The Role of a Hurricane in the Expansion of Disturbances Initiated by Motor Vessels on Seagrass Banks

Whitfield, Paula; Kenworthy, W.; Hammerstrom, Kamille; Fonseca, Mark

While investigating the ecology of physically disturbed seagrass and Porites porites coral banks in the Florida Keys National Marine Sanctuary (FKNMS), we documented the effect of Hurricane Georges on the recovery of injuries created by large motor vessels. Near the intersection of two main routes for vessel traffic in the Red Bay Bank region of the FKNMS, we found that blowholes caused by vessel groundings (n=146), as opposed to propeller scars (n=50), accounted for a majority of the vessel-related injuries on shallow seagrass banks. Evidence from four large motor vessel grounding sites that were mapped before and after a category 2 hurricane (Georges) with wind speeds of 96 knots (176 kmph) indicated an increase in injury size associated with the storm, At a site where a tugboat grounding had destroyed 100% of the macrophytes growing on 7,363 m^2 of a shallow bank, recovery was interrupted and reversed by Hurricane Georges. In January 1998, 4.5 yr after the original injury, macroalgae had recovered to 85% and seagrasses to 11% of the original 100% cover at the grounding site. In the second survey two months after Hurricane Georges, seagrasses "had declined to < 1% and macroalgae to 23% of the original cover. Twenty-one months later, seagrasses and macroalgae had recovered to near pre-hurricane levels, but cover was still much less than the original cover. Following the hurricane, the injured area increased from 1,576 m^2 to 2,514 m^2, a 63% increase. Three ather large vessel-grounding sites exhibited at least a 65% increase in the size of their blowholes following Hurricane Georges. Our data and observations suggest that large blowholes created by vessel injuries are unstable and will continue to be vulnerable to storm events many years after the original disturbance.

The Role of a Hurricane in the Expansion of Disturbances Initiated by Motor Vessels on Seagrass Banks

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