[Coral-List] Coral Bleaching in Miri

[Peter Heyes]

Hi,

 

As a group of divers we are struggling to understand why we have experienced
coral bleaching on the reefs offshore Miri, Sarawak in 2009. The bleaching
is unexpected in the sense that the NOAA-published sea surface temperatures
have not been more than 0.5C above the long term average and the DHW values
are all low, below the lowest NOAA thresholds. The last bleaching event was
in 1998.

 

The Miri reefs are in the proximity of N 4.4, E 113.8. The reefs are all
patch reefs varying in depth from 5m on the inshore reefs (8km from the
shore) to 30m on the further reefs (25km offshore). The closest inshore reef
is a mixture of hard and soft corals but most reefs are dominated by hard
corals. Generally hard coral cover is more than 50%. There is some silt on
the inner reefs.

 

The bleaching started in a small way in June 2009 and by the end of July it
was very noticeable. On the 20m reefs the extent of bleaching is 40% (Reef
Check method) and on the 12m reefs, 5 to 20%. The reef closest to shore has
experienced least bleaching. The corals affected are mainly encrusting and
plate corals. Some soft corals on the shallower reefs have also bleached. On
most reefs the branching corals have not bleached although we have seen
branched coral bleaching on a 20m reef. Some anemones have bleached as have
some sea whips. Details of the bleaching and substrate compositions are on
the ReefBase GIS bleaching reports.

 

We had unusually heavy rain in January but the rest of the year has been
more normal with a dry spell in June and July.

 

The closest river is the Sungai Miri which is quite polluted but has a
limited flow. We are quite close to the much larger Baram outflow and water
from the Baram does reach the reefs at some times of the year. 

 

Until 2009 the levels of nutrient indicator algae have been low,
particularly on the 20m reefs, 25km offshore. The inner reefs have seen some
increase in NIA in 2009 but the 20m outer reefs have seen a significant
change. Sea grapes and dictyota algae have appeared for the first time and
in substantial amounts. 

 

The early season diving did not suggest anything unusual was likely to
happen but we did see the outer reefs experience a significant "green water"
event all the way down to the reef in mid-February. This kind of water is
usually only in the top 5 or 10m and usually affects diving on the inner
reef.

 

Please let me have your thoughts on the likely causes. 

 

If you have seen similar bleaching, I'd appreciate hearing of the outcomes
in terms of coral recovery or decline.

 

Is the anything we should be measuring or assessing to better understand the
problem?

 

Thanks

 

Peter