**2. Use of remote sensing in coral reef ecosystems**

The observation of the earth using remote sensors is a most complete method for monitoring the most significant natural risks (Xin et al., 2007). In general, RS has proven to be a powerful tool in the overall understanding of natural and anthropogenic phenomena. It is particularly appreciated as a non-invasive, non-destructive technique with global coverage. Thus, satellite, airborne and *in-situ* radiometry have become useful tools for tasks such as characterization, monitoring and the continuous prospecting of natural resources.

Research using RS has been strengthened in recent decades as a result of the growing concern worldwide for the preservation of coral reef systems as natural reservoirs. This has been observed to be an excellent method for analysis, which aids in the holistic study of this complex ecosystem. In order to develop an approach that helps to safeguard these ecosystems, it is necessary to understand the physical, chemical, biological and geological dynamics that occur therein (Brock et al., 2006). Andréfouët & Riegl (2004) refer to RS as a technology that is now virtually mandatory for research where spatial and temporal precision is required. RS has gone from being a tool with no application to coral reef systems to one that is *per se* indispensable. Andréfouët & Riegl (2004) discuss four reasons why this change has occurred:


RS techniques offer an option for marine habitat mapping to determine not only the location and amount of different benthic habitats (Kirk, 1994) but also how these habitats are distributed and the degree of connectivity among them (Rivera et al., 2006). Nevertheless, the study of coral reefs using RS presents several important limitations. For example, intense cloud cover in optical images, optical similarities among spectral signatures of benthic communities, attenuation of the deep component (specific to each coral reef ecosystem) as well as the spatial and spectral resolution of remote sensors. In spite of these limitations, satellite sensors are highly useful for mapping the benthic bottom (Mumby et al., 1997), monitoring changes in its ecology (Krupa, 1999) and defining management strategies (Green et al., 1996).

#### **2.1 Determination of ecological characteristics of coral reefs using remote sensors**

Some of the characteristics of coral reefs that can be calculated using RS are temperature, wave height, sea level, turbidity, amount of chlorophyll and concentration of dissolved organic matter. In the case of atmospheric variables, it is possible to determine cloud cover, amount of seasonal rainfall, presence of contaminants and incidental solar energy (Andréfouët et al., 2003). All these factors directly and indirectly influence coral reefs and determine their health status (Andréfouët & Riegl 2004). In addition, it is possible to determine the different benthic ecosystems present in the coral reefs, such as seagrass, type of bottom, algae communities and different types of coral. If the reef is near a tourist or vacation area, anthropogenic impacts can be determined by calculating the growth of the

The observation of the earth using remote sensors is a most complete method for monitoring the most significant natural risks (Xin et al., 2007). In general, RS has proven to be a powerful tool in the overall understanding of natural and anthropogenic phenomena. It is particularly appreciated as a non-invasive, non-destructive technique with global coverage. Thus, satellite, airborne and *in-situ* radiometry have become useful tools for tasks such as

Research using RS has been strengthened in recent decades as a result of the growing concern worldwide for the preservation of coral reef systems as natural reservoirs. This has been observed to be an excellent method for analysis, which aids in the holistic study of this complex ecosystem. In order to develop an approach that helps to safeguard these ecosystems, it is necessary to understand the physical, chemical, biological and geological dynamics that occur therein (Brock et al., 2006). Andréfouët & Riegl (2004) refer to RS as a technology that is now virtually mandatory for research where spatial and temporal precision is required. RS has gone from being a tool with no application to coral reef systems to one that is *per se* indispensable. Andréfouët & Riegl (2004) discuss four reasons why this

The proliferation of new sensors for acquiring direct and indirect data for monitoring

 The proliferation and improvement of analytical, statistical and empirical approaches, Recognition of global climate change due to anthropogenic human impacts that are

Improved integration of technology for the conceptual design of coral reef research.

