This page describes some of the main pressures on marine water quality in Port Phillip Bay. They are barriers to achieving this strategy’s targets so planning and action will need to find ways to minimise their effects.
Whilst the condition assessment shows that the Port Phillip Bay’s water quality is generally good, pressures on water quality come from a variety of sources including the catchments, marine vessels and marine infrastructure.
The main sources of pollutants to marine water quality are from catchments, particularly from the Yarra River. Climate change also creates pressures to Port Phillip Bay’s marine waters.
Much of the information about pressures has been sourced from the Baywide water quality monitoring program (Milestone report no.9, Port Phillip Bay water quality: November 2007 – December 2011, March 2012).
The Western Treatment Plant supplies about half of the nutrients entering Port Phillip Bay, a smaller proportion of toxicants and discharges of treated water enter Port Phillip Bay on a year-round basis, with the highest flows in winter. Other nutrient and toxicant sources include rivers (principally the Yarra and Maribyrnong Rivers), streams and drains, with minor inputs from the atmosphere. Most of the riverine delivery of nutrients and toxicants to Port Phillip Bay occurs during storms.
The storm wash-off rate depends on the intensity of the surface run-off and on the mass of transportable chemicals available within the catchment that build up between storm events. Maximum concentrations of chemicals in catchment flows occur early in the first storm after prolonged dry periods.
The Western Treatment Plant discharge is predominantly wind-driven once it enters Port Phillip Bay, and may move toward Hobsons Bay under southerly and westerly winds or toward Corio Bay under northerly and easterly winds. The Yarra/Maribyrnong Rivers typically flow down the eastern coast of Port Phillip Bay. Past studies indicate that the increased plankton growth arising from these nutrient inputs might reflect these spatial patterns. Depending on the strength of the circulation, the impacts of nutrient inputs may occur some distance from the location of the input.
Low nutrient supply may lead to restricted growth, while too much may lead to undesirable impacts from explosive growth, including aesthetic, ecosystem and human health impacts. Nitrogen has been identified as the key nutrient limiting plant growth in the Bay. The Werribee and Hobsons Bay areas receive the largest nitrogen loads from land-based sources, and are thus two of the most highly productive areas of Port Phillip Bay.
Climate change is expected to reduce river flows to Port Phillip Bay but increase the frequency and intensity of storm flows. These will increase pollution risks when long periods of low runoff are interrupted by concentrated pollutant loads. Increase algal blooms are likely as nutrient-rich inflows mix with warmer bay waters.
Coastal Ramsar wetlands on Port Phillip Bay’s western shore will be vulnerable to sea-level rise. Global sea levels are projected to rise between 0.18 to 0.59m by 2095, with a possible additional contribution from ice sheet melts of 0.1 to 0.2m. Larger sea level rise from more rapid melting of polar ice sheets is possible.
Marine environments will be impacted by increased water temperatures and acidification. Warmer waters are already creating changes to the distribution of marine plants and animals. Acidification increases the energy marine crustaceans and planktons need to create their calcium carbonate shells. The loss of these animals could have catastrophic effects on marine food chains and ecological systems.
Water quality in Port Phillip Bay has been assessed (Method & Condition). Targets have been set (Targets) and lead organisations are committed to achieving the targets (Leadership). Arrangements are in place to monitor and report on progress and success (Monitoring & Reporting). Targets and leadership arrangements are determined in consultation with relevant departments, agencies and delegated land managers