Seychelles Lagoon Provides Corals with a Refuge from Bleaching

Abstract EN

An extensive bleaching event in the summer of the year 1997-1998 affected most reefs along East Africa's shores.

In the aftermath of that episode, the reefs of Île Alphonse in the Seychelles were examined and it was found that reefs along the seaward slopes of the island lost >95% of their branching coral colonies, with considerably higher survival of massive species.

Île Alphonse features a nearly circular shallow lagoon, with steep seaward slopes.

Contrary to our expectations, mortality in the warmer lagoon was far lower than of coral colonies on the surrounding slopes, bathed in deeper and cooler waters.

We suggest that corals in the lagoon were protected from UV radiation by leachate stemming from seagrass leaves steeped in the lagoon.

Our measurements in the lagoon showed a strong attenuation of ultraviolet radiation, not observed in the waters outside the lagoon, and laboratory examination confirmed that the strong UV absorption of substances leached into seawater from decomposing leaves of the seagrass Thalassodendron (=Cymodocea) testudinaceum.

Our findings demonstrate the synergism between elevated seawater temperature and UV radiation in triggering bleaching on shallow reefs.