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What does the Marine Water Quality Dashboard show?

The dashboard is primarily concerned with the remote sensing of optical properties (colour) of the ocean within the Great Barrier Reef Marine Park. These help us determine the type and amount of matter in the water and consequently how much light is available within the water. These factors are important because they can have a great impact on the health of the Great Barrier Reef ecosystem.

The optical properties displayed within the dashboard are

  • Concentration of chlorophyll-a (Chl) – an indicator of the amount of phytoplankton present in the water;
  • Light absorption due to coloured dissolved organic matter (CDOM) – a breakdown product of organic matter, such as phytoplankton. CDOM can also be used as a marker for inflows of fresh, low-salinity, river water,
  • Concentration of non-algal particles – particles other than phytoplankton present in the water. This is often referred to as the total suspended solids.

Note that the marine water quality indices shown in the dashboard do not provide information on pollutants or chemicals that may be present within the water. Laboratory analysis of water samples is required to determine this.

How deep does light penetrate into the ocean?

It is difficult to answer this question because the penetration depth depends on the constituents in the water. These can vary greatly with time and location. The penetration depth becomes shallower as concentrations of in-water constituents, such as phytoplankton and sediments, increase. In clear, blue waters, like those found in the reef lagoon, the penetration depth may be around 25 m. For waters that appear green-brown, like those found close to river mouths, the penetration depth may be less than one metre.

The majority of the ocean signal received by the satellite’s sensor comes from within this depth range, with water nearest the surface having the greatest influence.

Is there also a signal from the atmosphere?

The signal measured by the MODIS sensor contains contributions from both the ocean and the atmosphere. Nearly 90 per cent of the signal received by the sensor actually comes from light reflected by the atmosphere. To determine marine water quality we are only concerned with the signal from the ocean, so the atmospheric signal must be removed. This process of atmospheric correction uses a technique developed by CSIRO that uses an artificial neural network.

Further details can be found in the paper by Schroder et al, listed in eReefs Marine Water Quality Dashboard Scientific References.

What happens when there are clouds?

Clouds block the path of light, preventing satellite readings, so marine water quality can’t be measured.