Effects of suspended sediments on coral growth: Evidence from remote sensing and hydrologic modeling

Remotely sensed images derived from several sources were combined with in situ samples to estimate suspended sediments discharged from three rivers into Mayaguez Bay, Puerto Rico. Multispectral data were acquired using the Calibrated Airborne Multispectral Scanner (CAMS) and the Landsat Thematic Mapper (TM). CAMS data were used to produce spatial maps of suspended sediment concentrations in Mayaquez Bay during low to moderate discharge for 1990–1992. Images of land cover/land use during 1975 and 1985 were generated from a supervised density slicing technique applied to a mosaicked image of digitized aerial photos and from an unsupervised classification of the TM image. A geomorphic hydrologic model was used to simulate runoff and sediment yield from the drainage basin of the Guanajibo river; because land cover within the basin did not change significantly during 1975–1985, the hydrologic model provided estimates of sediment discharged by the Guanajibo river into the bay during this time period. Growth rates of the coral Montastrea annularis of Rodriguez reef were obtained from a previous study. Annual growth rates as determined from coral banding decreased significantly (r = 0.69) with increased sediment loading to the bay (tons/year). Evidence from remote sensing and hydrologic modeling suggests that sediment reaches the reef only during episodic, high-discharge events. During low to moderate discharge, sediments, both suspended within the river plume and resuspended sediments, are restricted to the near-shore regions by the prevailing circulation. Hence, a direct relationship between river runoff and coral growth rates was established in this study.