.; 

The Victorian Centre for Aquatic Pollution Identification and Management (CAPIM) was engaged by the Corangamite Catchment Management Authority (CCMA) to assess the ecological resilience of the Anglesea River and estuary in relation to acid sulphate soil and acidic flush events.

The Anglesea River estuary is part of the Anglesea catchment, a small catchment in south-west Victoria lying within the Otway basin. The Anglesea River is characterised by swampy tidal flood plains which are split by a ridge at the Great Ocean Road. The Victorian coast line is considered microtidal with tides on average reaching one metre and neap tides reaching

0.3 m. At Anglesea, the main process that affects the estuary is wave action. This wave action contributes to the net along-shore transport of sand. This can result in a build-up of sand within the vicinity of the Anglesea River estuary mouth, which in times of low river flow creates a sand bar which closes the river mouth. During high flow events, river water builds up until it spills over the sand bar, scouring a channel to the sea.

There are a number of significant man-made modifications that have occurred within the Anglesea River estuary. After the 1983 fires Coogoorah Park was created as a bush reserve and recreational area. A fifty year old brown coal mining operation is currently providing power to the Alcoa smelter in Geelong and is located in the the lower portion of the Anglesea catchment, above the estuary.

Soils that contain iron sulphides are commonly referred to as acid sulphate soils (ASS). In coastal environments, Sulphidic sediments are formed in vegetated, tidal environments when sulphate from sea water is reduced to sulphides by microbial activity; the most common being pyrite. In Victoria, these iron sulfide layers commonly occur well below the soil surface within the soil saturation zone or below the watertable. These soils were likely formed in the last 10,000 years.

When left undisturbed and submerged by groundwater, pyrite is chemically inert. When soil dries out during drought, pyrite oxidizes in the presence of oxygen and hydrogen to form sulphuric acid. If the acid-buffering and neutralising capacity of the soils is exceeded, soils will become acidified. The rate at which pyrite is oxidised tends to be closely linked with pH, with oxidation increasing as pH decreases, and is usually only limited by the rate of supply of oxygen.

When small acid discharges enter an estuary such as the Anglesea River, the natural alkalinity or acid-neutralisation capacity of the estuary can usually buffer these changes in pH, however after flood events or prolonged rainfall, drainage from acidic areas can reduce alkalinity of estuaries and acidify tidal reaches. The influence of tidal exchange on acidification of estuaries and receiving waters is important, since the tidal exchange can marginally buffer acidic events. When the estuary mouth is open, it enables the acidified estuarine waters to mix with seawater which contains calcium carbonate that buffers the low pH, returning it to a more neutral level. However, this report has not assessed the ecological or potential acid generation impacts of artificially opening the estuary which are likely to be significant.

The oxidation of sulphidic materials can lead to heavy metals such as cadmium and lead and metalloids such as arsenic becoming more available in the environment. Waterways can change colour due to the release of iron or aluminium when the acidity of the water increases. Flocculation of metals results in crystal clear waters due to soil particles falling out of the water column. The increase in clarity will create an increase in the temperature and this can also increase the amount of light penetrating the water; this in turn will increase algal production within the water column and on the sediment. In addition, flocculation may also smother benthic habitat and deplete dissolved oxygen, resulting in changes to the water chemistry at the sediment / water interface.

An obvious effect of ASS on waterways is mortality in resident fauna (ie. fish and macroinvertebrates), whilst secondarily, or less obvious impacts can include die back and growth inhibition of seagrass communities, death of smaller microinvertebrates and impaired health and condition of riparian vegetation due to poor water quality. The overall impact is a change in food web dynamics and ecosystem health.

A common impact of lowered pH on fish, macroinvertebrates and seagrass are physiological changes. For instance, damage to the outer epithelial layers of skin, gills or cuticle, mucus membranes and other external respiration organs can often occur. Such damaging effects can lead to increased susceptibility to disease and infection from pathogens and subsequent weakened immunity, which may cause reduced fitness and ultimately increased risk of mortality. In addition, if algal blooms occur when biota is stressed from damaged respiratory tissues, they are at an even greater risk of mortality. Reproduction is also likely to be affected when conditions change, through either a reduction in the number of viable gametes that are produced and resultant lowered fertility, or through adverse effects on the vulnerable early life stages (such as reduced hatch rates, increased rates of deformity and lower larval survival). In addition to changes in pH, exposure to elevated heavy metal concentrations can also cause toxic effects in biota. For organisms that have exposed gills filaments such as fish and some invertebrates, they are likely to be the most sensitive to changes in pH and are most likely to be negatively affected.

When metals such as aluminium settle out and flocculate, seagrass meadows and biota such as non-mobile macroinvertebrates, and low-mobility snails can be smothered leading to mortality. This biota is essential in maintaining healthy, stabilised habitat; therefore its loss is likely to lead to such areas becoming barren and anoxic.

Changes to water quality in estuaries such as the Anglesea River can lead to a reduction in sensitive species, resulting in an unbalance to the system. A situation may arise whereby predators are removed, and thus prey can become overly abundant and change the dynamics of that system until predators return. It is likely that species that have adaptation to low pH conditions will have increased survival as these conditions arise. Avoidance of low pH conditions depends on the mobility and chemosensory abilities of the species. In open estuaries, black bream have been shown to go out to sea during periods of high rainfall and flood events and this indicates for instance, that in Anglesea, when acidic conditions arise after large rain events, this species may be able to avoid the primary acid slug by going out to sea (assuming the estuary mouth is open).

The acid events in the Anglesea River are the result of natural process. If the water within the Anglesea River is frequently exposed to low pH conditions, it is possible that fish or other biota within the system may, over generational time periods, build up some resilience to those conditions.

This review identified knowledge gaps in relation to our understanding of how the ecology of a system will be altered in response to changes in water quality associated with ASS. Although we have a general understanding of what species exist in local estuaries, little research has been carried out on the impacts that episodic changes to water quality have on these species. Before changes to management practices are considered, the risk to the overall biological impact on the system needs to be considered, as this was not assessed in this report. Further research is recommended to increase the knowledge base in relation to acid sulphate soil impacts and the recovery and resilience of the local ecology.