R. Gijsman1*, S. Engel2, E.M. Horstman1
1 University of Twente, The Netherlands; 2 Mangrove Maniacs, Caribbean Netherlands;
* Corresponding author: r.gijsman@utwente.nl
Introduction
Mangrove forests are increasingly recognized for their environmental and societal values. Over the past century, however, the global mangrove forest cover has drastically declined due to their conversion to other land-use purposes and coastal erosion (Goldberg et al., 2020). In response to the increased awareness of mangroves’ value, the mangrove cover decline rates have reduced and several initiatives to restore previously lost mangroves have been initiated. To restore mangroves, it is paramount that knowledge of the geomorphological settings of the to-be-restored mangrove forests is considered, since mangrove forest dynamics vary substantially between settings (Balke and Friess, 2016).
This study considers the restoration of a mangrove forest in Lac Bay lagoon in Bonaire, Caribbean Netherlands. Lac Bay is characterized by a microtidal regime and a carbonate mangrove forest of multiple species. The mangrove forest in Lac Bay has been suffering from an excess of terrestrial sediment, causing that tidal creeks have been filling in over the past decades. The resulting clogging of tidal creeks has caused a reduction in tidal flushing and the degradation and mortality of mangrove trees in the most landward area. Mangrove restoration efforts in response to the degradation included the reopening of tidal creeks.
Objective and Methods
The objective of this study is to obtain quantitative understanding of the hydrodynamic and morphological processes in Lac Bay on a spring-neap tidal timescale. The focus is specifically on the flow velocities and sediment transport through the natural and restored tidal creeks. For this purpose, field measurements were conducted in Lac Bay lagoon between January and May 2022 (Gijsman et al., 2024). Flow velocities in the tidal creeks were measured with Lowell Instruments LLC Tilt Current Meters (TCM’s). Turbidity was measured with optical backscatter sensors (RBRsolo3Tu) and converted to suspended sediment concentrations through a lab calibration. The water flow and sediment transport through the tidal creeks were related to water level and wave fluctuations throughout Lac Bay.
Results
The field measurements revealed a substantial attenuation of water levels across the mangrove forest in Lac Bay. In addition to a daily reduction in tidal amplitude, a spring-neap tidal variation in water levels was measured in the landward degraded mangrove area, where water levels step-wise increased towards spring tides, to then step-wise decrease towards neap tide. The daily and spring-neap tidal variation in water levels forced the water flow through the tidal creeks. The suspended sediment concentration in the tidal creeks was also found to vary on a daily timescale as well as to increase gradually on a spring-neap tidal timescale. Consequently, flows were primarily lagoon directed during neap tide conditions, when suspended sediment concentrations were high. This combination caused the export of sediment to the lagoon on a spring-neap tidal timescale.
Hence, while the reopening of tidal creeks enhanced the tidal flushing in the degraded mangrove area, it also increased the export of sediment to the lagoon, where the mangrove forest can replace other pristine ecosystems such as seagrasses. The example of Lac Bay lagoon thus showcase the importance of local knowledge about the geomorhological settings and dynamics for informed restoration of mangroves.
References
Balke and Friess, (2016) Geomorphic knowledge for mangrove restoration: a pan-tropical categorization. Earth Surf. Process. Landforms, 41: 231–239. doi: 10.1002/esp.3841.
Gijsman et al., (2024) Field measurement data of hydrodynamic and morphological processes in the mangrove forest of Lac Bay, Bonaire, Caribbean Netherlands [Data set]. Zenodo. https://doi.org/10.5281/zenodo.13904523
Goldberg et al., (2020). Global declines in human-driven mangrove loss. Glob. Change Biol. 26, 5844–5855. doi: 10.1111/gcb.15275
