Metselaar, K.

Improving water circulation into backlands of Lac Bay through channel management

Lac Bay is the lagoon of Bonaire; in that lagoon a mangrove forest of red and black mangroves grows. The mangrove forest of Lac provides birds and fishes of the surrounding
area with several important ecosystem services, as breeding ground, nursery and feeding area. These eco-­‐services are threatened by a reduced water circulation which causes low
water heights in the backlands and thus a reduction of the area which is available to fishes. Water in the backland was measured hypersaline, the obtained result of low water circulation and high evaporation in the backlands.

This hypersaline values may reduce the area of viable mangroves in the backlands. The lagoon and the backlands are connected  through two feeder channels and those natural channels will overgrow when no maintenance is applied causing even less water flowing into the backlands. Calculating appropriate channel dimensions and formulating a maintenance plan a certain discharge into the backlands can be ensured, which may reduce the degradation of ecosystem services.

Data type
Research report
Research and monitoring
Geographic location

Modelling Mangrove growth and salinity: a semi-arid case study

of the mangrove forest in Lac Bay, Bonaire is an environmental issue. Mangrove forests
surround Lac Bay. Lac Bay has a status as RAMSAR site (RAMSAR, 2013), providing a defined legal
protected status for this wetland. The most common species in Lac are Rhizophora mangle (red
mangrove) and Avicennia germinans (black mangroves). The growth of Rhizophora mangle have
become so successful that it limits the water circulation from the open bay passage into the back
shallow ponds (Lott, 2001). This has led, due to the semi-aridity, to hypersaline water quality
conditions in the back shallow ponds. Avicennia germinans is better able to deal with these
hypersaline conditions. They dominate the interior area. The salinity at the Northern-outer borders
might become that high that mangrove growth is strongly limited, even impossible. Skeletons of
former mangrove trees can be seen at the outer borders, which suggests a dying-off process. Nutrient
limitation and frequent flooding leading to oxygen stress could also reduce the mangrove growth. The
main question to be answered within this study is: What could be the cause of the degradation of the
Mangroves in Lac Bay? More insight in which factors might limit the growth Rhizophora mangle and
Avicennia germinans is needed in order to answer this research question.

Bonaire is an island situated in the Southern Caribbean. It has a semi-arid climate according to the
Köppen classification. Average daily temperature ranges between 25 and 31 °C. Average annual
precipitation is 475 mm of which 55 % occurs in the rainy season which lasts from October to
December (Borst and De Haas, 2005). Lac Bay is a basin, positioned on the southeastern side of the
island. Mangroves forests border the bay.

SWAP (soil, water, atmosphere, plant model) will be used to model the mangrove growth depending
on the salinity. SWAP is a physical 1D-model and is able to simulate transport of water, solutes and
heat in the vadose zone in interaction with vegetation development (Van Dam, 2000). A new
subroutine, which simulates tree biomass growth, was added to SWAP. SWAP is unable to simulate
tides. Tides were given as an irrigation gift. The sum of the daily inflow equals the irrigation gift.
Biomass growth of Avicennia germinans and Rhizophora mangle was modelled with SWAP for the
period of 1989 to 2009.

Tides within Lac can be classified as a mixed dominant semidiurnal type (Brown et al., 1995). Tides in
Lac have a daily, two-weekly and annual tidal component. The maximum daily fluctuation within Lac
equals on average approximately 30 cm. The annual fluctuation equals approximately 10 cm. The tidal
fluctuation is delayed and attenuated towards the backland, given equal evapotranspiration. Salt stress
will increase towards the backland. Observations show a negative relation between the abundance of
Rhizophora mangle and salt stress. Avicennia germinans will occur at locations where the salt stress is
too high for Rhizophora mangle to grow. There is a peak in the salinity stress in March. Surface runoff
will reduce stress. There is a linear relation between the amount of precipitation and runoff. The
decline in salt stress if surface runoff is taken into account is on average approximately 28% for
Avicennia germinans and 30 % for Rhizophora mangle. Although, the exact influence of surface
runoff on the salt stress is strongly dependent on the locations and the annual precipitation rate.
Locations where more salt is stored within the soil will benefit more from the surface runoff. The
amount of saltwater inflow is also of major importance. Simulations at different locations show that
seawater flush salt out of the soil when the salt concentration increases above the salt concentration of
seawater. The saltwater inflow at the backland is relatively low, which will lead to hypersaline
conditions. The total seawater inflow should be increased with at least 130% so that salt stress will not
limit the mangrove growth.

The modelled peak seems to be relative low compared to field measurements done in other studies.
This study modelled a monthly mean of 120 mS/cm at Rooi Grandi (situated in the backland) in
March. Kats (2007) measured a monthly mean of 170 mS/cm at Awa Lodo di San José in March.
Regensburg (2012) measured electrical conductivities of 155 up to 160 mS/cm at this location in
April. The difference between the electrical conductivity modelled within this study and the mean
measured by Kats (2007) might be explained by the fact that SWAP simulate too low values for the
evapotranspiration or that barriers within Lac Bay limit the water circulation. Areas might become hydrologically isolated during the dry season. Mangrove trees might then die because of hyper
salinity. SWAP is currently unable to simulate this die-off process. The model could be improved by
simulating die-off of the mangrove trees, when the trees cannot transpire for a period of time.
The salinity increases towards the backland. There seems to be a negative relation between the
abundance of Rhizophora mangle and salt stress. Avicennia germinans will occur at locations where
the salt stress is too high for Rhizophora mangle to grow. Conditions at the northern outer shore
(around Awa Lodo di San José) become too saline for Avicennia germinans and Rhizophora mangle to
grow. The salt stress at the northern outer shore might be decreased by increasing the surface runoff or
by improving the salt water circulation. Increasing the salt water circulation seems to have the most
impact. The seawater inflow should increase with 60%, so that Avicennia germinans will not be
limited by salt stress and with 130% so that Rhizophora mangle will not be limited by salt stress.

Data type
Research report
Research and monitoring
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Groundwater recharge on the cultuurvlakte of St.Eustatius

MSc Thesis 

Groundwater is an important resource and an important factor to consider in pollution management. Accurate estimates of groundwater recharge are essential for effective management of groundwater. St. Eustatius is a small island of the Leeward Islands in the Caribbean Sea with a brackish groundwater lens with high nutrient concentra- tions at the inhabitated part of the island. In this study, groundwater recharge was estimated with a modelling approach, using the unsaturated zone model SWAP. A period of 30 years was simulated. Field work was performed on St. Eustatius to determine vegetation and soil parameters.

Model calculations show that for the inhabited part of the island, the Cultuurvlakte, long term average groundwater recharge ranges 75–210 mm year-1. Quantitative uncertainty bounds could not be determined. High groundwater nitrogen concentrations suggest that in some parts of the Cultuurvlakte a third to all of groundwater is infilrated sewage water.

A global sensitivity analysis was performed using the Morris method. The sensitivity analysis showed that rainfall is the most important factor in determining groundwater recharge, followed by refference evapotranspiration and vegetation characteristics that control evapotranspirative fluxes. Rainfall intensity and antecedent soil moisture play a key role in generating groundwater recharge; a simple conceptual reservoir model proved capable of reproducing the recharge simulated by SWAP. The results emphasize the importance of accurate rainfall measurements and the need to develop methods for estimating the evapotranspiration of natural vegetation.


Please contact Klaas Metselaar (

Data type
Other resources
Research and monitoring
Geographic location
St. Eustatius