Coastal ecosystem connectivity: watershed management, sedimentation and the response of coral reefs.

Abstract

Coral reefs are among the most biologically productive and economically important ecosystem on Earth. Nevertheless, unsustainable coastal development and watershed alteration have increased soil erosion and sediment influx to coastal waters and have been linked to marine habitat degradation. Sedimentation has been identified as a main local threat to coral reefs worldwide. However, there is a lack of information regarding sedimentation dynamics and the potential effects to near-shore coral reefs benthic communities. The goal of this study was to assess spatio-temporal differences in sediment dynamics and sediment characteristics as a function of changing coastal land use patterns, weather, and oceanographic conditions. Also, the benthic community structure response to sedimentation in two reef locations (Bahía Tamarindo (BTA) and Punta Soldado (PSO)) was assessed in the small semi-arid island of Culebra, Puerto Rico. Sediment traps were deployed across a distance gradient from the shore and were monthly replaced from February 2014 to April 2015. Sedimentation rate, sediment texture, and composition were analyzed by dry sieving and loss-on-ignition techniques and were contrasted with environmental variables. Permanent belt transects of 10m[superscript 2] were assessed through high-resolution photo-quadrats by seasons from February 2014 to 2015.

There were significant spatio-temporal differences in sedimentation rates over time (p=0.0001), and distance from shore (p=0.0040). At both sites, time series with increased sedimentation and terrigenous sediment rate were related to meteorological events with high rainfall and wave height. The spatial distribution of silt-clay and terrigenous sediments increased with rainfalls that exceeded 20 mm/hr, and sand resuspension was observed with wave height that exceeded 1.5m. The shallow reef areas, closer to shore were more exposed and vulnerable to sediment stress, suggesting that sediment influx had strong vii relationship with coastal runoff and changes in land use patterns. Coral diversity changed gradually through seasons as the reef experienced variations in sedimentation patterns. In the other hand, coral recruitment increased through seasons, even though it was lower in PSO zone B, area that received highest amount of terrigenous deposition after coastal watershed deforestation. Coral reef benthic community structure was significantly different among seasons (p=0.0020), site (p=0.0010), and distance zone (p=0.0010). Coral cover was significantly different among sites (pairwise, p=0.0010) with higher percent cover at areas less exposed to terrestrial sediment deposition.

Changes in water quality due to stochastic local sediment related stressors are a major factor influencing phase shift towards reefs dominated by macroalgae communities. Sediment dynamics and near-shore coral reefs are highly impacted by rapid transformation of coastal watershed land uses, combined with changes in weather, precipitation patterns and oceanographic hydrodynamics. There is a need of integrating scientific information on the multiple and complex interactions between terrestrial and marine ecosystems that influences sedimentation dynamics and coral reefs response into management and decision-making processes. Ecosystem based management provides an holistic framework that if combined with an inclusive and participatory decision-making process could assist in the identification and implementation of effective management actions that could eliminate local stressors and achieve the conservation of coral reefs worldwide. These processes should take into account future climate scenarios and the need to strengthen coral reef resilience to guarantee human coastal communities livelihoods.