Coral Reefs
General
Description Habitat
Occurrence
Most coral reefs are located within the tropics: in the Pacific ocean, the Indian ocean, the Caribbean sea, the Red sea and the Arabian gulf. Moreover, corals are also found farther from the equator in places where warm currents flow out of the tropics, such as in Florida and southern Japan. Wordwide, coral reefs cover an estimated 110 000 square miles (284 300 square meter) #2. See figure 2 #3. As above mentioned study #1 focused on the occurrence of coral reefs in the Arabian Gulf, the presented knowledge rules below are valid for this region and care should be taken to adapt them to other regions.
Environmental conditions
Coral growth in the Arabian Gulf is primarily controlled by the availability of suitable (hard) substratum, such as limestone rock, caprock with little resident sand, or rocky ledges that extend far enough above the surrounding sandy areas to be sand-free most of the time. Corals in this region are rarely found below 15 m and the most extensive formations occur at depths of less than 10 m. A total of 62 coral species have been recorded from the Arabian Gulf to date (Jones et al., 2002, #4), ranking it to one of the least species-rich sub-regions of the Indo-Pacific region. This is amongst other things caused by the extreme environmental conditions of high salinities, extreme fluctuations in water temperature and high maximum temperatures in summer, which limit coral development (Sheppard et al., 1992, #5). Coral species that are found in the Gulf, live at the edge of their physical constraints.
Increased salinities can greatly affect coral species and community composition. Species vary in their response to increased salinity. Coral species that occur in the Arabian Gulf are especially adapted to the higher salinities found there. Tolerance of the same species tends to be lower in less saline regions (Basson et al., 1977 #6). Wherever salinity rises above about 42‰ coral growth and diversity becomes severely reduced. There is a significant negative correlation of salinity with coral species diversity. Approximately 10 coral species in the Gulf thrive in salinities of up to 41‰. Up to 5 coral species in Bahrain were found to tolerate salinities up to 48‰ and three coral species tolerate salinities up to 50‰ (Sheppard et al., 1992, #5). At salinities above 48‰, a complete disappearance of corals, echinoderms and calcareous algae, together with most of the calcareous foraminifera, has been reported from Saudi Arabia (Basson et al., 1977, #6). Although larval forms generally are more susceptible to environmental stress, Edmonson (1946, cited in Johannes, 1975, #7) showed that coral planulae seemed more tolerant than adults to extremes of salinity and temperature. The response to lower salinities depends on the species. Sustained salinity below 27‰ have prevented coral reef establishment, but corals can tolerate short-term dilution to this value. Sublethal effects, such as loss of zooxanthellae and reduced settlement, in most coral species occur at salinities below 30‰ (Bell et al., 1989, #8). It is to be expected that when the salinity at a specific location is higher than 48 ‰ for more than 10% of the year, all coral growth will completely disappear. When the 90-percentile is 44‰, i.e. 90 % of the year the salinity is less than 44 ‰, the suitability of that location for coral is 50%. At salinities less than 40‰, but above 34‰salinity is not inhibiting coral growth.
The extent of enhancement of summer ambient water temperature is the critical factor in hyperthermal stress in corals (Hawker & Connell, 1989, #9). Field observations of coral mortality caused by thermal effluents from power plants have shown sublethal effects, such as depressed feeding responses, reduced growth and reproductive rates, increased zooxanthellae and mucus extrusion, and a decrease in the photosynthesis-/respiration ratio, at temperatures 3-4°C above ambient and near-total coral mortality at temperatures over 4°C above ambient (Neudecker, 1987; Grigg and Dollar, 1990). The duration for which a particular temperature prevails is as important as the temperature value itself (Sheppard et al., 1992, #5). Coral bleaching is linked to the presence of increased sea surface temperatures (SST), especially in so-called "hot-spots" where SST's have exceeded the expected yearly maximum for that location (Westmacott et al., 2000, #10). If a hotspot of 1°C above the yearly maximum persists for 10 weeks or more, bleaching is expected (Wilkinson et al., 1999, #11). This criterion can also be expressed in so-called Degree Heating Weeks (DHW). One DHW is equivalent to 1 week sea surface temperature at 1°C above the expected summer maximum. Two DHWs can indicate either 1 week of 2°C above the expected summer maximum or 2 weeks of 1°C above the expected summer maximum (NOAA, 2003, #12). Low temperature has also been shown to have sublethal and lethal effects on corals. Temperatures below 11°C in winter greatly reduce coral diversity (Sheppard et al., 1992, #5).
