Cardona-Maldonado, María A.

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    Assessment of coral reef community structure using water optical properties
    (2008) Cardona-Maldonado, María A.; Armstrong, Roy A.; College of Arts and Sciences - Sciences; García-Sais, Jorge R.; Gilbes-Santaella, Fernando; Aponte, Nilda E.; Department of Marine Sciences; Alfaro Lozano, Mónica
    Measuring and monitoring the underwater light field is essential for coral reef ecological assessments since the health of these communities are affected or influenced by nutrients, sediments, and other materials that are transported from land into the marine environment and affect water quality. Significant changes in the water optical parameters in coral reef areas usually result over large time scales. Therefore, long term monitoring of water optical properties and sediment nutrient loads are required to examine the impact of these factors on community structure, diversity, and health of coral reefs. Although remote sensing technology may be a promising tool for monitoring coral reefs, significant obstacles exist for its implementation; such as the spectral and spatial resolution of sensors and the confounding effect of variable water optical properties and bathymetry. Therefore, we tested an indirect approach to infer coral reef community composition and health based on the characterization of the optical properties of surrounding waters. A total of 31 sampling events that included 19 stations were conducted to measure apparent and inherent water optical properties for the study areas. Regressions were obtained between water transparency, as determined by the diffuse attenuation coefficient of visible light (Kd (PAR)) and coral reef ecological parameters including the percent coral cover (R= -0.96, R2=0.92, p < 0.0001), the distance of the sites from the shore (R=-0.66, R2= 0.44, p = 0.0020), the index of diversity (H’) (R=-0.96, R2= 0.92, p>0.10) and the mean rugosity index (R=--0.46, R2= 0.22, p=0.0455). The vertical attenuation coefficient was also related to secondary parameters including: evenness (R=0.42, R2= 0.17, p>0.10), richness (R=-0.47, R2= 0.23, p=0.0404), sponge cover (R=0.75, R2= 0.56, p=0.0002), octocoral cover (R=0.74, R2= 0.54, p=0.0003), uncolonized substrate cover (R=0.74, R2= 0.55, p=0.0003), diseased coral cover (R=-0.60, R2= 0.36, p=0.0065), macroalgae cover (R=-0.52, R2= 0.28, p=0.0212) and filamentous algae cover (R=0.79, R2= 0.62, p>0.10). Significance of the relation between percent coral cover and optical depth (R=0.97, R2= 0.94, p<0.0001), was taken into consideration when dividing the study sites into three optical depth groups. The relationship between the vertical attenuation coefficient (Kd) and the percent coral cover was then reassessed resulting in a stronger relationship (R=-0.9982, R2= 0.9964, p=0.0384). A strong inverse relationship exists between Kd and the percent coral cover for the areas of Ponce, Guayanilla, La Parguera and Mayagüez Bay. Although weaker, a tendency was also observed relating the vertical attenuation coefficients to the sites’ distance from shore. Euphotic zonation was considered a more efficient approach when determining similarities between the study areas. Future work should consider a more detailed evaluation of the specific biological parameters and the construction of a predictive model relating percent coral cover to Kd and optical depth by satellite imagery.