Protecting Corals while Maintaining Safe Navigation

Numerous ports, including Charleston SC, Savannah GA, and Jacksonville, Fort Lauderdale (Port Everglades), and Miami, FL, are expanding to accommodate larger ships.  A main driver for the expansion is the shipping industry building the large vessels, referred to as post-Panamax vessels, able to move through the recently expanded Panama Canal.  These vessels require deeper and wider channels and overall more space to safely navigate through marine traffic and strong ocean currents.  Allowing large port projects to continue while also protecting seagrass and coral reefs, designated Habitat Areas of Particular Concern for federally managed fishery species including snappers, groupers, and spiny lobster, can be challenging.  While each project presents a unique set of circumstances, it is critical to learn from each so that fishery habitat is better protected during the next large-scale dredging project.

 

At the Port of Miami, 5.5 million cubic yards of material (enough to fill over 500,000 dump trucks!) was dredged from the federal channel and inner harbor areas between late-2013 and mid-2015.  In order to widen part of the federal access channel in the ocean, over 3 acres of coral reef was dredged.  In addition, over one hundred acres of coral reef habitat surrounding the Port of Miami channel were affected by sediment stirred up in the water column, referred to as turbidity, or deposited on other parts of the coral reef, referred to as sedimentation.  While sedimentation can leave a distinct signature on a corals or other important sessile organisms, such as octocorals and sponges, turbidity generally affects corals physiologically in ways difficult to detect with field observations.  Both sedimentation and turbidity can cause total mortality of corals or a significant fraction of the live tissue can die within a colony (often referred to as partial mortality).  While partial mortality is commonly observed on corals in a healthy coral reef environment, the increased prevalence of new partial mortality may indicate a site has been impacted by dredging project.

 

When sedimentation is an environmental concern, it is important for the project to include environmental monitoring to identify sedimentation-associated stressors at appropriate spatial scales for the local oceanographic environment.  Currents, such as the Florida Current running predominantly north off the coast of southeast Florida, can transport sediment a considerable distance away from the dredging (Figure 1).  Once the spatial extent of monitoring sites is established, the presence of sediment-associated stressors of corals can be examined.  Indicators of dredge-related impacts for scleractinian (stony) corals include the presence of a “sediment halo,” defined by the somewhat concentric ring of dead coral tissue at the base of a coral colony that can result from a coral’s inability to completely remove sediment that accumulated on live tissue (Figure 2).  Similar to scleractinian corals, if a portion of the octocoral (soft coral) colony remains buried by sediment, it will not be able to heal.  The burial of octocoral attachment points (holdfasts) under a thick layer of sediment can kill the tissue in this area and make the holdfast more likely to break or detach from the reef leading to death of the octocoral (Figure 3). 

 

Coral reef impacts from the Port of Miami expansion went undetected by the monitoring that occurred while dredges were operating.  Application of lessons learned from the Port of Miami expansion project, in particular the methods for detecting sedimentation signatures on the coral reef and spatial extent of potential impacts, will improve the design of the environmental monitoring for future large-scale dredging projects near coral reefs thereby helping to conserve Essential Fish Habitats.  Adopting best practices from other regions where dredging near coral reefs also occurs, such as the implementation of “no dredging” windows in Australia to protect corals during spawning, could further help reduce coral reef impacts, while allowing these economically important projects to continue. 

 

Figure 1: Google Earth imagery from December 2014 showing the sediment plume at the Port of Miami extending over one kilometer north of the spider barge used to transfer material from the dredge to the scows that haul material to the disposal location.  The large plume is entirely over coral reef habitat.  The federal channel is depicted in red.

 

Figure 2: Illustration of the “halo” pattern of partial mortality originating from partial burial of the colony by sediment.  The Right photo was taken after a diver had manually removed sediment by fanning.  Note the ring of dead coral tissue (white skeleton) resulting from the burial depicted in the left photo.  These photos were taken 200 meters north of the federal channel.

 

Figure 3: An octocoral after a diver fanned away the sediment at the base of the colony.  Note the erosion of the live tissue, called gorgonin.  This photo was taken 200 meters north of the federal channel.