Hydrological characteristics of the catchments feeding the mangrove system Lac in Bonaire
The mangrove system Lac is located on the southeast side of the Caribbean island Bonaire. Since 1969 the mangrove system has been studied and since that time a significant die-back of the mangrove area has been observed. It is presumed that a decline in freshwater flow from catchments feeding the mangrove system and/or accumulation of sediments and/or a decrease of seawater circulation are causes of the mangrove die-back. In this research about ‘the hydrological characteristics of the catchments feeding the mangrove system Lac in Bonaire’ the freshwater fluxes are monitored during a fieldwork period from 25 February 2013 till 15 April 2013. The aim of the research is to get insight in the causes of the mangrove die-back of Lac. The fieldwork period was during the climatological dry period. During the fieldwork period only one rainfall event was measured with a low intensity and short duration. When the precipitation amount and intensity of the event is low no overland flow occurs because the precipitation can infiltrate in the unsaturated zone. The area that surrounds Lac is very complex because of its heterogeneous soil type and karstified limestone bedrock. Due to this karstified limestone the regional and local groundwater flow can follow complex flow paths. A wide range in correlation coefficients based on tidal fluctuation and hydraulic heads in wells and piezometers indicates a large variability in hydraulic conductivity. All wells showed a decline in hydraulic head during the dry period. During such dry periods the salinity values are increasing in the backwaters of Lac, and hyper salinity occurs. This is due to a lack of inflow of fresh- and seawater and poor water circulation in combination with high evaporation rates. The salinity levels are then predominantly influenced by inflow of seawater due to two channels that connect the backwaters of Lac with Lac Bay. These channels influence only a small area in the backwaters of Lac. The salinity levels therefore differ from 40 ppt till 125 ppt, this is more than three times as high as the average seawater salinity. The salinities are at some locations much higher than the critical values for mangrove species. As Rhizophora mangle occurs where salinities range from 60-65 ppt while Avicennia germinans are found at locations over 90 ppt. With more channels that connect the backwaters of Lac with Lac Bay the hyper saline conditions could be lowered, which would result in stable salinity levels in the backwater of Lac.