Water quality guidelines for the Great Barrier Reef

The purpose of this study was threefold: (1) to identify water quality guideline values based on existing ecological data; (2) to identify areas where guidelines are met or exceeded, and (3) to quantify the likely ecological changes associated with improving water quality to meet the guidelines.

Water quality guideline values were derived based on two separate approaches. Firstly, long-term average water qualty conditions were characterized for the six NRM regions within the GBR. Cape York, a region with little land use, was used as a Reference Condition Site, after controlling for latitudinal gradients along the shelf. Cape York has the lowest concentrations in the coastal zone but similar water quality to the other regions in the offshore zone. Second, the GBR water quality data were related to the spatial distribution of macroalgae, and the richness of hard corals and octocorals. Substantial increases of macroalgal cover and declines in the biodiversity of hard corals and octocorals were found with increasing turbidity and chlorophyll.

These two approaches were used to identify the following trigger values: mean annual values below 10 m Secchi disc depth, and mean annual concentrations exceeding 0.45 microgram / liter chlorophyll, 2.0 mg / liter suspended solids, >1.5 micromol / liter particulate nitrogen, and 0.09 micromol / liter particulate phosphorus.

The models display the regions within the GBR where the guideline values are exceeded. The models also show that up to 60% reductions in macroalgal cover and similar increases in coral biodiversity on inshore reefs are likely if the proposed trigger values were met (all other factors being equal).The study confirms that chlorophyll and water clarity, macroalgal cover and hard coral and octocoral diversity are useful as water quality indicators for monitoring inshore reef status.