Sediment plumes induced by the Port of Miami dredging: Analysis and interpretation using Landsat and MODIS data

Abstract Beginning in November 2013, large turbidity plumes were observed offshore the Port of Miami (Florida, USA), likely associated with a project to deepen and widen the Miami Harbor channels. Local coral colonies, including those considered threatened by the US Endangered Species Act, exacerbate the need for thorough assessment of these plumes. Without ruling out other causal factors such as wind storms and tidal currents, it is difficult to conclude whether the plumes were caused by the dredging. From current in situ monitoring programs, it is also difficult to estimate the size, duration, extent and historical context of these plumes. Satellite observing systems, in contrast, offer a means by which these plumes can be monitored and compared to previous events. As such, turbidity plumes visible in Landsat 8 and MODIS Aqua imagery were first manually outlined, and then refined (for MODIS only) using anomaly and normalized anomaly thresholds determined from pre-dredging data. Local environmental conditions were also considered and used to determine dates for which elevated reflectance data might be expected in the absence of dredging. In total, the spatial extent of all turbidity plumes observed from November 2013 to April 2015 was between 127 and 228 km 2 , at least 5 times that for January – October 2013. Furthermore, the frequency of observed plumes in images increased from 23% to 84% after dredging began. Coral areas were particularly affected after dredging began, with over 11 km 2 of coral area being within plumes on an average of 16% of images (maximum 39%). Temporal differences in location, severity, and size were also observed. Together, these results highlight widespread turbidity plumes associated with the Port of Miami dredging activities, which may cause large adverse effects on local coral communities. The approaches developed in this work, in particular the focus on historical norms after considering all perturbation factors, may be included in monitoring and assessment of this and future dredging activities, especially where fragile marine ecosystems may be impacted.