Unlocking the history of nitrogen cycling trapped in coral skeletons

A team of scientists will try to understand why coral reefs in the Pacific are in decline. The study, being led by Southern Cross University, will employ newly developed techniques for measuring nitrogen isotopes trapped in the skeletons of tropical corals.

The study, entitled ‘What is driving the decline in coral reef health on the island of Rarotonga?’, was recently awarded $44,600 from the Australia and Pacific Science Foundation (APSF) and is being led by Dr Dirk Erler, a geochemist from the University’s Centre for Coastal Biogeochemistry Research.

The project will be based in Rarotonga, in the Cook Islands, and aims to identify the major causes of declining coral reef growth in the Muri lagoon.

Tourism is the major economic driver for the country and declining coral reef health could jeopardise this crucial industry. The major anthropogenic (pollution caused by human activity) issues facing the lagoon are agricultural run-off, including soil erosion and fertiliser release, and discharge of groundwater contaminated with domestic effluent.

Climate variability could also be a major factor in the reef’s health.

“The challenge is to isolate the direct impacts of human activity from those of climate variability on reef health,” said Dr Erler.

Corals reefs lay down distinct seasonal growth bands and it was in these bands that an archive of local oceanic conditions was contained.

“We will use 100-year-old coral skeletons to reconstruct the history of the temperature, salinity, turbidity and nutrient status in the Muri Lagoon, Rarotonga. Furthermore the coral skeleton provides a direct measurement of growth and reef health. Putting these pieces together should yield a map of how and why the reef has changed over the last century,” said Dr Erler.

“We are particularly interested in the nitrogen isotopes contained within the coral skeletons. Nitrogen has an isotope signature that’s specific for the source of the nitrogen, for instance, wastewater and fertiliser nitrogen have different signatures. This enables us to isolate the source of nitrogen that may be having an impact on the environment.

“Declining reef health will have major ecological and economic implications for Pacific island nations in the coming decades. Identifying the causes is a key step in stemming the deterioration.”

Dr Erler said the project formed part of a larger plan to develop a global map of human activity on coastal nitrogen cycling.

“Scientists have been collecting coral cores for decades to reconstruct surface temperature changes in the ocean caused by climate change. Until now no one has been looking at the nitrogen content of those skeletons.

“We have forged a number of collaborations with international research groups and our grand plan is to construct a global map of changing nitrogen cycling over the past 100 to 1000 years.”

The international research team also includes Professor Daniel Sigman from Princeton University, Dr Sander Scheffers from Southern Cross University and Mr Ben Ponia from the Ministry of Marine Resources, Cook Islands.
Photo: Dr Dirk Erler drilling for a coral core on the reefs surrounding the island of Rarotonga, Cook Islands (Credit: Sander Scheffers).