The Peel-Harvey Estuary in south western Australia has suffered the effects of cultural eutrophication for a number of decades. The Dawesville Channel was constructed as a component of a three part management strategy, to increase flushing with the marine environment and thus alleviate some of the problems associated with eutrophic conditions. This year marks the five year anniversary of the opening of the Channel, and the objective of this report was to determine if there have been significant improvements in water quality since it was opened.

Water quality was monitored at three sites each in the Peel Inlet and the Harvey Estuary. Profiles of salinity, temperature and dissolved oxygen were measured in the field along with secchi depth and light attenuation. Surface and bottom water samples were collected at each site and chlorophyll aand phaeophytin concentrations were analysed, as well as inorganic and organic nutrients.

In order for meaningful comparisons to be made, years of similar rainfall were chosen to represent pre and post Channel conditions. The data was divided into winter, spring and summer seasons and the five years representing pre and post Channel were compiled. Statistical comparisons were made using t-tests, and data presented as means and standard deviations.

There have been dramatic reductions in the concentrations of chlorophyll a since the opening of the Dawesville Channel and no recorded blooms of Nodularia. The concentrations of dissolved oxygen have increased, as have salinity levels. This has resulted in a reduction of stratification and a decrease in release of inorganic nutrients from the sediments.

However, the flushing effects of the Dawesville Channel, especially during winter, do not appear to extend into the areas adjacent to river inflows. This, coupled with continued high (and possibly increasing) nutrient inputs from the catchment, means that this system is still partially eutrophic. It is possible that a phytoplankton species adapted to the increased salinity regime could replace the nuisance blooms of Nodularia. Also, it must be remembered that a year of high spring rainfall could create conditions conducive for the germination of Nodularia akinetes.

The Dawesville Channel was not meant to solve the problems of the Peel Harvey Estuary, but rather work as a component of a three part strategy. Without the reduction of nutrients entering the system, it is unlikely that the eutrophic conditions within the system will be completely remedied.