The Shallow and Deep Bathymetry of the Banco Chinchorro Reef in the Mexican Caribbean

BULLETIN OF MARINE SCIENCE, 73(1): 15–22, 2003

THE SHALLOW AND DEEP BATHYMETRY OF THE BANCO CHINCHORRO REEF IN THE MEXICAN CARIBBEAN

Alicia González, Daniel Torruco, Angeles Liceaga and José Ordaz

ABSTRACT This paper presents the use of two complementary remote sensing techniques in the Banco Chinchorro: satellite image and echosound. A cartographic relation was estab- lished between the shallower reef depths recorded by Landsat-MSS and those of greater depth recorded with echosound. A bathymetric map of the shallow lagoon areas and reefcrest was generated from the Landsat-MSS image using exponential correspondence between reef depths and corrected digital values in the image. The reef lagoon exhibits a bathymetric gradient from 1 m in the north to 25 m in the south. The echosound data show a pinnacle with depths greater than 300 m. Largely due to intense coral growth, the most pronounced slopes are found near the leeward reefcrest, and the more gradual on the windward margin. The use of these two complementary remote sensing techniques proved an efficient tool for characterizing reef morphology when depth strongly affects the efficacy of one or the other technique. The analytical delimitation of this complex reef is increasingly important because of its recent designation as a biosphere reserve area by the Mexican government. The present study is a pioneering effort that could be used as a pilot in future administrative, conservation, and sustainable management studies.

Bathymetry is one of the most relevant aspects in the dynamic ecology of coral reefs. Numerous reef studies show that coral species diversity tends to increase as a function of depth, reaching its maximum between 20–30 m (Loya, 1972; Sheppard, 1980; Huston, 1985), and diminishing with greater depth. This depth effect results in a marked zonation of the reef community. The upper depth limits of hermatypic corals are controlled by various physical and biological factors; whereas, their maximum depth depends largely upon light availability (Sheppard, 1982; Kühlmann, 1983). Important coastal and insular reefs are located in the Mexican Caribbean. Banco Chinchorro is Mexico’s largest reef complex as well as one of its least known, thus least altered. It has great regional and national importance due to its high biological diversity, its value to tourism, and its commercial uses. As a conservation and sustainable-use strat- egy, the reef has been declared protected under the biosphere reserve concept (Diario Oficial, 1996). In some parts of the Mexican Caribbean large-scale bathymetric soundings have been completed (Jordán, 1993) with greater detail in specific areas such as Cozumel Island (Muckelbahuer, 1990a). However, the only bathymetric soundings for Banco Chinchorro were done in 1888 (British Admiralty Chart No. 1796), and only for the reef lagoon. Additionally, the use of multispectral images in marine resources ecology evaluation has been a successful tool for obtaining biological and geomorphologic information in shallow, transparent waters (Jupp, 1983; Lyzenga, 1985; Kirkman et al., 1988). However, water-depth estimation accuracy using reflected radiance is influenced by suspended matter, waves, bottom reflectance, and other factors such as wind and illumination conditions (Luczkovich et al., 1993). In terms of physical principles the light that penetrates the water is a function of wavelength; a clear water column rapidly attenuates long wave-

15 16 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 1, 2003 lengths; therefore, reflected blue waves are used most often as they experience less attenuation in water and, in some cases, reach depths close to 30 m (Green et al., 1996). Within the context of conservation and sustainable use, and in the search for low-cost tools that synthesize information from large areas, a bathymetric study of the reef was made using multispectral images (MSS) in the shallow waters of the reef complex and echosound transects to 300 m in the external reef. The present bathymetric study provides a base for future studies of this very important reef complex.

