Dijksma, R.

Abstract
In recent decades, concern about mangrove die-back has intensified. One of the last remaining mangrove areas in the Dutch Antilles is the mangrove forest in Lac bay on the island of Bonaire.
It has a relatively large biodiversity and is essential for many bird species. Not only the inhabitants of the forest itself – also many other species outside the forest – depend on it. This is because the filtering effect of mangrove roots prevents excessive sediment deposition on sea grass beds and coral reef. Unfortunately, extensive mangrove die-back in Lac bay has been observed in recent years. The hypothesis is that this die-back is caused by hyper-salinity. This study focuses on subsurface freshwater flow from southern Bonaire into Lac bay as a source of refreshment to the mangrove area.
In the rainy season 2012-13 (October to January), the hydrological system in the area surrounding Lac bay was intensively investigated. Measurements included rainfall, hydraulic conductivity, groundwater levels and open water salinity in the backwaters of Lac bay.

The geo-hydrological properties of limestone were found to largely determine groundwater flow on both regional and local scale. On a regional scale hydraulic conductivity and degree of karstification determines gradients, hydraulic contact to Lac bay and responses to rainfall. 4 and 5km north of Lac bay, hydraulic conductivities were found to be 1.3 and 2.7m d-1, respectively. The relatively low values are accountable for: 1) steep gradients of 300-380cm km-1 of the southward groundwater flow; 2) poor hydraulic contact to Lac bay; and 3) strong groundwater responses to heavy rainfall. For instance, a rise in groundwater level of 4m was observed in only 9 days.
Further south, extensive karstification exists. Here, a relatively high hydraulic conductivity of 12.5m d-1 was found, resulting in: 1) low gradients of 2-10cm km-1 of the eastward groundwater flow west of Lac bay; 2) high tidal impact on hydraulic heads; and 3) weak responses to rainfall.

On a local scale, karstified limestone makes groundwater flow complex, because it facilitates underground streams with unpredictable flow paths. 700m northwest of Lac bay, a northeastern flow direction away from Lac bay was found. This may explain the relatively high salinity greater than 50p.p.t. observed in the northwest backwaters of Lac bay.

Msc. Thesis

Erosion can cause large problems for ecosystems on and around tropical islands. On site, erosion leads to a decrease of available nutrients, and off site, accumulation of nutrients can occur or it can leave the system by entering the sea and thereby disturbing the marine ecosystem. This is particularly bad for coral reefs surrounding the island since they are very vulnerable to nutrient enrichment, sedimentation, and turbidity. Change in erosion rates is mainly caused due to anthropogenic influences such as urbanization, land use change, and overgrazing. St. Eustatius is a small island (21 km2) in the Caribbean Sea, where the Cultuurvlakte (urbanized area) is especially sensitive to runoff and erosion due to paved areas, overgrazing, and relatively little vegetation. This research focusses on runoff and erosion on the Cultuurvlakte and especially the effect of land use types and slope gradients. Runoff and erosion were measured in the field with runoff plots of 1 m2 and modelled with the soil erosion model LISEM. The runoff plots were installed on slopes of 5 and 10% and on three different land use types (grass, shrub, and bare soil). All rainfall events that caused erosion at the plots were also simulated with LISEM. The model was also used to analyze to which parameters erosion and runoff are most sensitive and to do scenario analyses. The results of the plot measurements clearly show that both runoff and erosion were highest for bare soil. The difference between grass and shrubs was less clear, but generally there was more runoff and erosion at the plots with grass. Effects of slopes were not visible in the plot measurements, most likely due to differences in hydraulic conductivity between the plots. The model simulations show the same effect of land use on runoff and erosion (most at bare soil and least at shrubs) but also show that the steepest parts of the Cultuurvlakte, with little vegetation, are most vulnerable. With the measured rainfall events, up to 15% of rainfall was runoff at the plots while the discharge to the sea was only up to 4% for the model, which is (mainly) caused by scale effects. The scenario analyses show that both overgrazing and removal of shrubs have a significant effect on runoff and erosion on the Cultuurvlakte, and bare soil (which often occurs for longer time during construction projects) has the most significant effect. To reduce erosion as much as possible, land should lie follow as little and short as possible and overgrazing and removal of shrubs should be limited, especially at steep slopes.

Please contact: roel.dijksma@wur.nl