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Study confirms the ‘uniqueness’ of Kimberley reefs

27 June 2017

Water drains over the reef crest (left side of photo) at Tallon Island during ebb tide. The shape and roughness of Tallon reef (and other Kimberley reefs) allow water to 'pond' on the reef flat, which keeps reef organisms submerged even during low tide. (Ryan Lowe)

A new report confirms that reef systems in the Kimberley continue to produce life amid some of the most extreme conditions yet recorded for reefs worldwide, prompting scientists to ask the question: how much more pressure can the Kimberley reefs cope with?       

In the report for the Western Australian Marine Science Institution, scientists have documented what life is like on the Kimberley reefs and how that life responds to environmental variability, including extremes in temperature, oxygen and water levels.    

The research team, led by Professor Ryan Lowe and PhD student Renee Gruber from The University of Western Australia’s Oceans Institute, found the daily variability in temperature and dissolved oxygen that occurs on the reef platform is driven by semidiurnal tides (high and low tides that differ in height) and solar (daylight) cycles.

“The shape and friction of the platform causes water to ‘pond’ on the reef for up to 10 hours during each ebb tide (twice daily),” Professor Lowe said. “When these extended low tide periods occur near noon, extreme warming happens on the reef, with temperatures rising by 10°C over several hours and reaching up to 38oC.”

Researchers placed a scaffolding platform on the reef flat to hold a weather station and several sensors that monitored environmental conditions on the reef during their experiments. (Ryan Lowe)

 

“This high light availability also drives high rates of reef primary production (such as seagrass growth), which releases oxygen into the water column and results in extremes in oxygen saturation (~270%),” Ms Gruber said. “When low tide periods occur near midnight, community respiration (the consumption of oxygen as reef organisms create energy) causes oxygen levels to plummet, reaching very low (hypoxic) levels; low oxygen levels can harm or kill organisms in other ecosystems, and are not typically recorded on reefs.”

 

Despite extremes in temperature, light, and dissolved oxygen during low tide (seen here on Tallon reef), Kimberley reefs are able to support many species of producers including coral, coralline algae, macroalgae, and seagrass. (Ryan Lowe)

 

“Despite these extremes, the study found that overall rates of primary production of reef communities were not adversely affected,” Ms Gruber said. “This is a clear example of uniqueness of Kimberley  reef communities, which are well-adapted to conditions that would kill ‘typical’ reef producers.”

The report also highlights that productivity varied on a day-to-day basis, due to the timing of noon relative to low tide, a cycle that lasts about 15 days. It suggests that future studies shorter than this time frame may over- or under-estimate ecological processes (such as productivity). 

The study also found the overall average rates of productivity were similar to the global mean for tropical reefs, demonstrating that tide-dominated reefs can maintain moderate rates of production despite daily extremes in temperature.

The report concludes that, while the overall rates of productivity in the Kimberley reef system were comparable to other coral reef habitats worldwide, the environmental conditions under which primary producers survive and grow are extreme.

“This has implications for the resilience of these producers in the face of climate change across the Kimberley environment,” Professor Lowe said. “While these organisms appear well adapted to the local environmental conditions of the Kimberley, many are still likely operating at the edge of their capacity, as evidenced by coral bleaching in the inshore Kimberley associated with the El Niño heat wave in 2016.”

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The $30 million Kimberley Marine Research Program is funded through major investment supported by $12 million from the Western Australian government co-invested by the WAMSI partners and supported by the Traditional Owners of the Kimberley.

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Kimberley Marine Research Program