Mucher, S.

Coral reefs are rapidly degrading worldwide, due to  a combination of global and local stressors. While gloal stressors, such as ocean warming, cannot be managed, management of local stressors can increase the resilience of coral reefs to these global stressors. One such local stressor is overgrazing- induced terrestrial erosion causing sediment and nutrient run-off to coastal waters, which is associated with coral mortality and changes in benthic community composition. In this study we assessed the link between watershed-specific erosion hazard and coral reef health, using the coral reefs on the west coast of Bonaire, Caribbean Netherlands, as a case study. We first identified watersheds, waterflow and run-off sites into the sea based on a digital elevation model. Next, for each watershed we determined the mean erosion hazard (a proxy for the degree of terrestrial run-off based on slope and ground vegetation cover). Subsequently, we used multiple regression models to investigate whether mean erosion hazard of the nearest upstream watershed, and distance to the nearest upstream run- off point explained variation in marine benthic community composition at two depth zones (~5 m and ~10 m). We found a negative relationship between mean erosion hazard and coral cover (a proxy for coral reef health) at 5 m depth, but no such effect at 10 m depth. In addition, we found a positive relationship between mean erosion hazard and sand cover (a proxy for sediment run-off) for both depth zones, and a small but significant quadratic effect of mean erosion hazard on algae cover (a proxy for nutrient run-off) at 5 m depth. Moreover, distance to the nearest upstream terrestrial run-off point was positively related with coral cover at 10 m depth, and negatively related with sand cover at 10 m depth. Our results provide direct evidence for a negative relationship between watershed-specific erosion hazard and coral reef health, and highlight the need for a nature-inclusive “watershed-to-reef” based coastal management approach that integrates terrestrial and marine conservation to preserve the island's valuable coral reef and related ecosystem services.

Publication referenced in BioNews 40 article "Watershed-to-Reef: New Approach for Coral Reef Management"

Smalls islands are especially vulnerable to climate change and land  use changes due to the competing needs for limited resources. To support the NEXUS approach we need evidence based monitoring tools that can provide policy makers, conservation managers, entrepeneurs, scientists and the general public with information on the state, pressures and associated changes in the environment. Satellite imagery can provide synoptic information at appropriate
spatial and temporal resolutions that can support evidence based monitoring. Only at very detailed levels information might be added by using airplanes or drones. Remotely sensed information can help to provide information on e.g. land cover and associated dynamics such as urban sprawl, mapping habitats such as mangroves and coral reefs, surveying terrain conditions such as soil moisture conditions and erosion hazards associated within catchments, sea level rise and changing coastlines, and on many aspects of the vegetation (natural and agriculture), such as plant traits, phenology and plant growth. Remotely sensed information can in general make field surveys and monitoring more effective, and can thoroughly support decision making.

In order to update the 1985 atlas of Bonaire’s coral reefs (Van Duyl, 1985), a hyperspectral mapping campaign was performed in October 2013 using the Wageningen UR Hyperspectral Mapping System (HYMSY) with 101 spectral channels. In June 2016, with the help of STINAPA Bonaire, Sander Mücher and Erik Meesters were able to perform an extensive diving campaign to collect in-situ information in 20 transects perpendicular to the coastline across the western coast. Detailed photographs of the sea bottom were taken by diving to a depth of 20-30 meters, then progressing shallower towards the coast. Photographs were geotagged by another person snorkelling exactly above the diver with a GPS in a waterproof box and making overview pictures of the sea bottom as well.

This in-situ information was used to interpret the hyperspectral imagery made by the HYMSY camera. In order to interpret the data more consistently, the hyperspectral data were corrected for the water depth into at-ground-reflectance factor units. A bathymetric model was used for the calibration of the hyperspectral imagery based on a former field campaign by measuring water depth at specific locations along the western coast. The final bathymetric model was based on extrapolation of the terrestrial digital elevation model by fitting the DEM with additional in-situ bathymetric measurements at sea. A more detailed bathymetric model may improve the calibration of the hyperspectral data with a 1-meter spatial resolution.

Due to the limited penetration of green and red light through the water, it was decided to use only the first 15 hyperspectral bands in the violet-blue -cyan till green range (Band 1 = 450.0 nm to Band 15 = 520.0 nm). This means that spectral measurements can be made to a maximum depth of 20 to 30 meters. The hyperspectral reef classification defines 12 classes including sand, pavement, rubble, soft and hard corals, and various mixtures of these at a pixel level of 1-meter spatial resolution. Due to their spectral similarity, corals and algae remain difficult to separate. 

Access the data here.

The Dutch Caribbean island Bonaire has been protecting its marine resources for more than 35 years and is ranked as one of the Caribbean’s top dive destinations thanks to its easily accessible diving sites, clear waters and the relatively healthy coral reefs around the island. Coral reefs are important for the ecosystem services they provide including jobs and income to local economies from recreation and tourism. They serve as a natural barrier, protecting the coastlines and as a habitat for many economically important species. 

The general consensus is that the extent and biodiversity of Bonaire’s coral reef is decreasing due to local and regional anthropogenic and global climate pressures. However, the last extensive study of the coral coverage of the reef ecosystem was performed in 1985 by Van Duyl who created an underwater atlas of Bonaire and Curaçao. In order to update this atlas of Bonaire’s coral reefs, a hyperspectral mapping exercise was performed in October 2013 using the Wageningen UR Hyperspectral Mapping System (HYMSY) with 101 spectral channels. 

This news article was published in BioNews 27

BioNews is produced by the Dutch Caribbean Nature Alliance and funded by the Ministry of Economic Affairs.