Darwin Harbour water quality report 2020

New to the 2020 report!

Transitioning to a Water Quality Index - WQI

All water quality data is aggregated using a methodology that generates a WQI which can be calculated easily and used to provide an overall description of the quality of water.  It provides an approach to summarise water quality using a single value and a corresponding scale.

Each measured value at each location (either fixed sites or positions along Continuous Flow Monitoring transects) is compared to a guideline value (specific to that measure and sampling Zone) to yield an index value that expresses the difference between the observed value and the guideline on a standardized scale of 0 to 1 via a process called fixed cap scaled modified amplitude (Logan, 2016).

(Adapted from: Logan, 2016)

The scores at each reporting level (measures, sub-indicators, indicators, zones, regions and whole of harbour) are thereafter calculated as the mean of the data aggregated at each of those levels and these scores are further expressed as grades according to a simple classification of A: Very good (greater than or equal to 0.80), B: Good (greater than or equal to 0.6, less than 0.8), C: Satisfactory (greater than or equal to 0.4, less than 0.60), D: Poor (greater than or equal to 0.2, less than 0.4) and E: Very Poor (less than or equal to 0.2).

Data used to inform the index will be drawn from the DEPWS marine water quality monitoring program and other stakeholder data.

How data are collected and grades 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 circumstance, or because a very slight exceedance of the water quality objectives for a particular indicator is deemed to be of no ecological consequence.

Reference: Logan, M (2016). Pilot 1 - Upgrade of Darwin Harbour Report Card Analytics. Report prepared by the Australian Institute of Marine Science for Aquatic Health Unit, Water Resources Division, Department of Land Resources Management.

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 100,000 measurements were made in 2020 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.

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.

*Water quality guideline value (WQGV)