MATERIALS AND METHODS

STUDY SITE.—The Banco Chinchorro reef complex is located approximately 24 km off the south- east coast of the Yucatán Peninsula, between 18∞47'–18∞23'N and 87∞14'–78∞27'W (Fig. 1). The complex is atoll-like with four islands: two in the north (both known as Cayo Norte), one in the center (Cayo Centro) and one in the south (Cayo Sur), all separated from the coast by a channel approximately 1000 m deep. It is the largest reef system in Mexico with 814.2 km2 total area, including the reef lagoon and reefcrest. The great variety of habitats found in the reef complex include a shallow lagoon reef (Torruco, 1995), physiographically complex fringe reefs, a network of mangrove islets, and a shallow lagoon microsystem on the largest island (Cayo Centro). The greatest coral reef growth is present south of the windward zone. It is a commercially important fishing area for species such as queen conch (Strombus gigas), spiny lobster (Panulirus argus), and various fish. Cayo Centro supports a number of ecologically important relic species, among them American crocodiles (Crocodylus acutus). LAGOON AND REEFCREST BATHYMETRY.—A Landsat-MSS image from 2 April 1997 was used to generate the bathymetry for the lagoon and reefcrest. These images did not include the blue electro- magnetic spectral band-like TM image, which has the highest light penetration. Consequently the green band and first principal component were used. The geographic reference for the image was done using the British Admiralty Chart No. 1796, preliminarily corrected with a GPS geo-refer- enced map. Using a false-color analysis of the map, spectrally different areas were identified. Ran- dom points were then selected for each area, and each point’s depth verified in the field and compared to the map. The error estimate for the depth classification was evaluated using 400 random depth verification points and both data sets were compared with a student’s t-test (P = 0.95) and did not show significant differences. Map points were then selected in areas of homogeneous and heterogeneous depths and treated as accurate data. The bathymetric model classifies a spectral band assuming exponential correspondence between depth (Z) and the radiation values (VS) corrected for reflected and/or atmospheric radiation (Lyzenga, 1985), and the water column and surface (Eastman and McKendry, 1991). The equation used is:

zbm- Ve=+() VS Eq. 1

and the linear expression

Zm=- ln() VVSb+ Eq. 2 where z is depth in meters, m is the coefficient function of water attenuation, VS is the reflected and/ or atmospheric radiation values, V is the digital image radiation values, and b is the sensitivity factor due to solar radiation and the sensing equipment. For sensing purposes atmospheric scattering is subtracted from V. The approach selected for this study was a model of a single linear regression using the first principal component. PCA has been used previously to integrate radiometric variance into a few components (Chuvieco, 1990). GONZÁLEZ ET AL.: BATHYMETRY OF BANCO CHINCHORRO 17

Figure 1. Location map of the study area.

EXTERNAL REEF BATHYMETRY.—To record the bank area bathymetry in 1997 (Fig. 2A), 40 echosound transects with 10 stations each were performed using an Echosound Lowrance X-16 with 0.3–760 m range, an object resolution of up to 0.1 m, and a recording resolution of 1000 lines (250 dpi). Recording accuracy depended upon depth, signal frequency, turbulence, and other physical factors of the water mass, such as transparency and thermoclines. The depth scales and inferior and supe- rior margins can be adjusted with interval amplitude. A graphic range of 0–30 m with pulse longi- tudes of 100 s-1 was used until this depth had been reached, when the interval was changed to 30– 100 m, and the automatic pulse longitude used. The geographic position of each reading was recorded with a portable satellite locator Magellan GPS 5000 D. To obtain a topological representa- 18 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 1, 2003

tion of the reef, the bathymetric data were processed with the Surfer v. 6.1 graphic program. Uni- versal kriging with linear variogram was selected as the gridding method, because this is one of the best methods for the interpretation of 400 observations, considering that each transect can be treated as a gradient (Keckler, 1994). This technique provided the best unbiased lineal estimate with variance similar to zero and a high statistical robustness (Trangmar et al., 1985).

RESULTS

LAGOON AND REEFCREST BATHYMETRY.—An analysis was made of the MSS image results in eight radiometric classes. Through various ecological studies and field data it was possible to satisfactorily corroborate the bathymetric correspondence with the British marine chart for these reef zones (Novak et al., 1992; Torruco, 1995; Torruco et al., 1996). The satellite image base map facilitated analysis by making available a spectrally heterogeneous sample (Ahmad and Neil, 1994; Mumby et al., 1997a). The British marine chart shows evidence of little change in the northeast lagoon area, which was confirmed during the depth classification. Given that the depth evaluation is point-based, that is to say the calculation only involves the radiometric value, the islands Cayos Norte, Centro and Sur were masked. Consequently the interior lagoons depths were not obtained for Cayo Centro. The VS values were obtained from the same image and were calculated using radiation values for water less than 30 m deep. Then the bathymetric map was compared to the results obtained with the green band and first principal component. The lagoon reef bathymetry is heterogeneous, with depths ranging from 1– >15 m, and the classification of the spectral analysis recorded a total of 6 depth intervals (Fig. 3). The most frequent depth intervals were those between 1–7 m (396.87 km2 of the total area) and 8–13 m (260.84 km2), and the least frequent those less than 1 m (40.28 km2) and greater than 13 m (32.16 km2). The islands cover 6.38 km2 of the area, and the image cloud cover was a small area 2.72 km2. However, the north–south bathymetric gradient was quite conspicuous. The shallow depths are defined by extensive sandy bottoms in the north portion of the lagoon, whereas the southern lagoon area is deeper, with more frequent coral masses and patches. There is a large fissure within depths of 7–13 m in the central zone that extends to mid-lagoon and even into part of the northern zone. EXTERNAL REEF BATHYMETRY.—In almost the entire bank area and a short distance from the reefcrest, the bathymetric profiles show steep submarine slopes with depths greater than 100 m. The eastern margin slope is more gradual than the western as it has an extensive plane with intense coral growth, this side also presents profound valleys and wider peaks (Fig. 2B). In contrast, the western margin has an abrupt scarp approximately 300 m from the reefcrest. The northeast bank is less drastic as a sandy plain attenuates the submarine cliffs. On both sides the picture already showed narrow fractures with great depth.

