Evaluating land–sea linkages using land cover change and coral reef monitoring data: A case study from northeastern Puerto Rico

Abstract:

Land cover change that leads to increased nutrient and sediment runoff is an important driver of change in coral reef ecosystems. Linking landscape change to seascape change is necessary for integrated land–sea management of coral reefs. This study explored the use of freely available satellite products to examine long-term patterns of change across the land–sea continuum. We focused on northeastern Puerto Rico, where a widespread decline in live coral cover has occurred despite concomitant watershed reforestation that was expected to reduce land-based threats. The aims of this study were (1) to examine whether these land–sea trends continued in 2000–2015 and (2) to assess the opportunities and limitations associated with using satellite data to inform land–sea management. We applied a Random Forest classifier on Landsat-7 satellite imagery to assess changes in land cover and landscape development intensity, a spatial index to estimate land-based pressure on nearshore marine ecosystems. We used field monitoring data to quantify benthic community change. We found that reforestation continued in 2000–2015 (+11%), suggesting reduced land-based pressure on adjacent reefs in both northern (Luquillo) and eastern (Ceiba-Fajardo) watersheds. Concomitantly, coral cover continued to decline, and a new aggressive expansion of peyssonnelid algal crust was recorded. Clustering analysis indicated that benthic monitoring sites in the same geographic regions (nearshore/offshore, north/east) followed similar community composition trajectories over time. Our results suggest that continued reforestation and the expected reduction in land-based pressure have not been sufficient to halt coral cover decline in northeastern Puerto Rico. To improve the characterization and monitoring of the full causal chain from changes in land cover to water quality to benthic communities, advances in satellite-based water quality mapping in optically shallow waters are needed. A strategic combination of remote sensing and targeted field surveys is required to monitor and mitigate land-based stressors on coral reefs.