Water quality pressures threaten coral reefs in the Caribbean Netherlands
- dr. G (Gülşah) Bonenkamp-Dogruer
- WR Onderzoeker
Coral reefs around Bonaire, Saba and St Eustatius are facing increasing pressure from nutrient enrichment and chemical contamination. New data show that water quality around the islands is deteriorating as a result. This reduces the resilience of coral reefs at a time when climate change is already putting them under increasing stress.
Researchers monitored water-quality indicators between 2022 and 2025 and analysed contaminants in marine sediments. Because sediments accumulate pollutants over time, they provide valuable insight into long-term ecological risks. Together, these data form the first integrated assessment of water quality and chemical contamination across the three islands. The data collection began as a project of the Ministry of Agriculture, Fisheries, Food Security and Nature. Since 2024, the monitoring programme has been commissioned by Rijkswaterstaat and carried out in close collaboration with local partners on Bonaire, Saba and St. Eustatius.
Eutrophication
The assessment shows that eutrophication - the excessive input of nutrients like nitrogen and phosphorus into water bodies - is a risk on all three islands. Bonaire displayed the highest risk with frequent nutrient enrichment across much of the island, while St. Eustatius showed the strongest deterioration in water quality during the monitoring period. Saba generally showed lower overall risk levels, although several local hotspots were identified. However, regional effects from South American river systems or upwelling can also contribute to the high nutrient levels that were measured.
The findings indicate that land-based sources, such as wastewater, groundwater and runoff, play an important role in nutrient enrichment in coastal waters across all three islands.
Gülşah Doğruer, marine researcher at Wageningen Marine Research, says:
"This is the first time we have been able to look at water quality and chemical contamination together, across all three islands and over several years. We built this up jointly with the island nature organisations, STINAPA on Bonaire, STENAPA on St. Eustatius, the SCF on Saba and other local partners — into a strong, shared monitoring network. That joint effort and continued data collection are what make this assessment different from earlier studies: it does not depend on a single snapshot, but on consistent, comparable data that the islands themselves helped to collect and can continue using to guide management."
Chemical pollution
In addition to nutrient enrichment, the assessment identified chemical contamination at several locations across the Caribbean Netherlands. Elevated concentrations of substances including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), perfluorooctanesulfonic acid (PFOS) and globally banned tributyltin (TBT) were detected. These contaminants are persistent man-made substances - such as industrial chemicals, combustion by-products and anti-fouling compounds used on ship hulls - that break down very slowly in the environment. They are known to affect marine organisms in different ways, including impaired growth, reproduction and development. Some substances can also accumulate in marine food webs, potentially increasing ecological risks over time.
According to the researchers, nutrient enrichment, chemical contamination and climate change should not be viewed as separate challenges. Nutrient enrichment can lower coral bleaching thresholds and promote the growth of pathogens, while chemical contaminants may impair immune function and reproduction. Rising ocean temperatures can further amplify these effects.
Improving local water quality is therefore not only an ecological priority, but also an important climate adaptation strategy.
Erik Meesters, coral reef ecologist at Wageningen Marine Research, says:
"Improving water quality is the most important local action we can take to strengthen coral reef resilience. Clean water is to coral reefs what clean air is to us: essential for survival. By reducing water-quality stressors, which our results show are frequently above healthy thresholds, we give reefs a better chance to withstand the increasing impacts of climate change."
The study also suggests that local pollution may be one of the factors affecting coral restoration success. At three coral restoration sites on Bonaire, the site with the highest sediment contamination showed the lowest survival of outplanted young corals.
Supporting future management
The assessment identifies the need for continued long-term monitoring and targeted measures to reduce nutrient inputs and chemical pollution. The researchers also point to the absence of a dedicated water-quality assessment framework for the Caribbean Netherlands, and a governance gap that disconnects monitoring results from environmental targets and management actions.
According to the authors, strengthening monitoring, improving wastewater and waste management, and addressing land-based sources of pollution are important steps towards improving water quality and giving coral reef ecosystems in the Caribbean Netherlands a chance to withstand the impacts of climate change. Without such measures, reef degradation is likely to continue, restoration costs may increase, and opportunities to strengthen reef resilience in the face of climate change may become more limited.
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