Because most tropical organisms, including corals, are living near their critical tolerance levels for dissolved oxygen (5 mg/l), depressed oxygen levels may constitute a significant stress (Johannes, 1975, #7). Depression is most critical at night when oxygen levels are usually at their daily minima. Thus, any environmental perturbation which lowers the oxygen concentration (such as thermal pollution or increased biological oxygen demand) exerts an effect on tropical biota. Mortality of corals is expected to occur at DO concentrations around 2 mg/l.
Control and growth opportunities
Coral reproductive methods vary according to the species. Some species are hermaphrodites other corals are gonochoric #2.
Dose-effect relations
The dose-effect relations are copied from #1.
Flow chart
Dissolved oxygen (mg O2/l) |
HSI |
|---|---|
0 |
0 |
2 |
0 |
3 |
0.5 |
5 |
1 |
10 |
1 |
Reference: #1
Uncertainty and validation
The dose-effect relations have been validated for the Arabian Gulf #1.
Applicability
These dose-effect relations can be applied to the Arabian Gulf, and probably to a broader region. However, care should be taken and the knowledge rules should be validated firstly.
Exemplary project
Arabian Gulf #1.
References
1 Erftemeijer, P.L.A and J.W.M. Wijsman (2004): "Az-Zour North hydraulic studies and marine environmental impact assessment", Delft Hydraulics repart Z3420.30
2 http://www.coral.org/resources/about_coral_reefs/coral_overview and last viewed on 11/07/2011
3 http://oceancolor.gsfc.nasa.gov/cgi/landsat.pl and last viewed on 11/07/2011.
4 Jones, D.A., A.R.G. Price, F. Al-Yamani and A. Al-Zaidan (2002): "Coastal and marine ecology", in N.Y. Khan, N. Munawar, A.R.G. Price (Eds.): "The Gulf ecosystem: health and sustainability", Backhuys Publishers, Leiden, pp. 65-103.
5 Sheppard, C., A. Price and C. Roberts (1992): "Marine ecology of the Arabian region. Patterns and processes in extreme tropical environments", Academic Press, London, 359 pp.
6 Basson, P.W., J.E. Burchard, J.T. Hardy and A.R.G. Price (1977): "Biotopes of the Western Arabian Gulf. Marine life and environments of Saudi Arabia", published by the Aramco Department of Loss Prevention and Environmental Affairs, Sharan, Saudi Arabia, 284 pp.
7 Johannes, R.E. (1975): "Pollution and degradation of coral reef communities", in E.J. Ferguson Wood and R.E. Johannes (Eds.): "Tropical Marine Pollution", Elsevier Science Publishers, Amsterdam, pp 13-51.
8 Bell, P.R.F., P.F. Greenfield, D. Hawker and D. Connel (1989): "The impact of waste discharges on coral reef regions", Water Science Technology (21), p 121-130.
9 Hawker, D.W. and D.W. Connel (1989): "An evaluation of the tolerance of corals to nutrients and related water quality characteristics", International Journal of Environmental Sciences (34), p 179-188.
10 Westmacott, S., K. Teleki, S. Wells and J.M. West (2000): "Management of bleached and severely damaged coral reefs", IUCN, Gland, Switzerland and Cambridge UK, vii and 36 pp.
11 Wilkinson, C.R., O. Linden, H. Cesar, G. Hodgson, J. Rubens and A.E. Strong (1999): "Ecological and socio-economic impacts of 1998 coral mortality in the Indian Ocean: an ENSO impact and a warning of future change?", Ambio (28), 188-196.
12 NOAA (2003): "Tropical ocean coral bleaches indices, Degree Heating Weeks methodology", National Oceanic and Atmospheric Administration (NOAA) - Coral Reef Bleaching Page. Available online at : www.osdpd.noaa.gov/PSB/EPS/method.html, last viewed on 11/07/2011.
