«——[·Preface·] ——»
For studies of corals at different depths (1-42 m) in the northern Gulf of Aqaba and northern in the Gulf of Aqaba, all corals have clear and regular skeletal bands.
The results showed that a high-density annual growth zone was formed in winter, and a high-density annual growth zone was formed in low-density annual growth.
Through research, it can be known that the annual average exposure rate of corals has large inter-population variation, and the variation range is 236~200mm/year.
With the increase of depth, the coral stretch rate showed a downward trend, and the simple index model explained the best effect, but in the water depth range of less than 10m, the coral extension rate can be divided into two categories: 1086254mm/year and 523199mm/year below 12m.
Light intensity appears to be the main environmental factor that reduces coral elongation rate with increasing depth, but because other environmental parameters in the Gulf of Aqaba can be ignored, the average annual rate of coral expansion recorded by time series shows a slight linear trend of increase, which may be related to the increase in sea temperature over the time period represented.
«——[Coral Research in the Gulf of Agaba.] ——»
The abundance of stony corals from the modern rocky reefs of tropical and subtropical oceans provides an important archive of past climate and ocean changes.
These corals form aragonite (CaCo) skeletons that grow at a rate of millimeters to centimeters per year, and the status of coral growth records has improved over the past few decades due to the wealth of environmental information that can be identified in coral skeletal material.
The annual density band created in the bones during growth is widely used for this purpose, as it provides a long history of coral growth and calcification (ChroNology).
Coral growth is an important parameter for assessing the effects of natural and environmental changes, which can reflect several parameters such as temperature, nutrients, food availability, water transparency, and sediment inputs.
The environmental and ecological factors that affect coral growth are almost limitless, and some of them may reduce growth rates, such as increased latitude, increased water movement, decreased nutrients, light availability, increased water depth, increased turbidity, and lower water temperature.
In the Gulf of Aqaba, the coral reef community represents the northern limit of Lake Jiaoshan (2932N) in the western Indo-Pacific region, as there is no true continental shelf in the Red Sea and Gulf of Aqaba.
And the offshore profile is very steep, so the reef is very narrow and clings to the coastline.
In this study, corals from different depths of Aqaba Reef were investigated, as well as annual bone elongation over 181 annual growth zones.
Due to their wide distribution, they represent a major reef construction for coral reefs at all depths in the Gulf of Agaba and the Red Sea, also showing clear annual growth zones and continuing to grow at higher rates.
The study area is located at the northern end of Agaba Bay, which is the direction of the desert-enclosed Red Sea extending northward, and the bay is a semi-isolated basin.
It is separated from the Red Sea by the Strait of Tiran, which is about 240 meters deep and the bay has a maximum depth of 1,830 meters, a length of 180 kilometers and a width of 26 kilometers.
The Gulf is located in a very warm part of the Sahara bioclimatic zone, with a dry climate, high evaporation, negligible precipitation (400 cm/year of rainfall), runoff (-2.2 cm/year), an annual average sea surface temperature of 23.5°C, and an average salinity value of 40.4%-40.6% of the upper water.
Due to the arid climate and due to the withdrawal of water from the surface of the nutrient-depleted Red Sea through the Strait of Tiran.
Extremely nutrient deprivation prevails in the bay, where low amounts of recycled material, high freshwater fluxes and high light transmittance have led to an extension of the depth limit of massive vegethatory corals such as porous corals to 40-50 metres.
«——[Collection and study of coral samples.] ——»
All corals surveyed in this study were obtained from deep sections of the reef complex in front of the Aqaba Marine Science Station (2927"N, 34''58"E).
Corals are found in types such as AQB-G, AQB-18H, etc., and alkaline phosphatase transponds are arranged on reefs between the reef flats and the deep front reef to a depth of 42 meters, while corals are distributed along this section at all depths.
A total of 32 polypore flora were collected along this transect at different time periods (April 1999, June 2000, May 2005 and June 2010), from 1 to 42 m, and colonies were harvested by scuba diving, removing hemispherical colonies (5-10 cm in diameter).
Sampling is taken underwater using hammers and chisels, after which the corals are washed under high-pressure tap water to remove residual organic matter and then dried in the sun.
The corals are sliced longitudinally into 4 mm thick parallel axis plates for maximum growth, and the slabs are cleaned and X-rayed with an industrial X-ray machine to obtain black-and-white positives of X-ray pictures for measuring elongation.
The annual skeletal elongation of all corals is the distance between the top edges of the low-density zone, measured directly from the positive film of the X-ray.
Each pair of high/low density zones showed an annual growth trend, and as a double test of the method, the annual extension rate of corals was calculated using So (or Sr/Ca) as the seasonal cycle.
That is, the distance from the maximum value of 0 (Sr/Ca) in a given year (corresponding to the minimum seasonal temperature) to the maximum value of a given year.
The annual mean elongation (mm/year), standard deviation (SD), minimum (min), maximum (Max) and number of growth zones (N) collected from Aqaba Reef were summarized.
X-rays showing shallow (AQB-T-10-D2 left) and deep (AQB-T-30-AL) orthoindium erupting from Abalho Reef, clearly showing the skeletal density band pattern of alternating high- and low-density bands, the main growth axis is indicated by black arrows, the black bars of which are 1 cm long.
«——[Discussion of the Coral Annual Growth Zone.] ——»
The corals surveyed in this study revealed clear and regular bone density bands of alternating high- and low-density zones, with all coral samples found between April and June.
They show high-density annual growth bands in the outermost part, suggesting that they formed during the winter.
The formation time of the annual density band obtained in this study is similar to the results of Rosenfeld et al.
Rosenfeld et al. found that low-density bones are produced in summer, while high-density skeletons are grown in the same coral in shallow and deep growth stages (6 to 40 m in the coral transplant period) in winter.
