Darwin Harbour Report Card 2019

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.

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 27,000 measurements were made in 2019 using this method.

Water samples were also collected from over 100 sites and analysed for nutrients and chlorophyll-a.

Four key water quality indicators are assessed for the Darwin Harbour report cards.

Icon	Indicator	Parameter	Why is this important?
	Water clarity	Turbidity	Light is important for microscopic algae and other plants, such as seagrasses, to grow and photosynthesize. Stormwater and dredging can reduce water clarity and affect plant growth.
	Dissolved oxygen	Oxygen saturation	Oxygen dissolved in the water is essential for fish and other animals to survive. The amount of oxygen in the water can be reduced by water pollution.
	Nutrients	Nitrogen oxide, ammonia and soluble phosphorus concentrations	Nitrogen and phosphorus are nutrients required for algae to grow. However, high nutrient concentrations can increase the amount of algae, resulting in algal blooms which affect the broader environment.
	Algae	Chlorophyll-a concentration	These are microscopic plants that may appear as specks in the water, or as a scum floating on the surface. Algae are an important part of the ecosystem in the harbour, but excessive amounts have a detrimental effect on the waterway.

The Darwin Harbour Water Quality Objectives provide locally derived guideline values for water quality indicators in Darwin Harbour.