The Regional Estuaries Initiative supported the ongoing water quality monitoring of the Peel-Harvey estuary since 2016 with monitoring continuing through Healthy Estuaries WA. Key water quality indicators are monitored fortnightly including salinity, temperature, oxygen, chlorophyll, phytoplankton and monthly nutrients at 12 sites in the main estuary basins and the estuarine reaches of the Murray and Serpentine rivers. This condition assessment covers the 12 months from October 2016 to September 2017.
For the 2016-2017 monitoring period, the main basins of the Peel-Harvey estuary had satisfactory water quality free from persistent eutrophic symptoms. All mean nutrient concentrations were generally below the relevant guidelines. In contrast, the estuarine parts of the Murray and Serpentine rivers showed significant eutrophic symptoms. These included high algal activity, harmful algal blooms, low dissolved oxygen and fish kills.
The Dawesville zone contains two estuary sites closest to the Dawesville Channel, and is the most marine-influenced zone. Dawesville had the best water quality (with marine salinities): it was well-oxygenated, and had low nutrient concentrations and low phytoplankton activity throughout the year.
In the Peel basin, mean salinities were slightly below marine levels. Waters were well-oxygenated and had good clarity. Nutrients and chlorophyll concentrations were mostly below relevant guidelines, except after a significant summer rainfall event in February 2017 which delivered high loads of nitrogen, primarily from the Murray River. This event elevated phytoplankton activity in the Peel basin and caused a significant fish kill extending from the Murray River into the Peel Inlet.
The phytoplankton flora was dominated by diatoms and cryptophytes. Harmful species were relatively few in number and generally in low cell densities.
The Harvey basin had salinities slightly above marine levels, indicating the tendency towards hypersalinity in summer and autumn due to low freshwater inputs and summer evaporation. Waters were well-oxygenated and had good clarity for light penetration. Nutrients and chlorophyll concentrations were mostly below relevant guidelines.
Phytoplankton composition was dominated by cryptophytes, diatoms and dinoflagellates. Several harmful species were detected, mostly at low densities. The harmful dinoflagellate species Dinophysis acuminata was observed throughout the year – particularly at the southern-most site.
Murray River – estuarine reach
The lower reaches of the Murray River had persistent salinity stratification with poor oxygen status in the bottom waters. Also recorded in the Murray were six harmful species of phytoplankton comprising mostly dinoflagellates and diatoms.
Four fish kills occurred in the Murray River; these were associated with catchment inflows, stratification, low oxygen and at times high densities of a harmful dinoflagellate (Karlodinium). The February 2017 rainfall event triggered the worst fish kill in the Murray River. Fish kills in the Murray have occurred approximately once a year during the past 15 years, so the four observed during the monitoring period represented a relatively high frequency.
The possible causes of the February 2017 flood fish kill event include an abrupt change in salinity and/or pH, high organic loads causing low oxygen conditions, or toxins that came in from the catchment.
Serpentine River – estuarine reach
The Serpentine River, which is shallower and has less flow than the Murray, tended to be longitudinally stratified, having extreme ranges of salinity from fresh to hypersaline at the end of summer. The highest concentrations of nutrients, chlorophyll a, and phytoplankton were observed here. The Serpentine was one of the most eutrophic zones of all the monitored estuaries for the monitoring period. Harmful algae were present in all estuary zones, however the greatest cell densities and number of harmful species were observed in the Serpentine. The Serpentine had 18 harmful algae species, predominantly cyanophytes (blue-green microalgae/bacteria).
While the Peel and Harvey basins are considered to have relatively good water quality, during high rainfall/flood events, high organic loads and poor quality brackish waters are transferred from the river systems to the estuary basins. This creates the potential for harmful algal blooms, fish/fauna deaths and smothering of seagrass. The February 2017 rainfall event, for example, caused fish kills and was likely responsible for the subsequent Karlodinium bloom.
The 2016-17 results of the estuary monitoring program underpin the importance of
continued commitment to actions already underway in the Peel-Harvey catchments, such as use of soil amendments in agricultural catchments, clay trials for agricultural drains, stream restoration and fencing, improved dairy effluent management, and drainage works for better water quality outcomes.