Analysis of coral reveals long-term climate impact in Southeast Asia

Leicester-led research into coral reefs fringing the iconic island of Borneo in Southeast Asia has revealed the impact of cyclical climate patterns on the local marine ecosystem.

An international team of scientists including Professor Jens Zinke and PhD student Hedwig Krawczyk, both from our School of Geography, Geology and the Environment, studied coral reefs off the northwest coast of Borneo.

By taking and analysing core samples, they were able to determine both sea surface temperature (SST) and the levels of oxygen isotopes in the sea water (indicative of salinity), covering a period from 1982 to 2016.

“Corals incorporate geochemical tracers from the surrounding water into their skeleton, leaving behind a type of record of the marine environment at specific instances in time,” said Hedwig, who is lead author on the paper, published recently in Scientific Reports.

“In coastal regions where there is limited, long-term environmental data, such as in Borneo, coral cores provide a critical record of local changes in river runoff and rainfall that affect coral reefs.”

Southeast Asia is affected by a periodic but irregular change in climate called the El Niño Southern Oscillation (ENSO); El Niño is the warm, dry phase, while the cooler, wetter phase is known as La Niña. This study provides the first long-term data on how the ENSO-driven climate cycle impacts on both terrestrial and marine ecosystems in northern Borneo, offering new insight into regional environmental changes, both natural and human-driven.

“The ways that humans use the land can severely impact soil contents and soil erosion patterns, which then discharge sediments into freshwater systems and nearby marine environments, ultimately altering the water quality on coral reefs,” said co-author Dr Nicola Browne, a coral ecologist from Curtin University, Australia.

“Weather patterns that bring heavy rainfalls, such as those seen with the La Niña, can exacerbate these erosion patterns even more, bringing more sediments into the local marine environments, which then ultimately end up affecting the coral reef ecosystems. El Niño leads to droughts, forest fires and less river run-off in Borneo.”

The research was conducted in the Miri-Sibuti Coral Reefs National Park in Sarawak, Malaysia, a protected area and popular diving destination with nearly two square kilometres of ecologically rich coral reef. Two locations at slightly different depths, charmingly named Eve’s Garden and Anemone Garden, were sampled in September 2016.

Under a permit from the regional government, the team took 30cm x 4cm cores from Porites coral at both locations, which were then transported, under a CITES license, for analysis in Germany. Since coral grows continuously, these cores provided environmental data going back a third of a century.

At the University of Kiel, the historical sea surface temperatures were calculated by measuring the ratio of strontium and calcium in the cores, sampled every 1.1cm, while a mass spectrometer at the Natural History Museum in Berlin was used to measure stable oxygen and carbon isotopes, an index of salinity.

The results show that salinity decreases during La Niña periods as increased river run-off dilutes the coastal sea water, with a commensurate increase in salinity when El Niño is in effect. Surface temperature on the other hand is increasing steadily at a rate of about one quarter of a degree Celsius every decade.

Identifying historical trends and matching them to climate cycles such as the ENSO and the Pacific Decadal Oscillation (PDO) provides a baseline against which future changes, resulting from both direct human activity and long-term climate change, can be measured.

“Over the past 40 years, nearly all of Southeast Asia’s marine coastal ecosystems have experienced intense pressures, due to large-scale economic development, urbanisation and deforestation,” said Hedwig.

“These records help us to understand the types of pressures these reefs have been exposed to over the last 30 years and beyond, and their level of resilience to future environmental changes.” The coral records are now in the process of being expanded to the 19th century, pre-dating current rainfall and river flow measurements by several decades, by Leicester PhD student Walid Naciri at the University of Leicester funded by Professor Zinke’s Royal Society Wolfson Fellowship.

Professor Zinke, whose expertise was recently acknowledged when he was named a Fellow of the International Coral Reef Society, concluded: “Our study testified that both marine and terrestrial environments in Borneo are massively affected by changes in the hydroclimate associated with El Niño, and longer term cycles in regional rainfall and temperature.”

• Corals reveal ENSO-driven synchrony of climate impacts on both terrestrial and marine ecosystems in northern Borneo, DOI: 10.1038/s41598-020-60525-1