Fresh water and sediment dynamics in the catchment of Lac at Bonaire.

Deterioration of mangrove forests is occurring globally. The loss of mangrove habitats causes a decline in fishery resources, livelihood and biodiversity loss. Deterioration is also been seen in the mangrove forest of Lac Bay on Bonaire. Especially at the landside of the mangroves hyper saline conditions are found and Rhizophora mangles as well as Avicennia germinans trees are dying. The deterioration of the mangroves at the landside is partly compensated by expansion of the mangroves at the sea side. The main causes for the seaward shift seem to be, 1) less fresh water inflow during rainy season, 2) increased sediment transport towards Lac, 3) reduced interaction (tidal flow) between sea side and land side. This project has focussed on the dynamics of the fresh water fluxes toward the mangroves and the associated sediment transport. During two major rain events water discharge and sediment transport was measured for two catchments. Based on these measurements the total runoff amount and amount of sediments transported during the rainy season was determined. It can be concluded that runoff have brought only relatively minor quantities of fresh water to Lac. However, extreme events or extreme annual rainfall sums (1980, 1981, 1985, 1988, 2004, 2005) might have caused extreme runoff amounts and in addition extreme sediment load transports toward the mangrove system. Along with the deterioration of the channels and the associated water circulation, this might have caused root smothering and mangrove die off. This mangrove die off is probably strengthened by a relatively dry period with relatively less freshwater transport by surface runoff toward the mangrove system which enhances the hyper saline waters in the back of the system. Although deterioration of the landscape occurs since the 17th century, due to cutting down forests and overgrazing by goats and sheep’s, heavy rainfall events will not only lead to an increase in water supply but also to more extreme sediment transport rates toward the mangrove system. In addition, also removal of dams would cause an increase in surface runoff towards the mangrove system and an increase in sediment transport. Monitoring rainfall, surface runoff, and sediment transport over a large number of years together with monitoring the growth and the health of the mangrove forest would give more insight in how the mangrove systems reacts on hydrological changes over a number of years. 

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