History of water quality monitoring in the Peel Harvey Estuary

Water quality has been monitored in the Peel Harvey Estuary since the 1970's. During the last 40 years, local government, state government and universities were responsible for the sampling at different times, and although water quality variables and sampling frequency changed over the years, the six sites in the Peel Inlet and Harvey Estuary created in 1979 are being used in the current monitoring program managed by the Department of Water. The continuity of long term data is important for monitoring changes in the Peel-Harvey Estuary.

General description of water quality in the Peel Harvey Estuary

The Peel Harvey Estuary System is a broad, shallow water body covering an area of 130km2 and receives the Serpentine, Murray and Harvey Rivers. Tidal exchange is via the Mandurah Estuary to the north and Dawesville Channel to the west.

In 2009, Peel Harvey Estuary System was generally well oxygenated due to its shallow depth exposure to high winds, although some riverine sites did suffer from low dissolved oxygen during the warmer months. The temperature ranged from 10-18oC during winter to 20-30oC in summer and salinity ranged from fresh to hypersaline. Conditions at each site changed depending on a number of factors.

Two important factors influencing estuarine water quality are the variations in season and tides. In general, winter precipitation delivers fresh water via the rivers to decrease salinity throughout most of the estuary. Since most freshwater flows are greatly reduced during summer, salinity is higher in the estuary and lower reaches of the Serpentine and Murray Rivers than during winter. Poor tidal water exchange and evaporation can cause hypersaline conditions to arise in sites further from the Dawesville Channel and Mandurah Estuary, which may be pushed into the lower reaches of the Serpentine and Murray Rivers by a flooding tide.

A combination of tides and fresh inflows may cause saline stratification in the Peel Harvey Estuary System. Saline stratification commonly occurs during summer in the lower reaches of the Murray River and may occur in the Harvey Estuary and Peel Inlet when conditions are suitable. Saline stratification develops when a layer of water sits on top of another body of water with different salinity. These two water layers don't mix so the water quality can vary dramatically in the water column. In the Peel Harvey Estuary, saline stratification typically occurs when fresh or brackish water sits over the top of saline water. As well as salinity differences between the two layers, temperature differences occur with occasional higher temperatures in the deeper saline water. High dissolved oxygen is common in the less saline surface component compared to poorly oxygenated, saline component near the bottom.

Information presented on the fortnightly water quality vertical maps for dissolved oxygen, salinity and temperature will reflect the season and tides as described above. Snapshot maps representing nutrients and algal activity also collected in the water quality monitoring program can be viewed below.

References

Peel-Harvey: The Decline and Rescue of an Ecosystem, Bradby K. (1997)

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