Loss of habitat
Two important water environment habitats that are threatened are in-stream habitat and seagrass meadows.
Degradation of in-stream habitat
Changes to habitat such as removal of woody debris and instream vegetation, constructing instream barriers and alteration of river channels can significantly affect aquatic ecosystems.
Removal of woody debris and instream vegetation reduces available food sources and the habitat available for aquatic animals such as fish and invertebrate organisms (for example, insects, snails, worms and shrimps). Fish require woody debris as sites for spawning and rearing juveniles, and for protection from strong currents and predators. Removal of woody debris and instream vegetation also reduces the stability of streambeds and banks.
Barriers to fish movement can severely limit the survival and distribution of many native fish and can create situations that are more favourable to introduce species such as carp. For example, about 70 per cent of native fish need to migrate so they can spawn and reproduce. Barriers to fish migration can be anything from a culvert under a road, to a small weir or a large dam.
Rivers channels have been modified to improve drainage for agriculture and reduce flooding. These modifications change the natural course and form of the river. This can result in increased water flows which increase bank erosion and sediment deposition.
Under these conditions, much of the habitat for aquatic animals can be washed away or covered with sediment. When a river is completely channelised (as often happens to urban rivers and streams), all the instream habitat is removed which results in a uniform fast flow that prohibits aquatic animals moving upstream and, in some cases, may wash them downstream.
Loss of seagrass
Seagrass provides a critical part of the coastal and marine landscape. It is a vital part of the ecosystem and plays an important role in maintaining the ecological processes and protecting the range of beneficial uses that can be supported in areas such as Western Port. Marine vegetation such as seagrass communities maintain water clarity through stabilising coastal zones and mud banks and preventing erosion, and assist in the recycling of nutrients.
During the last 30 years, seagrass populations in Western Port have declined. During the 1970s a dramatic loss of seagrass (approximately 45km2 or approximately 85 per cent loss) was reported in Western Port, much of this occurring on the intertidal banks.
The causes for the seagrass loss in Western Port have not been clearly identified but are likely to be attributed to a number of factors including increased sedimentation and nutrients from catchment activities. Direct losses of seagrass in Western Port have also been attributed to other activities including the dredging of channels and harbours.
The loss of seagrass cover is likely to have resulted in long-term changes to the physical and biological characteristics of marine environments. For example, where seagrass is lost, sediment is frequently suspended, creating poor water quality which in turn inhibits seagrass re-establishment and the return of seagrass reliant communities.
While some limited recovery (approximately 13 per cent) of seagrass has occurred, the conditions outlined above make it particularly difficult for seagrass regenerate.
EPA, in conjunction with the community based Western Port Seagrass Partnership (WPSP) and the Natural Heritage Trust (NHT), is implementing a seagrass restoration project in Western Port.