Turbidity in coral reef habitats during Port of Miami Expansion dredging

Coral reefs are invaluable to Florida's ecology and economy, yet turbidity and sediment deposition generated by major dredging projects threaten coral health, survival, and recovery. The Port of Miami (POM) Expansion, completed in 2015, resulted in severe sediment impacts to nearby coral reefs as shown in prior studies, despite virtually no recorded exceedances above the regulatory turbidity threshold: 29 Nephelometric Turbidity Units (NTU) above background. Data from POM Expansion permit-required turbidity monitoring reports were mined and analyzed. Mandated turbidity monitoring data was compromised by lack of consistency in time intervals and locations, lack of near-bed measurements, and dependence on real-time, sea-surface visual observation to discern the location of turbidity plumes. Spatially interpolated turbidity measurements showed substantially different levels and spatial structure of turbidity conditions between background and turbidity plumes due to dredging (i.e., impact of construction conditions). Hopper dredging was associated with significantly higher turbidity than other operation types. The mandated pre-construction baseline was compromised by dredging. This "quasi-baseline" had significantly higher turbidity than the rest of the project, violating the most basic principles of impact assessment. Lastly, turbidity values were used to hindcast the effects of more stringent turbidity standards. Over a third of reported turbidity values were < 7 NTU, and ~ 80% of values were < 15 NTU. Assuming data accurately reflect conditions, adopting a lower NTU standard could better protect corals and still allow for timely completion of major dredging projects. Our study highlights the need for a stricter turbidity standard than 29 NTU over background, and we suggest additional improvements to environmental monitoring based on international best practices.