Clean rivers could make coral reefs healthier

Great Barrier Reef, Australia. Credit, Peter Nijenhuis.A new study by Australian scientists indicates that cutting water pollution in rivers that drain into the ocean may provide a significant health benefit to large parts of the Great Barrier Reef.

The researchers found that a fifth of the Great Barrier Reef suffers from low water quality, which reduces the diversity of corals living on the reef and favors a takeover by seaweeds.

Minimizing agricultural runoff and other forms of river pollution could reduce seaweed coverage by 39% and raise coral diversity by up to 33% in the presently impaired portions of the Great Barrier Reef.

Many other studies have linked water quality and marine ecosystem health on a local scale, such as in a bay receiving runoff from an urbanized watershed, but it has been unknown if the linkage holds true at regional scales.

In the case of the 1,200-mile-long Great Barrier Reef, for example, if people in northeastern Australia improved water quality in their rivers, would the whole reef, including offshore parts, reap the benefits? Or would the benefits be confined to local areas near river mouths?

Glenn De’ath and Katharina Fabricius of the Australian Institute of Marine Science addressed these questions with a regional study that used long-term datasets on water quality and biodiversity.

As indicators of water quality, they used Secchi disk depth data from 2,058 sites and chlorophyll data from 4,067 sites spanning most of the Great Barrier Reef.

To analyze patterns of biodiversity, the researchers used data on seaweed coverage and octocoral diversity along 1,106 transects, as well as the diversity of hard corals along 599 transects.  

In their regional analysis, De’ath and Fabricius found that diversity of hard corals and phototrophic octocorals was lower in areas with poor water quality, whereas seaweed coverage increased.

Because the highest coral diversity and lowest seaweed coverage occurred in places where Secchi disk depth averaged 10 meters and chlorophyll averaged 0.45 micrograms/L, the scientists used these guideline values to designate 22.8% of the Great Barrier Reef as impaired.  

Based on statistical analysis and modeling, the scientists determined that if agricultural runoff and other pollutants carried into the ocean by rivers were minimized, hard corals could increase by 16% and phototrophic octocorals by 33% in impaired areas of the Great Barrier Reef, assuming all other factors remained constant. According to the authors,  

“Such guidelines may help focus efforts to implement effective pollution reduction and integrated coastal management policies for the GBR and other Indo-Pacific coral reefs.”

-- By Peter Taylor

De'ath, G., & Fabricius, K. (2010). Water quality as a regional driver of coral biodiversity and macroalgae on the Great Barrier Reef Ecological Applications, 20 (3), 840-850 DOI: 10.1890/08-2023.1