The Exchange of Water and Sediment Across a Microtidal Caribbean Mangrove Forest

Mangroves have high value for inhabitants of small islands, yet, in the Caribbean they face disproportionate levels of ecosystem degradation. On small islands, mangroves are exposed to high salinity levels due to high evaporation rates, and to the clogging of mangrove creeks following excess influxes of terrestrial sediment. Tidal flushing - a mechanism that can mitigate salinity levels and prevent clogging of creeks – is limited in Caribbean islands’ mangroves due to microtidal ranges. This study presents the findings of a field measurement campaign of 4 months on tidal flushing and the exchange of water and sediment. The campaign was carried out in a microtidal mangrove forest in Bonaire, a small island in the Southern Caribbean. The results show that tidal flushing was minimal during smaller tidal ranges. However, tidal flushing increased rapidly with the tidal maximum water levels in the bay. It was found that a natural tidal creek through the mangroves was flood dominant, implying the import of sediment into the mangroves. In contrast, a restored tidal creek was flood dominant with larger tidal ranges, but ebb dominant with smaller tidal range – implying the export of sediment. These findings show that tidal flushing and the import and/or export of water and sediment through tidal creeks in microtidal mangroves are sensitive to small differences in the tidal creek configuration, their connectivity, and the tidal range. Tidal flushing influences salinity levels, a key stressor for mangroves, and should be considered in the management and restoration of microtidal mangroves. • Tidal flushing in microtidal mangroves varies with sea level and tidal range. • Tidal flushing can mitigate salinity levels in hypersaline lagoons. • Tidal flushing may cause export of sediment due to sluggish and delayed outflow. • Lagoon evaporation may have a substantial contribution to tidal creek flows. • Variability in tidal flushing important for mangrove management and restoration.