Annual skeletal extension rates of the sclera-actinian corals Porites species were investigated in 32 colonies from the northern Gulf of Aqaba fringing reef at various depths (1 - 42 m). All corals reveal clear and regular skeletal density banding patterns. Results showed that the high-density annual growth bands were formed during winter and the low-density annual growth bands during summer. The mean annual extension rates of the studied corals reveal a large inter-colony variability with values ranged between 2.36 to 20.0 mm/year. While a general trend of decreasing coral extension rate with depth was observed and best explained by a simple exponential model, the rates clustered into two groups: 10.86 ± 2.54 mm/year in water depths less than 10 m, and 5.23 ± 1.99 mm/year below 12 m. Light intensity seems to be the primary environmental factor responsible for decreasing coral extension rate with depth since the effect of other environmental parameters could be neglected from the Gulf of Aqaba. Time series record of the mean annual coral extension rate showed a slight increasing linear trend which could be linked to increase seawater temperature over the period of time represented.
Massive stony corals from modern and fossil reefs of the tropical and sub-tropical oceans provide an important archive of past climate and ocean variability [1-6]. These corals build skeletons of aragonite (CaCO3) and grow at rates of millimetres to centimetres per year.
Due to the enormous environmental information that can be recognized in coral skeletal materials, the understanding of coral growth records have been increased over the last decades [6-11]. The annual density bands in massive corals that are produced in the skeleton during growth [12,13] have been widely used to achieve this purpose as it provides long-term dated histories (chronology) of coral growth and calcification [14-16].
The coral growth is an important parameter in assessing the impact of natural or anthropogenic climate and environmental changes [15,17]. It can reflect several parameters such as temperature, nutrient and food availability, water transparency and sediment input [17-19]. The environmental and ecological factors that could influence coral growth are almost unlimited in number (reviewed by [
In the Gulf of Aqaba, coral reef communities represent the northern limit (29˚32′N) for reef corals in the western Indo-Pacific region [24,25]. They are highly diverse and mostly of the fringing type, because the Red Sea and the Gulf of Aqaba are devoid of a true continental shelf and the offshore profiles are very steep, therefore, the reefs are narrow and they closely follow the shorelines [
In this study, I have investigated the annual skeletal extension rate obtained from 181 annual growth bands in Porites corals from various depths in Aqaba reef. The Porities coral spp. was chosen because it is widely distributed and represents a major reef building scleractinia at the reef throughout the Gulf of Aqaba and Red Sea at all depths, it also shows a clear annual growth bands and grows continuously at high rates. The environmental variables that may affect coral extension rates from the study area were also discussed.
The study area is located at the northern end of the Gulf of Aqaba (
The Gulf is located within the very warm portion of the Sahara bio-climatic zone. The climate is arid with high evaporation (~400 cm/year) and negligible precipitation (~2.2 cm/year) and runoff [
Extremely oligotrophic conditions are prevailing in the Gulf due to the arid climate and because it receives its waters from the nutrient-depleted Red Sea surface waters through the Straits of Tiran [
All investigated corals in this study were collected from a depth transect from the reef complex in front of the Marine Science Station in Aqaba (29˚27′N, 34˚58′E), (
A total of 32 columns of Porites spp. colonies (
The corals were sectioned longitudinally into slabs of 4 mm thickness parallel to the axis of maximum growth. The slabs were cleaned and X-rayed using industrial X-ray machine. Black-and-white positive prints of the x-radiographs were obtained for extension rate measurement.
The annual skeletal extension rates of all corals were directly measured along the major (vertical) growth axes (
lowing year [31-33]. The mean extension rate of a given colony was calculated as the average of its annual values.
