The monitoring program has assessed the Ecological health of Wallis and Myall Lakes, Manning and Karuah River Estuaries and the Khappinghat Estuary. There are a number of steps taken to determine the score for each zone and subsequent Report Card grade:
- Selecting the indicators.
- Identifying the trigger levels.
- Collecting the data.
- Calculating the zone score.
- Allocating the Report Card grade.
Selecting the indicators
Chlorophyll and turbidity were chosen as the indicators as they are proven to be very informative and responsive measures.
What we do on the land impacts on the quality of water that runs off. If the quality of the runoff is poor it puts stress on the environment. Stressors are changes to the environment that result from activity; these can lead to ecological harm. Stressors can include nutrients, acid leachate and sediment in the water (turbidity).
Ecological condition grades are a combination of turbidity (water clarity) and algae (measured as chlorophyll) scores.
Identifying the trigger levels
A healthy ecosystem refers to a system which has normal ranges of diversity and function. These ‘normal’ ranges have been established from extensive monitoring of estuaries across New South Wales.
A trigger value is the value which indicates that a variable is outside the ‘normal range’ and could trigger further investigation. In our context, we have used the trigger value to indicate conditions which are not desirable for continued waterway health.
A trigger value is specific to different types of estuary. In this study, Wallis Lake, Pipers Creek, Charlotte Bay, Bombah Broadwater and Myall Lake were all classified as ‘Lakes’, Wallamba River, Karuah Estuary,
Wallamba Cove, Branch Estuary, Lower Myall River and Upper, Mid and Lower Manning Estuary River were classified as a ‘River estuary’ and Khappinghat was classified as a ‘Creek estuary’.
Collecting the data
The MidCoast Council region has been divided up into six different reporting zones.
Samples were collected on six occasions between summer and autumn from December to March. This represents the part of the year when the highest chlorophyll concentrations are expected.
At each of the selected sites, samples were taken in accordance with the New South Wales Monitoring, Evaluation and Reporting protocols.
Allocating the Report Card Grade
Defining the Report Card grade is an important step in the development of the Report Card. The grade definitions below are linked to the environmental values outlined above and are structured to allow easy comparison between each system and over time.
It is important that the cut-off values for each grade reflect the condition of each zone in comparison to a broader scale of condition across all New South Wales estuaries (i.e. an ‘Excellent’ grade represents an excellent condition for a New South Wales estuary).
To assist with the derivation of cut-offs, scores were calculated for 130 zones across a wide range of New South Wales estuaries using the same triggers and worst expected values as the MidCoast analyses. Cut-offs were then defined as representing a percentage of the scores for the state.
For example, a zone score less than 0.07 defined the 20% of best zone scores in the state and this became our ‘Excellent’ grade. We did not use a score of 0 as ‘Excellent’ because, as a consequence of how the trigger values are calculated, we expect that even pristine reference sites will exceed trigger values 20% of the time.
The amount of rainfall that occurs around the period of sampling for the Report Card (September – March each year) influences the Report Card results.
If there is more rain, there is more runoff in the catchment resulting in greater quantities of sediment and nutrients entering our waterways.
In general, the sampling period in 2017/18 was very dry, except for a significant storm in March 2018. If it wasn’t for the March 2018 rainfall event then the rainfall between September 2017 and March 2018 would have been the lowest since 2007.
The total rainfall for the sampling period was slightly above the long term average but half of that fell in one month.
The extended dry period resulted in good water clarity due to the minimal amount of catchment runoff entering the estuary. At some sites chlorophyll-a concentrations (algae) increased during this time period, as the dissolved nutrients in the system and the abundant light would have provided suitable growing conditions.