HIGHER MORTALITY BUT INCREASED GROWTH RATES OF CORAL ON IMPACTED REEFS AT JEDDAH, SAUDI RED SEA

The coast of Saudi Arabia contains many of the best developed fringing reefs in the Red Sea. However, coral communities on reefs closest to central Jeddah have been badly affected by a wide range of impacts including extensive coastal infilling, discharge of untreated as well as treated sewage, unregulated fishing, coral bleaching and disease, and outbreaks of Crown-of-thorns starfish. Coral growth has been widely used as a proxy for reef health. We used an experimental approach to investigate the effects on coral growth rates and mortality of the various impacts affecting different portions of the coast.

Abdulmohsin Al Sofyani1, Rupert Ormond1,2, Khalid ZubIer1, Adnan Turki,1 Mohammed Mudarris1, Jerald Wilson1, Salman Al Ahmadi1, Jaffar BaOmer1, Adam Porter1,3 and Martin Eaton 1.

1 = Faculty of Marine Science, King Abdulaziz University, Jeddah, 21859, SAUDI ARABIA; 2 = Centre for Marine Biodiversity & Biotechnology, Heriot-Watt University, Edinburgh, EH14 4AS, UK; 3 = College of Life & Environmental Sciences, University of Exeter, Exeter, EX4 4QD, UK

Work was undertaken at 6 different sites, stations A & B subject to little if any impact, stations C & D subject to infilling and domestic sewage, and stations E & F subject to discharge of municipal sewage and pollution from the city’s port. Growth rates of fragments at each station were monitored using both photographic and weighing methods and growth rates of adjacent corals, growing naturally on the reef, using both direct measurement of branches and alizarin staining. Concentrations of nutrients (nitrate, nitrite, phosphate) and some pollutants were measured in water samples taken monthly from each station at 1, 5, 10 and 20 m depth, and compared with those at matching locations 2km further offshore . The bacterial fauna of the corals was also investigated using next generation RNA analyses.

Results revealed that at the more impacted sites, where nutrient and pollution concentrations were up to 100% higher than at control or offshore sites, the mean rates of coral growth, especially of P. verrucosa, were up to 50% higher. However, coral mortality rates at impacted sites were also up to two to three higher (especially for A. hemprichi) than at the unimpacted control sites. The bacterial studies revealed a shift in bacterial fauna at the control sites, even on apparently healthy corals.

Sharm Obhur Creek

reef edge

original shore line

A

B

C

D

E

F

Sharm Obhur Creek

Nitrate Levels In & Offshore at Stations

A B C D E F

Station

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0.1

0.2

0.3

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0.5

NO

3 µ

mo

l-1

In/Offshore Reef Inshore

In/Offshore Reef Offshore

F(5, 276)=1.57, p=0.2

Vertical bars denote +/- standard errors

The coast extending north from Jeddah showing (left) the extend of coastal infilling over the fringing reef, and (b) the location of six experimental sites: A & B = control sites well north of Jeddah, C & D = sites adjacent to residential area subject to infilling and domestic sewage discharge, E & F = sites opposite city centre subject to pollution from port and discharge of municipal sewage.

Mid-water culture of coral fragments undertaken at experimental sites. Left: part of rig showing corals, sediment traps and temperature logger. Upper right: healthy colony of P. verrucosa after considerable growth. Lower right: A. hemprichi colony showing disease like mortality.

Contrasting coral assemblages occurring on adjacent reefs at Sites A and C. Note hard coral dominated community at unimpacted Site A, and soft coral dominated community at Site C. Sites E & F, subject to the greatest range of impacts, had low levels of both hard and soft corals.

The differences in survivorship between impacted and control sites seems likely to reflect a reduced health of the cultured corals, coupled with increased growth rates of competing turf algae. The shift in associated bacterial fauna at the impacted sites seems to be a consequence of declining coral condition, that may in turn allow pathogens to become more prevalent, so causing increased coral mortality.

The increased growth rates of corals at impacted sites could be either a direct effect of increased nutrient levels perhaps leading to increased productivity of symbiotic zooxanthellae or the result of heterotrophic feeding on the greater amounts of particulate organic material (POM) associated with input of domestic sewage. The latter seems the likely cause of the shift to the soft coral-dominated community observed at stations C & D.

Left: mean nitrate concentrations at stations A to F and at matching stations 2 km offshore (note high concentrations at stations C & D). Right: survivorship of cultured A. hemprichi fragments at stations A to F (note greater mortality at stations C and E).

Left: cumulative growth of P. damicornis fragments at stations A to E (note increasing growth at stations C through E). Right: growth rate of selected P. damicornis colonies growing naturally on the reef at stations A to F, measured using the alizarin method.

Acknowledgements: The work was funded by His Royal Highness Prince Khaled bin Sultan bin Abdulaziz, as part of the programme of the Chair in Conservation of the Coastal Marine Environment at King Abdulaziz University, Jeddah, Kingdom of Saudi Arabia. The authors are most grateful to His Royal Highness for his sponsorship.