Corals investigated in the present study reveal clear and regular skeletal density banding patterns of alternating bands of high and low-density (
The timing of annual density bands formation from this study is similar to that obtained by Rosenfeld et al. [
It was suggested that seawater temperature and light intensity are the major factors influencing the density patterns in Porites corals from the Red Sea [
The annual extension rates measured using the x-radiography and the δ18O (or Sr/Ca) curves reveal identical results. Mean annual extension rates of the 32 Porites spp. corals as obtained from 181 annual growth bands reveal a large inter-colony variability with values ranged between 2.36 to 20.0 mm/year (average 9.48 ± 4.98 mm/year). The maximum extension rate value of 20.0 mm/year was recorded in corals growing at a depth of 19 m and the minimum of 2.36, 2.40 mm/year in corals from 27 and 42 m depth, respectively (
Most interesting, is the highest extension rates (15.2 - 20 mm/year) measured in corals growing at depths around 20 m (Aq19-A, Aq19a, Aq19b, AQB-T20-B1 and AQB-T-20-C1). This could be attributed to the position of these colonies, since they stand in the upper fore reef and the top of these colonies is about 1.5 m above the sea bottom. This will elevate the colony to a position of either more light, less abrasion by moving sand or both [
high extension rate value (16.3 mm/year) obtained from coral AQB-Big which is a huge colony and lies only 1 m below seawater surface. Similarly, Heiss [
In Caribbean and Indo-Pacific regions, Porites corals exhibit annual growth increments of about 10 mm/year for shallow water region and a decrease to 4 mm/year in more deeper waters. However, it is noticed that even in the deeper water environment (e.g., 10 - 25 m), the annual growth rates is still high with up to 7 mm/year [
In general, coral extension rates from this study decreased with increasing water depth (excluding the high values from 19 - 20 m depth). The inverse relationship between mean annual extension rates and depth for Porites spp. corals is non-linear and can be explained by a simple exponential model as follows (
where coral extension rate in mm/year and d is the water depth in meter.
The values tended to cluster around two groups, one at 10.86 mm/year in water depths less than 10 m (range between 7.8 - 16.3 mm/year), and the other around 5.23 mm/year below 12 m depth (range between 2.36 - 9.63 mm/year). The analysis of variance (one way ANOVA) showed that there is statistically significant difference in the mean extension rate of corals between the two groups (p < 0.0001).
The environmental factors that may control extension rates in massive coral include water depth, sedimentation and turbidity, nutrient concentration, seawater temperature and salinity [17,44,45]. In the Gulf of Aqaba, environmental factors such as precipitation, cloud cover and freshwater discharge (terrigenous sediment input) and its effect on salinity, sedimentation and turbidity can be eliminated since the area is a desert-enclosed sea, and annual precipitation is less than 2.2 cm/year [
Depth is considered as a mixed environmental variable including the effect of light, water movement and resuspension of sediment and organic matter, and in some places temperature and/or salinity [10,14]. Seawater temperature effects are most probably not the reason for the decrease in coral extension rate with depth in this study since the Gulf of Aqaba is isothermal over the studied depth range. In the study area and from the same reef transect, the annual water temperature decreases with depth between the depths 7 and 42 m by less than 0.25˚C, and the salinity variations was less than 0.3‰ [
At 40 m depths, light intensity drops to about 50 mE·m−2·s−1 in which extension rate of corals is minimum (2.40 mm/year). Therefore, light seems to be the primary environmental factor responsible for decreasing coral extension rate with depth and other parameters are fairly constant throughout the water column from the Gulf of Aqaba. Light enhanced calcification is responsible for most of the skeletal growth and carbonate production of reef building corals [47,48].
In inter-annual time scale, coral extension rate varies within the same colony over the studied time span. The time series record showed a fluctuated pattern with slightly increasing trend of the extension rate in both shallow and deep corals for the period of time represented despite the significant difference in extension rate between them (
Local sea surface temperature record (SST) from the study site shows similar increasing trend between 1988- 2009 (
The reef building Porites spp. corals from Aqaba show clear and regular skeletal annual density banding patterns, the high-density growth bands seems to be laid down during winter and the low-density bands during summer. The average extension rates for Porites corals are around 10.86 ± 2.54 mm/year in water depths less than 10 m, and decreases to 5.23 ± 1.99 mm/year between 12 - 42 m. The relationship between mean coral extension rate and depth is best explained by exponential model as a response to decreasing light availability and photosynthesis with depth. However, the effect of other environmental parameters such as sedimentation and turbidity could be neglected throughout the water column and over the studied depths. On inter-annual time scale, seawater temperature variability seems to have an effect on the mean annual coral extension rates suggesting the sensitivity of Porites spp. corals from the Gulf of Aqaba to global warming effects.
I wish to thank Abdullah Al-Momany and the Marine Science Station diver staff in Aqaba for their assistance in coral samples collection. Dr. Riyad Manasrah kindly provided seawater temperature data from Aqaba. Special thanks are also due to Dr. Thomas Felis for facilitating
coral slicing and X-radiography work at the University of Bremen, Germany.