The economy of Bonaire is highly dependent on tourism. Tourists are drawn to Bonaire because of the beautiful nature and biodiversity in the coastal ecosystems, e.g, mangrove forests, seagrass meadows, coral reefs. Therefore, it is important to protect these coastal ecosystems. Currently, eutrophication and pollution are serious threats to Bonaire’s mangrove forests and seagrass meadows through terrestrial run-off and influx of Sargassum. Seagrasses are known to be sensitive to local environmental changes by bioaccumulation of chemicals and nutrients through absorption in their tissues. In this study, we used turtle grass (Thalassia testudinum) as a bioindicator of spatial and temporal variation in eutrophication and pollution in five different bays on Bonaire. We found that T. testudinum is a good bioindicator of eutrophication and pollution. Analysis of stable isotope signatures (δ13C, δ15N), nutrient (%N, %P, %S) and trace metal content (%As, %Cd, %Co, %Fe, %Mn, and %Ni) in T. testudinum leaf samples
revealed that Lagun was the most eutrophic and polluted bay on Bonaire. The high eutrophic and polluted state in Lagun is mainly due to a nearby landfill, large catchment area, and influxes of pelagic Sargassum spp.. In Lac Bay, higher sulfide stress was observed in T. testudinum leaf tissues in 2019 compared to 2015, which may have hindered the uptake of N and P by T. testudinum. The difference in sulfide stress between 2015 and 2019 is due to the massive influx of Sargassum that occurred in 2018. However, we suggest a possible recovery given better uptake of N and P and lower sulfur content in T. testudinum leaf tissues in 2022. This may indicate less sulfide stress in 2022 compared to 2019. In Lac Bay in 2022, biochemical content of T. testudinum leaf tissues collected at fixed sampling sites where the direct cumulative effect of Sargassum influxes was assumed to be highest (i.e., west of Lac Bay), were similar to tissues collected in areas with no or intermediate direct impact of Sargassum. This may also suggest recovery of T. testudinum that has survived the most severe influx in 2018. We showed that Bonaire’s coastal ecosystems are threatened by eutrophication and pollution through land-based run-off and the influx of pelagic Sargassum. Hence, we want to encourage the local government with this study that nature restoration measures need to be taken immediately to protect their coastal ecosystems.