How grades are calculated

For each indicator the median value of all the data within a zone is calculated and compared against the Darwin Harbour Water Quality Objectives. These guideline values are locally derived benchmarks to evaluate water quality condition in Darwin Harbour.

The nutrient indicator comprises three sub-indicators, an exceedance for one of the sub-indicators results in a “fail” for the overall indicator.

Report card grades are assigned depending on the number of indicators that are exceeded:

How data are collected and grades are calculated

In some exceptional circumstances, expert opinion may override the calculated water quality grade. This may occur because data collection in a particular year was affected by unusual circumstances, or because a very slight exceedance of the water quality objectives for a particular indicator is deemed to be of no ecological consequence.

Report card data collection

Water quality data for the Darwin Harbour Report Cards were collected during the dry season. Data collection was restricted to neap tides to minimise variability due to tidal influence.

Surface water quality (pH, dissolved oxygen, salinity, temperature and turbidity) was measured at one-second intervals along continuous transects across the nine reporting zones, using a flow-cell method that continuously pumps water through a water quality instrument. More than 3000 measurements were made in 2016 using this method.

Water samples were also collected from over 75 sites and analysed for nutrients and chlorophyll-a.