How El Niño and related sea level fall impacted indonesian corals

Image of the Month - April 2017

Photos of coral reefs in the Bunaken area. Top (A) a living Heliopora coerula (blue coral) community in 2015 in a keep-up position relative to mean low sea level, with almost all the space occupied by corals. In that case, a 15-cm sea level fall will impact most of the reef flat. Bottom: (B) Healthy Porites lutea (yellow and pink massive corals) reef flat colonies in May 2014 observed at low spring tide. The upper part of colonies is above water, yet healthy. (C) Same colonies in February 2016. The white line visualizes tissue mortality limit.(Credits IRD)
Trends in absolute dynamic topography over the Bunaken marine reserve, a broader area over North Sulawesi, and the whole Indonesian seas (Credits IRD)
Saral sea level anomaly trends for 2013-2016, over the Bunaken area and over Indonesia. There's a clear drop in sea surface height over that period, linked with El Niño (Credits IRD)

Coral reefs play an important role in the marine and coastal ecosystems. Indonesia is at the core of the so-called "coral triangle", a vast area spanning Malaysia to Solomon Island, where the number of marine species is maximum.

One of the known El Niño effect on coral is the bleaching due to higher-than-usual water temperatures (the living part of the coral dies, leaving only the mineral (white) part of the coral, hence the bleaching).
However, variations in sea surface heights also occurs during an El Niño episode: increasing in the East Pacific, but decreasing in the West, close to Indonesia. Corals living just under the water surface can survive being uncovered (they usually are at low tide), but not for too long. When sea surface height is decreasing, the coral can be out of water longer than usual, thus leading to a higher coral mortality rate. An in situ survey led in 2016 showed a clear sharp horizontal limit of tissue mortality in some colonies.

Altimetry is measuring sea surface heights. Looking at the time series for the past 12 years (and more), September 2015 is the lowest sea surface height observed during those twelve years, lower of about 8 cm from the level in 2014. Using the higher resolution brought by Saral to get closer to the coasts corroborates the results obtained with gridded (open ocean) data. The same phenomenon probably occured in 1997-1998, but was not noticed at that time.

 Altimetry, by providing with an important aspect of the physical environment of corals can help understanding their mortality. With more and more coastal-processed data, more local studies should be possible, waiting for Swot high-resolution coastal imagery.

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