What corals tell us about the impacts of poor water quality

By Dr Chris Brown

Poor water quality threatens the health of many coral reefs. But we often don’t know how much of a reef is impacted by poor water quality.

Our new research shows the extent of pollution can be mapped by looking at changes in the types of reef habitats on the ocean floor. Surveys of reefs can then inform us about the impacts of new developments and bolster efforts to avoid pollution.

Healthy coral reef in the clear waters of Lady Elliot Island on the Great Barrier Reef. Photo: Chris Brown

In the Solomon Islands, logging frequently causes run-off of sediments to coastal seas. Here the sediments can smother types of corals that are important habitats for fisheries.

We looked at how the types of coral habitats change as you move further away from logging activities. Branching corals were more common in clearer water far from logging. Whereas, sand, dead coral and stress-tolerant brain corals were more common in murkier waters near to logging.

So, high levels of sand, dead coral and brain corals are indicative of poor water quality near to logging, whereas high levels of branching corals is indicative of good water quality.

We then developed a new statistical approach used the diversity of habitats as an indicator of pollution impacts.

Ecologists often measure the diversity of habitats to figure out the overall health of ecosystems. They commonly measure two types of diversity.

The first ‘alpha diversity’ is just the overall number of habitat types at a site. We were interested in a second type of diversity called ‘beta diversity’. Beta diversity measures how much change there is in the types of habitats as you move from place to place.

We analysed the change in types of habitats at different distances from logging. This measure of beta diversity tells us how quickly the impacts of pollution dissipate.

Once we knew how fast pollution dissipates, we could estimate how far away a coral reef had to be from logging to not be affected by pollution. We estimated that nearly 50% of reefs in the Kia region of Solomon Islands had likely been affected by pollution from logging.

The extent of impacts in the Kia region is of concern to local people, because these reefs support important fisheries that are sensitive to pollution.

The evidence from the coral surveys has helped our research partners in The Nature Conservancy work with local communities to protect the remaining forests.

More details on this work can be found in the publication: Brown and Hamilton. 2018. Estimating the footprint of pollution on coral reefs with models of species turnover Conservation Biology (Link to open access pre-print)

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