Characteristics and assessment of dredge related mechanical impact to hard-bottom reef areas off northern Dade County Florida

Abstract

Beach erosion control measures have become a necessity along the south Florida coast due to the continued erosional loss of protective beach and dune areas. The primary method to date, for beach reconstruction in south Florida involves dredging of sand from an offshore "borrow area" and pumping the sand to the shoreline. Although the method results in a "new" beach with greatly enhanced erosion protection and recreational use, it is often not without impacts (both unavoidable and avoidable impacts) to the environment. In the summer of1988, Dade County sponsored a beach erosion control project to renourish a 2.5 mile segment of northern Dade County shoreline. Near the time of completion of the project, areas of mechanical impacts to the reef adjacent to the borrow area were discovered. The physical characteristics of the impact indicated it was associated with the dredging operations. A subsequent survey of the reefs bordering the borrow area identified nine sites of impact. At each site physical evidence was found consistent with the dredging equipment making contact with and scraping the reef. Two of the nine locations exhibited substantial (i.e., orders of magnitude greater in size) and severe impact. These sites were chosen for detailed assessment. The assessment involved mapping the extent and magnitude of the impacted area via evenly spaced transects which were evaluated by biologists using scuba. The assessed impact at the two sites was spread over an area of 2.2 acres. Approximately 1.5 acres of benthic hard-bottom communities within the impacted area were destroyed (destruction of 75-100% of the benthic organisms). It was estimated that over 25,000 hard coral colonies, 24,000 soft coral colonies and over 2,000 barrel sponges were among the organisms destroyed by the dredging equipment. This destruction represents a significant impact to the hard-bottom community within the region by reducing habitat quality, density of organisms, reef structural complexity and the overall productivity of the area.