Bio-optical water quality trends and back-reef benthic community structure in southwestern Puerto Rico

Guánica Bay and the La Parguera Nature Reserve (LPNR) are ecologically and economically vital to Puerto Rico. Over time, however, these sites have faced a reduction in live coral cover presumably in part due to the combined effects of climate and impoverished water quality related to local land-based sources of pollution. The main objective of this multidisciplinary study was to analyze benthic composition data collected between 2014 and 2018 of six back-reef zones within the context of a ten-year (2012-2021) analysis of remotely sensed coastal water quality (chlorophyll-a [Chl-a]) and diffuse vertical attenuation coefficient at 490 nm [K_d490]) influenced by runoff of nearby rivers. Back-reef zones were mostly dominated by macroalgae (turf and filamentous) and octocorals (sea whips and sea fans). Most sites showed less than 10% calcifying Cnidarians (Scleractinian/Milleporids) cover. Coral cover at most sites was limited to weedy sediment-resistant species. Chl-a concentration and K_d490 were above proposed water quality thresholds for Puerto Rican coral reefs at most virtual stations and have apparently remained stable over the study period of 2012-2021 suggesting that the studied reefs have been exposed to these water quality conditions for more than a decade. This is a concerning factor as benthic composition is indicative of a phase-shift occurrence (or in process) at most back-reef areas from a Scleractinian/Milleporid dominance to macroalgae and/or octocorals. The incorporation of a larger temporal scale using additional remote sensing datasets may provide further insights into the seasonality and trends of coastal water quality linked to both natural and anthropogenic causes in southwest Puerto Rico.