Historical changes in intertidal oyster (Crassostrea virginica) reefs in a Florida lagoon potentially related to boating activities

Research in the late 1990s showed that some intertidal eastern oyster (Crassostrea virginica, Gmelin) reefs in Mosquito Lagoon within the Canaveral National Seashore, Florida had dead margins consisting of mounded up, disarticulated shells. It was hypothesized that boating activities were the cause of the damage because all the reefs were adjacent to major navigation channels. To investigate this, we characterized the history of the appearance of dead margins and other reef changes using aerial photographs taken between 1943 and 2000. Imagery analyzed included prints (black & white, color, or color IR) from 1943, 1951, 1963, 1975, 1988, and 1995, and digital imagery from 2000 (USGS 1:12,000 digital ortho-quarterquads), at scales from 1:6,000 to 1:24,000. Prints were scanned at a resolution sufficient to yield 1-m pixels. After scanning, each set of images was georeferenced to the year 2000 imagery using ArcView and ArcInfo GIS software. All reefs found to have dead margins based on 1995 and 2000 aerials were visited in 2001 and 2002 to confirm the presence and extent of dead areas. This provided ground-truthing for the "signature" (a highly reflective, light-colored area adjacent to darker-colored live reef) to be used to detect the appearance of dead margins in the historical aerials. The earliest appearance of dead margins was in the 1943 aerials on one reef adjacent to the Intracoastal Waterway (ICW), a major navigation channel. The total number of reefs affected and areal extent of dead margins steadily increased from 1943 through 2000. In 2000, 60 reefs (of a total of similar to400 in the Park) had dead margins, representing 9.1% of the total areal coverage by oyster reefs in the Park. Along the ICW, some reefs migrated away from the channel as much as 50 in and in 2000 consisted mainly of empty shells mounded up a meter above the high water mark. In contrast, many reefs in areas away from navigation channels showed little change over the 57-year period. This historical analysis provides strong (although correlative) evidence that boating activity has had dramatically detrimental effects on some oyster reefs in the study area. Ongoing studies are aimed at further testing this hypothesis and elucidating the causal mechanisms involved.