**2.1 Determination of ecological characteristics of coral reefs using remote sensors**  Some of the characteristics of coral reefs that can be calculated using RS are temperature, wave height, sea level, turbidity, amount of chlorophyll and concentration of dissolved organic matter. In the case of atmospheric variables, it is possible to determine cloud cover, amount of seasonal rainfall, presence of contaminants and incidental solar energy (Andréfouët et al., 2003). All these factors directly and indirectly influence coral reefs and determine their health status (Andréfouët & Riegl 2004). In addition, it is possible to determine the different benthic ecosystems present in the coral reefs, such as seagrass, type of bottom, algae communities and different types of coral. If the reef is near a tourist or vacation area, anthropogenic impacts can be determined by calculating the growth of the

RS techniques offer an option for marine habitat mapping to determine not only the location and amount of different benthic habitats (Kirk, 1994) but also how these habitats are distributed and the degree of connectivity among them (Rivera et al., 2006). Nevertheless, the study of coral reefs using RS presents several important limitations. For example, intense cloud cover in optical images, optical similarities among spectral signatures of benthic communities, attenuation of the deep component (specific to each coral reef ecosystem) as well as the spatial and spectral resolution of remote sensors. In spite of these limitations, satellite sensors are highly useful for mapping the benthic bottom (Mumby et al., 1997), monitoring changes in its ecology (Krupa, 1999) and defining management strategies (Green

characterization, monitoring and the continuous prospecting of natural resources.

**2. Use of remote sensing in coral reef ecosystems** 

change has occurred:

coral reefs,

et al., 1996).

lethal to coral reefs and

urban stain, vegetation coverage, the structure of the hydrographic basins, etc. Intrinsic conditions of coral reefs can be described, which are largely defined by the inflows and outflow and their transport of sediments and export of dissolved organic matter. This enables us to understand the patterns involved in coral whitening, among other events (Brock et al., 2006).

The coral reefs—located in relatively clear water—allow us to use passive optic sensors (Benfield et al., 2007). The more common satellite sensors that have been used to study this are SPOT, Landsat TM and ETM+ (Andréfouët & Riegl 2004; Benfield et al., 2007; Mumby 2006; Mumby et al., 2004; Mumby and Harborne 1998). Studies previously conducted (Green, 2000; Mumby et al., 1999) have observed that Landsat and SPOT images are suitable for mapping corals, sands, and seagrass, depending on their resolution. Nevertheless, it is important to note that various types of habitats can be represented in one Landsat image pixel (or others with less spatial resolution), which may limit classification abilities (Benfield et al., 2007). Previous studies conducted (Green, 2000; Mumby et al., 1999) have observed that according to the resolution of Landsat images, they are suitable for mapping sea corals, sands and seagrass. Based on this assumption, the data obtained from Landsat and SPOT are adequate for simple complexity mapping (3-6 classes, such as seagrass, sand, dead corals and some species of corals) but for more complex targets (7-13 classes) they are limited by their spatial and spectral resolution. (Mumby, 1997; Andréfouët et al., 2003; Capolsini et al., 2003). To a lesser extent, SeaWiFS (seaviewing wide field of view sensors) have also been used, as well as IKONOS with higher spatial resolution, LIDAR and SONAR, among others (Andréfouët & Riegl 2004; Andréfouët et al., 2003; Brock et al., 2006; Elvidge et al., 2004; Liceaga-Correa & Euan-Avila, 2002; Hsu et al., 2008; Lesser and Mobley, 2007). It is important to note that analytical methods as well as spatial modeling, statistics and empirical methods at different scales and for different applications have been used in direct relation to ecological processes of reefs (Andréfouët & Riegl 2004). The use of airborne remote sensors, such as CASI (Compact Airborne Spectrographic Imager) with a high spectral or hyperspectral resolution, has gradually been increasing in this type of studies, to the extent that the specialists mention that mapping reefs using air or satellite sensors have proven to be more effective than fieldwork (Mumby, 1999). Nevertheless, field measurements cannot be discarded, since they provide us with the basis for corroborating the information obtained from satellite images. In addition, images from satellite sensors provide the opportunity to conduct multi-temporal monitoring (Helge et al., 2005) in order to identify the status of an ecosystem and predict possible future changes.

According to the above, it can be stated that studies applying RS in coastal ecosystems and, specifically, in coral reef ecosystems provide information and knowledge that can successfully be applied to define management strategies for these important ecosystems, as well as to design viable alternatives for their conservation.
