Saar Ellerbroek

The effects of salinity, temperature and soil composition on mangrove abundance at Lagun, Bonaire

Mangroves are important intertidal forests that exist in tropical and subtropical areas around
the world. They are important coastal protectors, providing coastal stabilization and wave
mitigation. Besides their protective ability, they fulfill a number of ecosystem services like
nursery grounds for juvenile fish, carbon sequestration and pollutant filtering. However,
mangroves have been under heavy threat in many countries due to anthropogenic pressure,
causing major degradation rates. In Lac bay, Bonaire, a big lagoon consisting of mangroves
and seagrass for example, high sedimentation due to erosion has caused channels to close,
creating hypersaline conditions in ponds together with high temperatures. The forest around
Lac bay therefore experienced high mortality rates and a strong decline in the systems’
health. Another bay, Lagun, showed similar degradation in the back pond, but due to lack of
documentation the actual cause is still uncertain. It is important to find out whether siltation,
hypersalinity and high temperatures are also an issue at Lagun in order to create a
successful restoration plan.

Vegetation, soil composition, conductivity and temperature were analyzed for several plots at
different location types around Lagun, including the degraded part in the pond. Results
showed that the silt layer in the pond was significantly higher than the other location types,
which indicates root burial caused by high sedimentation rates. Conductivity (as a measure
for salinity) and temperature were, contrary to the hypothesis, not higher in the pond
compared to other locations, and showed no significant relationship to mangrove abundance.
This could be explained by fresh water input due to heavy rainfall during the rainy season
that had just ended. For now, restoration measures should focus on mitigating external
pressures through habitat regeneration. Sediment trapping in and around watersheds and
runoff areas connected to Lagun could decrease the sedimentation rate and prevent burial
and floor elevation. Channel connection to Lagun could increase water circulation and might
help to prevent a larger part of the pond from drying out during the dry season. Active
planting can be combined with habitat restoration measures to increase survivability of
mangrove seedlings. Besides restoration efforts, more research needs to be done on the
existing pressures at Lagun, including Sargassum and trash inflow from the ocean, erosion
rates, pond water and soil quality and the effect of the nearby landfill on mangroves.
Documentation and monitoring of Lagun should be continued in order to obtain more
information about the processes at hand. With sea levels rising due to climate change, it is
crucial for Bonaire to have stable coastlines that can mitigate effects of storm surges, in
which mangrove forests play an important role.

Data type
Research report
Research and monitoring
Geographic location