Model development BES coastal waters : progress report 2024-2025

The Nature and Environment Policy Plan (NMBP) for the Dutch Caribbean 2020-2030 defines objectives for sustainable development and protection of nature and the environment in the BES islands: Bonaire, St Eustatius and Saba. It also proposes research on threats for ecological health in coastal waters and coral reefs. The Ministry of Infrastructure and Water Management (IenW) is responsible for developing a monitoring plan of coastal waters around the BES islands as part of the implementation of the NMBP. This task is coordinated by Rijkswaterstaat and by DGWB (Directoraat-Generaal Water en Bodem) at the Ministry, in collaboration with local partners at the islands. The implementation of the NMBP requires more insight in the sources of nutrients and suspended matter in the coastal waters of the BES islands. This would help to evaluate which measures would be most effective in improving water quality and the health of coral reefs. Rijkswaterstaat has asked Deltares to develop a model instrument that can provide this insight in the dominant sources and pathways of nutrients and suspended matter in coastal waters of the BES islands. This report describes the progress that has been made in 2023 and 2024 in the development of this model instrument. It combines catchment modelling of water, sediments and nutrients on the islands with transport of nutrients and suspended matter in marine waters. Although the NMBP only covers the 3 BES islands we included St Maarten, Aruba and Curaçao as well, to support existing collaborations on marine ecological research in these areas. For each of these islands catchment models for hydrology and suspended matter have been developed. These hydrology and sediment models were coupled with inland emissions and water quality models for all six islands. Nutrient inputs across the islands are primarily derived from four sources: local population, tourism, livestock pressure, and atmospheric deposition. The D-Emission model estimates show that Curaçao experiences the highest nutrient inputs, followed by Aruba, Bonaire, Sint Maarten, St. Eustatius and Saba. Wastewater discharges from the local population contribute the majority of nutrient loads, with Bonaire and St. Eustatius having a notable contribution from livestock. Tourism is an important source of nutrient contributions in Aruba. While the models provide insights into dominant nutrient sources and transport pathways, the current estimates and model outputs should be interpreted considering the uncertainties attached with data sources and model assumptions as explained in the report. For the transport modelling in coastal waters, two model domains have been developed: one for the northern islands: St. Eustatius, Saba and St Maarten and one for the southern islands: Bonaire, Aruba and Curaçao. Both model domains have been nested in the CMEMS global models, to provide a finer resolution around the islands than the global model. As a first step in model development, the nutrients and suspended matter concentrations in coastal waters are simulated as conservative tracers. This means that they behave as inert dissolved substances, that are only transported with the water and are not affected by water quality processes such as sedimentation, uptake by phytoplankton or denitrification. Large-scale spatial distribution patterns have been compared with available satellite data to enhance the understanding of the relative importance of local sources compared to sources from the South American mainland. Where in 2023, the catchment models only included water and suspended sediments, the nutrients were added in 2024. The hydrodynamic models for coastal waters reproduce temperature and salinity patterns from the CMEMS model and observations by WMR reasonably well, especially with the spatial refinement around the islands and the parametrization with regards to vertical mixing performed in 2024. The analysis of results of the coastal water quality model combined with satellite data indicate that for suspended matter local rainfall events are the dominant source. For nutrients, sources seem to be more dominated by offshore sources. This can be linked to the outflow of larger rivers on the South American continent. However, comparisons with observations also indicate missing sources for both nitrogen and phosphorus throughout the year. These missing sources, and the resulting fluctuation over time in nutrient seen in observations, can be an important gap in our model. The current model setup shows that on a yearly basis very few of the nutrients and silt particles originate from the islands, even close to the shore. However, the question is whether the model resolution allows for such an assessment and if a focus on yearly averages, instead of events, is required. More research is needed on this regard. The project results was shared with the research and policy community for the BES-islands during a hybrid workshop in February 2025. The model instrument will be available for external users in 2026, as part of the maintenance program of national model schematisations of Rijkswaterstaat (including a protocol of transmission and factsheet). The 3D model schematizations of the Caribbean Netherlands in D-HYDRO Suite have been developed for the following applications: 1. Tracer dispersion studies (in which, among other things, layering and horizontal and vertical exchange play a role). 2. Simulation of water movement and flow under different hydrological conditions. With regards to the applications it can be used for, a recommendation is to further investigate and develop the exchange and transport in the near-shore area This model schematization has not been developed for the applications below and therefore a reservation is made regarding the use of the model schematization for the following: 1. Hydraulic adjustments in the management area. 2. Morphological studies (in which, among other things, the bottom position varies dynamically). 3. Shipping guidance (in which, among other things, depth-varying currents and cross currents play a role).