Assessing the Relationship between Coastal Currents and Water Quality Indicators on Bonaire: ADCP & CTD approach
MSc internship report
75% of coral reefs worldwide experience degradation of which 60% is caused by local (anthropogenic) stressors. The human welfare of a small islands like Bonaire strongly depends on coral reef ecosystem services. On a global scale the carbon contribution of Bonaire is neglectable, thus their best course of action is to reduce their local stressors. These local stressors include terrestrial runoff of wastewater, sediment and nutrients to the sea. The Project Resilience Restoration of Nature and Society in the Caribbean Netherlands aims to quantify local stressors by monitoring water quality indicators along the leeward coast of Bonaire. Due to their efforts, the site-specific water quality dataset is expanding. However, research on the nutrient transport between the monitoring sites was lacking. To gain insight on local currents, a boat mounted Acoustic Doppler Current Profiler (ADCP) was deployed along the leeward coast of Bonaire. By sailing transects orthogonal to the shore, the ADCP captured the currents in 3D up till 40 m deep. In addition, temperature, salinity and chlorophyll-a were measured using a CTD. Combined the ADCP and CTD data was used to link currents to water quality indicators. The dominant flow was found to be a longshore at all sites. For Bonaire the current is predominantly northward, yet in the Kralendijk area a southward current occurred closer to shore. Around Klein Bonaire, the flow was counterclockwise. Generally, the water masses closest to shore had the highest temperature, salinity and chlorophyll-a concentrations. Local elevated chlorophyll-a layers were observed either in (1) less saline seawater at the surface near shore or (2) in equal saline seawater at 7m depth further offshore. The former could be attributed to terrestrial runoff (local stressor), whilst the latter may originate from open sea. Surface layers of chlorophyll-a were found in both longshore currents on the main island, implying the transport of land-based effluents to the north and to a lesser extent to the south. Exchange between Bonaire and Klein Bonaire seems limited. These results will provide a foot hold on how terrestrial effluents are (re)distributed around the coastal environment. And, hopefully, contribute to successful management practices and monitoring of local stressors.