Mixing over shallow coral reefs in relation to the reef rugosity

Wageningen University, INREEF & WMR (MSc thesis)

Abstract  

Increasing inflows of nutrients to shallow coral reefs are an important stressor in ongoing global reef degradation. Previous research has indicated that there are often major nutrient flows from terrestrial sources to the degrading coral reefs. To better understand the importance of nutrient flow to the coral reefs, a clearer understanding of the coastal hydrodynamics and mixing processes transporting these nutrients, is necessary. This research has aimed to assess the role of reef rugosity on the mixing processes in shallow coral reefs. Two 25h ADCP measurements were performed on four different sites on the west coast of Bonaire, together with various rugosity measurements on each site. The sites were chosen based on an assumed differences in rugosity. Friction factors were calculated to assess the influence of rugosity on the turbulence and mixing in the water column. The collected data showed the currents to be wind dominated, with low average velocities of 0-0.1 m/s. The currents did not seem affected by the rugosity, with changes in current velocity similarly correlated to changes in wind forcing for both high and low rugosity areas. Higher rugosity did seem to give higher friction factors, although only weak correlations were found due to the low flow velocities. The results suggested that during high flow velocity periods more nutrients could be trapped in the coral reefs, due to increased turbulence around the reef surfaces. This could be even more relevant considering the fact that high flow velocities often occur during rain and storm events, which are important nutrients inputs through freshwater discharge. However, due to high uncertainties in the friction factor calculations, these relations cannot be taken as scientific proof but rather as indications for further research.