Eutrophication of the Peel-Harvey estuarine system has been occurring for many decades and is associated with a number of human and induced factors, agriculture and other catchment land uses. Eutrophication is generally associated with poor water conditions resulting from high nutrients in a water body. The nutrients phosphorus and nitrogen are generally linked, so both are described in this section.

Phosphorus and nitrogen are essential for plant growth, so increases in these nutrients are linked with increases in algae in the Peel-Harvey Estuary. A brief history of algal activity is provided in Algal activity on the Peel-Harvey Estuary. Occurrences of odours, weed fouling and nuisance insects are also associated with high nutrient inputs.

Historically, poor land use management combined with an efficient drainage system in the catchment has resulted in nutrient enrichment of the lower reaches of the Harvey, Serpentine and Murray rivers. Nutrient concentrations in the Swan coastal plain reaches of these rivers are considered high and they discharge into the Peel-Harvey Estuary. Limited tidal exchange between the estuary and the ocean has resulted in retention of nutrients in the system. This provides an opportunity for algae to utilise the nutrients and grow at higher than normal rates, resulting in an 'algal bloom' when conditions are favourable.

The majority of phosphorus comes from agricultural sources and accounts for 39 per cent of the load in the Peel Harvey estuarine system. Similarly, phosphorus from combined rundown (release of stored nutrients from past poor land use management) and the atmosphere is 34 per cent. While the residential contribution was proportionately lower at 20 per cent in 2003, the area designated residential is quite small in comparison to grazing, indicating that phosphorus contributions are much higher per hectare in a traditional urban context than a rural one.

Since moving water transports nutrients, phosphorus and nitrogen are generally high in winter and low in summer for the Peel-Harvey Estuary. However, significant stores are held in the sediments and groundwater contributing to the system all year round. After the opening of the Dawesville Channel in 1994, the volume of nutrients in the Harvey Estuary had decreased due to improved tidal flushing.

The nutrient load entering the Peel-Harvey Estuary has increased over the last few decades. Between 1977 and 1988 an average of 1200 tonnes of nitrogen and 140 tonnes of phosphorus annually entered the Peel-Harvey Estuary. The current estimated phosphorus winter load has increased to 145 tonnes per year, which is 49 per cent above the current phosphorus reduction target described in the Water quality improvement plan for the rivers and estuary of the Peel-Harvey system - phosphorus management.

The current snapshot is building on the historical data and provides valuable information about the current condition of the Peel-Harvey estuarine system.

References

DA Lord & Associates Pty Ltd 1998, Dawesville Channel monitoring programme: technical review, Report to the Waters and Rivers Commission, Perth, Western Australia.

Environmental Protection Authority 2008, Water quality improvement plan for the rivers and estuary of the Peel-Harvey system - phosphorus management, Environmental Protection Authority, Perth, Western Australia.

Hale J & Butcher R 2007, Ecological character description of the Peel-Yalgorup Ramsar site, Report to the Department of Environment and Conservation and the Peel-Harvey Catchment Council, Perth, Western Australia.

Humphries RB & Croft CM 1984, Management of the eutrophication of the Peel-Harvey estuarine system, Department of Conservation and Environment Bulletin, No. 165, Perth, Western Australia.

James RN & Dunn JC 1996, Water quality in the Peel-Harvey estuarine system for the period 1977-1993, Waters and Rivers Commission, Perth, Western Australia.

URS 2007, State of Play Peel-Harvey eastern estuary catchment environmental assessment discussion paper, Report prepared for Department of Water, Perth, Western Australia.

Water and Rivers Commission 2000, Nutrients in tributary inflows to the Peel-Harvey estuarine system: status and trend (1983-1998): Status and Trend, Waters and Rivers Commission, Water Resource Technical Series No WRT23.

Water and Rivers Commission 2004, Condition statement for the Peel-Harvey estuarine system south-western Australia, Water Resources Management Series, Waters and Rivers Commission unpublished report.