International Dredging Review

International Dredging Review

As part of the overall environmental mitigation requirements for the deepening and widening of Miami Harbor in October CSA Ocean Sciences Inc. (CSA) completed the transplantation of more than 115000 seagrass plants into the newly filled dredge hole north of the Julia Tuttle Causeway in Miami Florida.

CSA was part of the Great Lakes Dredge and Dock team awarded the prime contract by the U.S. Army Corps of Engineers. For more on the Deep Dredge project see page 18.

During August and September 2015 CSA staff systematically planted 14.3 acres of the 17-acre mitigation site using donor manatee grass (Syringodium filiforme) harvested from a nearby healthy seagrass community in Biscayne Bay. CSA utilized proven methods developed and published by Dr. Mark Fonseca a worldrenowned marine ecologist and vice president of science at CSA.

CSA said regular coordination with the GLDD team and federal and state agencies was necessary during the planning and implementation phases due to the location of the mitigation site (situated in a state Aquatic Preserve) the high-profile nature of the project and low success rates associated with prior large-scale seagrass restoration projects.

Sensitive to desiccation and temperature extremes seagrass plants were carefully extracted from the sediments of an approved seagrass bed sorted and assembled into 29000 individual bare root “planting units” while ensuring they remained bathed in ambient seawater using a custom made on-board circulating system. Scientific divers meticulously planted each planting unit by hand to ensure the growing tips of the plant were buried to the appropriate depth below the sediment surface. To provide passive fertilization to the plants through the introduction of coastal bird feces more than 1150 bird roosting stakes were installed within the planted areas of the mitigation site.

In early October CSA conducted the first monitoring survey to assess the success of planting and to verify that the planting units remained firmly anchored in the sediment. A comprehensive evaluation of the entire planted area and all planting units was conducted and despite frequent feeding by manatees on the planted seagrass the percent survival was documented at 97.6 percent—much higher than the mandated 70 percent survival. Dr. Fonseca noted that “barring any unforeseen disturbances this is on course to be one of the largest and most successful actively planted commercial (seagrass mitigation) projects to date.”