Unsustainable Land Use, Sediment-Laden Runoff, and Chronic Raw Sewage Offset the Benefits of Coral Reef Ecosystems in a No-Take Marine Protected Area

Abstract

Unsustainable land uses may result in poor watershed management, increased soil erosion, poorly-planned urban development, increased runoff, and sewage pollution, creating an environmental stress gradient across coastal coral reefs. This study was aimed at: 1) Evaluating water quality within and outside the Canal Luis Peña Natural Reserve (CLPNR), Culebra Island, Puerto Rico; 2) Determining if there was any significant environmental stress gradient associated to land-based non-point source pollution; and 3) Characterizing shallow-water coral reef communities across the gradient. Strong gradient impacts associated to sediment-laden and nutrient-loaded runoff pulses, in combination with non-point raw sewage pulses, and sediment bedload, impacted coastal coral reefs. Water quality showed significant spatio-temporal fluctuations (p<0.0001), largely responding to heavy rainfall and subsequent runoff pulses. Benthic community structure showed significant spatial variation along the environmental stress gradient (p=0.0002). Macroalgae, dead coral surfaces, algal turf, and low coral species richness, species diversity index (H’c), and evenness (J’c) dominated benthic assemblages across reefs frequently impacted by runoff pulses and sediment bedload. The combination of fecal coliform and enterococci concentrations were correlated with variation in benthic community structure (Rho=0.668; p=0.0020). The combined variation in salinity, dissolved oxygen and enterococci concentrations explained 75% of the observed spatial variation in benthic assemblages (R²=0.7461; p=0.0400). Local human stressors affected coral reefs within no-take CLPNR and risk analyses suggest it may offset its ecological benefits. There is a need to design and implement integrated coastal-watershed management strategies to address multiple land use activities, including erosion-control best management practices, watershed reforestation, and sewage pollution control.