Thermal variations always affect the natural phenomenon whether it is land or sea. These variations bring changes on the earth features such as reduction in vegetation with increase in temperature.
At the same time these thermal variations affect the coastal ecosystem and its habitats. Coral reef, which is also the part of coastal ecosystem, will also change with the variation in temperature.
The ideal temperature for coral growth is 18oC, however many coral species can survive in water temperatures between 23oC-29oC. Bleaching can be induced by short-term exposure (i.e., 1-2 days) to temperature elevations of 3oC to 4oC above normal summer ambient values or by long-term exposure (i.e., several weeks) at elevations of 1oC to 2oC.
The rise in sea surface temperature can cause stress on Coral reef ecosystem, which may result in coral bleaching followed by coral mortality.
The role of remote sensing and GIS become very significant in such type of mapping and analysis. As remote sensing data products could be integrated in GIS with in-situ observation data and numerical simulation results for analysis and application.
Remote sensing data and GIS can be used to examine the detailed characteristics of spatio-temporal variation of thermal stress in the inner reef area and their relationship to the coral bleaching event.
To assess impact of temperature, different bleaching indices have been devised to understand the severity of stress on coral cover. So parameter can be designed to ascertain the spatio-temporal variations in the inner reef area. The input parameters included were
Observations of wind velocity
Apart from this, field observations are also required for better interpretation and analysis. The spatio-temporal variations in the water temperature for the reef area were simulated.
Case Study Example to Understand Thermal Stress Spatial Variations
For the explanation of thermal stress on coral reef, an example of Ishigaki Island has taken in this article.
To analyze data of thermal stresses, the simulation region was divided into three parts (north, middle and south), the input parameters used for the temperature simulation included the results of a hydrodynamic simulation.
The detailed temporal variations at the bottom and surface in the inner reef area and outer reef area were analyzed for the southern part of the southeast coast of Ishigaki.
Figure 1- Case Study Area
In-situ observation results indicate that temperature fluctuated greatly in late July. Hence, the numerical simulation computations were carried out for the period from July 21 to July 26, 2007, to provide a detailed and quantitative understanding of spatio-temporal variation in the fringing coral reefs.
The values of sea-water temperature recorded at 10-minute intervals during July (July 21 to July 26) were used to validate the temperature simulation results. The temperature simulation results were later converted to grid format for further analysis in the GIS environment.
In this process, bathymetry data was also used and converted to DEM. The coral cover thematic layer was restructured using temperature and bathymetry data. The temperature and bathymetry data were then interleaved in the coral cover layer using GIS functions.
Analyzed Information of Thermal Stresses
The numerical simulation results (figure 2) indicate the variation in the spatial distribution of daily sea-water temperature from July 21 to July 26, 2007,
whereas figure 3 represents the spatial distribution of inter-daily average water temperature on July 26, 2007. The in-situ results indicate that in 2007, the daily mean temperature outside the reef was approximately 29oC until July 20 and increased to 30.5oC in late July.
In the inner reef area the daily lowest temperature exceeded 30oC from July 21 and climbed up to 33.6oC in late July. The maximum average temperature was on July 26 and rose up to 37.6oC during day time (between 12:30 to 14:30).
The simulation results show an instantaneous temperature rise on July 26 and an increase in maximum average temperature to 37.6oC between 12:30-16:30 near the coast and at the reef crest (Figure 3).