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.