Anne Fleur Mineke van Leeuwen

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.

For full report or more information,  please contact erik.meesters@wur.nl or gulsah.dogruer@wur.nl

Final presentation 

Abstract

Worldwide, coral reef health is declining rapidly due to both global stressors (climate change) and local stressors (pollution). Reef maintenance on Curaçao focusses on reducing local stressors including terrestrial pollution. One f low path for terrestrial pollution is via submarine groundwater discharge (SGD). Yet, on the island of Curaçao these hotspots of submarine groundwater discharge have not yet been located, let stand quantified. With this research we aimed to increase our understanding of groundwater flow on the island. This will serve as a foothold for future SGD research. To do so, we have conducted 9 electrical resistivity tomography (ERT) measurements to understand (1) the heterogeneity of the lava formation, the main aquifer, (2) the saltwater-fresh water interface at the coast, and (3) the groundwater flow at geological interfaces. To assist in the interpretation of the data we measured groundwater levels and the discharge of Hato spring, in addition to observing local geological outcrops. We discovered that the lava formation is a very heterogenous aquifer due to variety in degree of weathering of the pillow basalt. As a result of the heterogenous permeability, the extent of seawater intrusion in the coastal lava formation is strongly variable. Yet, the lava formation does form a better barrier for sea-fresh water mixing than the limestone terraces. The midden formation (sedimentary rock) forms a thin aquifer near the surface. And, at the interface of lava and midden formation, the groundwater flow is hampered. As a result, the groundwater is confined within the lava formation until the lava formations reservoir “overflows”. The same occurs at the interface of the lava formation and the diorite intrusion. The next step in SGD research is quantifying the seaward groundwater flux where the lava formation is in direct contact with the sea or limestone formation. Here seaward groundwater flow is not hampered by the midden formation or the intrusion and thus forms hotspots of Submarine (polluted) Groundwater Discharge.