Bleaching in Southern Arabia

[Simon Wilson]

BLEACHING IN DHOFAR PROVINCE, SOUTHERN OMAN, ARABIAN SEA

BACKGROUND
The greatest development of coral communities along this mainland coast of
southern Oman occur at the Mirbat peninsular centred at Rahaa (N16° 56.6
E054° 48.3) where incipient fringing reefs have formed. Elsewhere in
Dhofar, coral communities grow on rocky substrates without accreting reef
framework. This region is affected by intense upwelling of cold,
nutrient-rich water during the SE monsoon season between June and September
when surface temperatures drop as low as 18º. The Dhofar region supports
the greatest diversity of corals in Oman waters, where a total of 42 genera
are known to occur.

BLEACHING
Extensive bleaching was observed at eight sites around the Mirbat peninsula
between 22nd and 26th May 1998 just prior to the onset of the SE monsoon
season. Recorded surface sea water temperatures at these sites varied
between 29.5 and 31.5°C. Approximately 40  kilometres to the east at Sudh
(approx N16° 59.5 E055° 10.5) surface sea water temperature was recorded at
25°C during the same period. No bleaching was observed at this site or any
other site (N=6) in this area where surface seawater temperatures varied
between 25 and 25.5°C.

Genera affected included Acropora, Porites, Favia, Favites, Goniopora,
Coscinarea, Pavona, Montipora, Pocillopora, Leptastrea, Cyphastrea and
Stylophora. Water depths to a maximum of 5 metres were surveyed. Stylophora
appears to be one of the most abundant of coral genera growing in shallow
waters (1-2 metres depth) in the area. All colonies of this genus were
found to be bleached. Between 75% and 95% of all other coral genera
observed were bleached. 50% of large colonies (>1m diameter) of Porites
were partially bleached. Epiphytic algal growth was observed on recently
dead Porites, Acropora and Cyphastrea colonies. Approximately 95% of coral
colonies examined were still living despite the expulsion of zooxanthellae.

No bleaching has been observed or reported in the Muscat Area, Gulf of Oman
(N23° 37.7 E058° 34.9, approx 1000km to the NE) where surface seawater
temperatures at 2 sites on 4th June were recorded as 30.5 and 30 °C.

Robert Baldwin and Simon Wilson 
PO Box 2531 
CPO 111
Seeb 
OMAN