Historical and modern data sources reveal long-term declines in Caribbean coral reef water quality

Although it is suspected that the widespread death of Caribbean corals is associated with declining reef water quality from land-based pollution, this link has been difficult to quantify due to a dearth of reef water quality data. To assess the role of land-based pollution in recent coral declines, we synthesized paleoecological, historical, and modern data on reef water quality across the Caribbean region from 1970-2021. We focused on two aspects of water quality that have been most consistently collected in the Caribbean and have been shown to have direct impacts on corals: nutrients (nitrogen and phosphorous) and water clarity. We analyzed trends in water quality across the wider Caribbean and within individual well-studied countries or reefs, providing insights into how the differing histories of land use and contrasting oceanographic conditions have influenced the varying trajectories of reef water quality change and ecosystem health. On a regional scale, nutrient concentrations had already exceeded established thresholds beyond which coral health was compromised by the 1970s, nutrient levels increased and water clarity decreased between the 1970s-2000, and nitrogen to phosphorous ratios were negatively correlated with coral cover. Case study sites revealed that coral cover declines were often related to water quality declines even in marine protected areas and at offshore reefs previously thought to be less influenced by land-based pollution. This synthesis of long-term historical data confirms the urgent need to mitigate land-based pollution to improve reef health and resilience in the face of increasing alteration of coastal zones and climate change impacts.