DISCUSSION

Although the present study results are only partially satisfactory when a simple error estimation is applied, the model is valid and the depth classification reflects the actual bathymetry of this area, as shown by an almost continuous depth gradient in the lagoon. To create a more detailed bathymetric classification, the spatial resolution of the image would need to be amplified (scale of 1: 250,000) and coupled with extensive fieldwork (Mumby et al., 1997b). It is important to mention that this work provides evidence of the GONZÁLEZ ET AL.: BATHYMETRY OF BANCO CHINCHORRO 19

Figure 2. Map of the study area: A) number and positions of echosound sampling sites are indicated; B) the contour map of the Banco Chinchorro depth. The map continues until the 120 m contour level. 20 BULLETIN OF MARINE SCIENCE, VOL. 73, NO. 1, 2003

Figure 3. Banco Chinchorro map that showing depth classes and the magnitude of the area occupied. GONZÁLEZ ET AL.: BATHYMETRY OF BANCO CHINCHORRO 21

persistence of reef formations, since the differences found in the depth values of the reef lagoon show little change to the data of 100 years ago. An important aspect of multispectral analysis is that, in addition to its precision in shallow zones, it facilitates the joining and calculation of actual areas within required depth intervals, and detects anomalies, making interpolation techniques obsolete (Congalton, 1991). The external reef bathymetry shows that the reef emerges from depth as a single plate, with a well-developed coral barrier along its eastern margin. The latter results from the clear, oxygenated Cayman Current, which supports strong coral growth in a scenario similar to that of Cozumel Island (Muckelbahuer, 1990b). However, the western margin bathymetry does not have a well-developed coral barrier even though it also receives the waters of the Cayman Current. This is attributed principally to the abrupt, steep slopes along the channel between the Banco Chinchorro and the continent, which considerably limits coral growth. Low wave turbulence is present on the leeward side, which is also protection for that reef. Additionally, deep and shallow reef areas differ significantly in this reef complex, largely due to the effect of depth, though this interacts with other environmental parameters such as light penetration and turbulence (Liddell and Ohlhorst, 1988). In view of this complexity, it is vitally important to begin an adequate base charac- terization of the reef complex in order to continue with studies that aid in understanding its dynamics (Green et al., 1996; Mumby et al., 1997b). The external reef bathymetric profile has helped to delimit areas of conservation and sustainable use. The abrupt, steep slopes of the western margin do not allow for continual and diverse use, which proves beneficial for the protection of this complex. Zones of intermediate depth are generally the most biologically diverse, and in terms of utility can serve as propagate contribution centers for damaged or deteriorated areas. In its own right, the reef lagoon experiences the heaviest human use. However, with an analysis of its specific bathymetry together with a detailed reef zonation, it is possible to define different conservation, exploitation and sustainable-use alternative scenarios, and provide for complete and periodic monitoring (Strong et al., 1997).

ACKNOWLEDGMENTS

This research was financially supported by the Wildlife Conservation Society, CONACyT and the Coral Reef Laboratory at CINVESTAV-Mérida. We would like to thank the boatmen who worked with us for their invaluable assistance during the fieldwork.

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ADDRESSES: Cinvestav- ipn. Laboratorio de arrecifes coralinos. AP. 73 Cordemex, 97310 Mérida, Yucatán, Mexico. (A.G.S.) E-mail: . Tel: (99) 812903 ext. 513, Fax: (99) 812917.