Local reefs given all-clear on water quality
CLEAN water helps coral reefs thrive, according to the largest study even undertaken on the Great Barrier Reef to examine the relationship between water quality and reef health.
The study found reefs in clean water had greater diversity and were more resilient to temperature changes.
On the other hand, 22 per cent of reefs on the GBR do not meet water quality guidelines.
“While the amount of coral cover on a reef doesn’t appear to suffer from poor water quality, the bio-diversity of those reefs does,” coral reef ecologist Dr Katharina Fabricius said.
While tourists wouldn’t notice the difference, reefs can lose a lot of their most sensitive corals in dirty water.
The study measured water clarity and nutrient levels at more than 2000 sites across the Great Barrier Reef, and crossreferenced those with levels of coral cover and diversity on 150 reefs.
The team measured two factors to determine how clean the water was - chlorophyll levels, which are used to determine nutrient loads, and clarity.
The study found reefs were at risk when visibility dropped below 10m and chlorophyll levels rose over 0.45 microgram per litre.
Clean water not only results in healthier reefs, they are also more resilient to shocks.
Dr Fabricius said unpublished data showed reefs in clear water were better able to bounce back from disruptions such as bleaching events.
“Reefs with turbid water have five times more seaweed than nearby reefs in clean water, and this additional seaweed can compromise the ability of coral to settle and reclaim areas of reef after bleaching events,” she said.
Dr Fabricius said it was too early to the Reef Rescue program to have had a noticeable impact on the health of the reef.
“The fact state and federal governments have got together to tackle the issue is good,” she said.
“River loads of nutrients and sediments are four to ten times higher than in pre-European times.
“We should start to see a decline in pesticides and fertilisers, because they have a short half-life.
“It won’t be making a measurable difference to water clarity in the short term, however.
“We’ve still got a lot of sediment built up in the rivers so it will probably be a decade or two before we see a marked difference.”
Dr Fabricius said the study had not looked at algal blooms, which can smother corals on a reef.
“One thing we’re really concerned about, which wasn’t covered in the study, is blooms of filamentous seaweed which usually only last for a couple of weeks a year.
“Typically, these are late winter blooms, they occur in August and September.
“I’ve seen it at Green Island, great carpets of green slimy sludgy stuff up to a foot thick.
“They may only last for a few weeks before they break down, but afterwards the patch of reef where these carpets had smothered the corals look pretty sick.”