However, Klein et al. documented the deposition of high-density zones only in shallow water (3 m) colonies and abyssal (51 m) corals on the opposite shore in winter.
The study shows that sea temperature and light intensity are the main factors affecting the density distribution of coral in the Red Sea, and the sea temperature records in the study area have strong seasonal characteristics, with the highest temperature in August (26.32°C) and the lowest temperature in March (21.18C).
Annual spread, measured using radiography and SO (or Sr/Ca) curves, shows the same result.
The average annual expansion rate of 32 spotted corals, obtained from 181 annual growth zones, showed large interpopulation variations, with values ranging from 2.36 to 20.0 mm/VEAR (mean 9.48498 mm/VEAR).
The maximum elongation values for corals grown at a depth of 19 m are 200 mm/year, while the minimum elongation values for corals grown at depths of 27 m and 42 m are 236 and 240 mm/year, respectively.
These values fall within the global average of 12 mm/year for large coral under normal conditions, as well as those proposed by other authors in the Gulf of Aqaba.
Hayes et al. found that coral growth rates in shallow water in Agaba (1-3 m) vary between 864-1226 mm/year, which is high for coral reefs at this latitude.
Of greatest interest are the highest elongation rates measured (15.2 to 20 mm/year) grown in the lagoon at a depth of about 20 meters (Aq19-A, Aq19aAql9b, AQB-T20-Bl and AQB-T-20-C1).
This can be attributed to the location of these areas, as they are located in the upper part of the front reef, and the top of these colonies is about 1.5 meters above the seabed, which will improve the position of the colonies and the corals will have less wear and tear.
Scatterplot and regression relationship (standard deviation of mean) between the mean annual average extension rate (mm/year) of polyte corals and the depth of Aqaba reef (M).
Excluding the high value, high elongation value (16.3 mm/year) of coral AQB-H obtained by coral growth at a depth of 20 m, this is a huge group that only exists in seawater below 11 meters.
In the Caribbean and Indo-Pacific regions, corals are growing at an annual rate of about 10 mm/year in shallow waters and falling to 4 mm/year in deeper waters.
It is worth noting, however, that even in deeper water environments (e.g. 10-25 meters), the annual growth rate is still as high as 7 mm/year.
As Chacker et al. proposed earlier, the decline in the rate at which corals expand in deeper waters may reflect the location of a light value below which calcification decreases.
For Caribbean coral reefs, 15 meters is cited as the control depth of coral reef formation after the Holocene sea level rise has slowed, which is also the dividing line between shallow and deep reef species of many Caribbean reefs.
Overall, the rate of coral expansion obtained by this study decreases with increasing water depth (excluding high values of 19-20 m depth), and the inverse relationship between the annual average elongation and depth of coastal coral speckle corals is nonlinear and can be explained by a simple exponential model.
Coral elongation = 13.06 xexp(-0.039xd), F=0.82, coral elongation is measured in mm/year, d is the water depth in meters.
Values tend to be concentrated in two groups, one with a water depth of less than 10 m (range between 7.8-163 mm/year) and others about 5.23 mm/year and a depth of less than 12 m mm/year (range between 236-9.63 mm/year).
One-way ANOVA showed that there was a statistically significant difference in mean elongation between the two groups (p<0.0001), and environmental factors controlled the spread rate in large-scale coral populations, including water depth, sedimentation, turbidity, nutrient concentration, sea temperature and salinity.
In the Gulf of Aqaba, due to the fact that the area is a desert-surrounded sea, annual precipitation is less than 2.2 cm/year.
Thus, environmental factors such as precipitation, cloud cover and freshwater flow (land-based sediment input) and their effects on salinity, sediment and turbidity can be eliminated, so the Gulf of Aqaba is characterized by high clarity of sea water and strong light penetration.
Depth is considered a mixed environmental variable, including the effects of light, the movement of water, the residue of sediment and organic matter, and temperature and salinity in certain places.
Since the Gulf of Aqaba is isothermal over the depth range studied, it is likely that the seawater temperature effect is not the cause of the decrease in coral elongation with depth.
In the study area and the same reef transect, annual water temperature decreased by less than 0.25°C with depth between 7 m and 42 m, salinity variation was less than 0.3%, and light intensity (uE-m2s) was measured monthly by Kempman on a steep reef slope (3-40 m depth) in the study area.
Light levels have been recorded to decrease exponentially with depth, and the coral expansion rates derived from this study follow the same trend.
At a depth of 40 meters, the intensity of light drops to about 50 ms, with the smallest extension of Ruihu (2.40 mm/year).
Light therefore appears to be the main environmental factor, responsible for reducing coral elongation with a fairly constant value of depth and other parameters, and light-enhanced calcification is the main cause of reef-building coral bone growth and carbonate production.
On interannual time scales, coral expansion rates within the same group differed over the time span studied, and time series recordings showed a fluctuating pattern of elongation for shallow and dark corals, with a slight upward trend, indicating presence, although there were significant differences in elongation between them.
«——[·Conclusion·] ——»
Local sea surface temperature records (SSTs) at the study sites showed a similar upward trend between 1988 and 2009, and the correlation between SST records and the rate of expansion of the two corals was not strong (AQB-H and AQB-T-42-A2, respectively).
Several studies have documented the sensitivity of corals to temperature over a wide SST range (22°C-29C) and reported correlations between the two variables.
In our study, the interannual variation of SST over the overall period (1988-2009) was relatively small (fluctuation less than 1.2C),
The environment is the primary factor influencing the pre-mortal tension rate of corals, so a long time series record of SST and annual coral expansion rates is needed to validate this relationship in the northern Gulf of Aqaba.