June 28, 2007

Reef Check Egypt: Extreme low tide caused coral death at South Sinai’s coast

Text and photos by Christian Alter
Red Sea Environmental Centre, Dahab
Tidal events belong to the most predictable natural fluctuations in coral reef habitats. They determine intertidal zonation patterns and limit the vertical growth of corals, but are rarely reported to cause mass mortality among corals. Corals are reported to tolerate a certain time of aerial exposure while enhancing mucus production to prevent desiccation. However, the combination of extreme low tides and high solar irradiances has the potential to cause widespread damage among corals. A report from the Great Barrier Reef reveals 40-75% of corals on reef flats were either bleached or suffered partial mortality from such an event.

We observed a similar phenomenon on reef flats in Dahab at the end of March this year and assume an additional factor has contributed to the coral mortality during this event. Within four days (March 19-22), absolute calm conditions coincided with extreme low tides and high solar irradiances. Moderate to strong wind speed would produce waves, surf and spray which may prevent corals from drying out and decrease the effect of strong insolation. We observed the coral mortality on reef flats of various sites in Dahab. Most likely this natural disturbance affected the whole coastline of the Gulf of Aqaba.

Many of the smaller coral colonies were killed completely whereas most of the larger ones only suffered partial mortality. It seems that coral tissues disintegrated and formed shreds hanging from the coral before getting washed away. First, the wall of coral skeleton became visible while tissue was still left inside. Understandably, the upper portions of colonies affected were more heavily damaged. Coral bleaching, in the sense of corals having ejected their symbiotic algae while retaining their elsewhere intact tissue, was not observed.

Partly affected colonies certainly are able to recover to a certain extent but have to struggle against algae quickly taking possession of any part of stripped skeleton. After one week all the affected colonies were tinted in shining dirty yellowish-green hues covering the white. We estimate the natural damage to coral colonies on the reef flat to be in the order of 25-75%. The first survey we did after the event revealed a mortality of 50 % at a known reef site south of Dahab.

Supplementary note:

We wish to underline that the observed decimation of coral cover has been caused entirely by natural processes. The observed phenomenon may not be distributed equally among reef flat zones due to natural variations in geomorphology and, thus, various degrees of exposure. The observed event and its assessment described in this article is restricted to relatively shallow reef zones. However, the observed decimation may not be very obvious and only recognized by trained persons.

Coral mortality, caused by the same low tide event was also observed by Dr. Moshira Hassan during an excursion with biology students from the American University of Cairo to Marsa Ghozlani (Ras Mohammed National Park) in early April this year. Similar events have been described earlier by the scientific community e.g. for the Red Sea (Fishelson, 1973) and the Great Barrier Reef (Anthony & Kerswell 2007).

Anthony, K. R. N. & A. P. Kerswell (2007) Coral mortality following extreme low tides and high solar radiation. Springer, Marine Biology, Vol. 151: 1623-1631.

Fishelson, L. (1973) Ecological and Biological Phenomena Influencing Coral-Species Composition on the Reef Tables at Eilat (Gulf of Aqaba, Red Sea). Marine Biology 19: 183—196.

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Aerially exposed coral colony (Pocillopora verrucosa) on the first day Affected colony (Platygyra daedalea) on the second day. Bare skeletal parts are clearly visible on the upper part.
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Almost entirely killed colony (Pocillopora verrucosa) after three days Four affected colonies on the reef flat in Dahab
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After one week, the bare skeleton of affected colonies is covered with green algae After 2 weeks, a layer of algae covers the recently killed corals