oyster, restoration, boat wakes
In recent years, intertidal reefs of Crassostrea virginica (eastern oyster) along central Florida's east coast have suffered extensive losses due to wakes from recreational boats. These wakes have caused extensive shell movement and sediment resuspension which results in large piles of disarticulated shells along the seaward edges of reefs. Dead margins extend up to one meter above mean high water. The creation and enforcement of "no wake" zones in the area are unlikely. Thus, there is an urgent need for an alternative restoration strategy before these oyster reefs decline any further. The goal of this project was to develop a scientifically-based restoration technique that minimized wake damage from recreational vessels on intertidal reefs in Canaveral National Seashore. To accomplish this, I tested a range of restoration measures to identify a design that best increased: 1) oyster recruitment, 2) three-dimensional structure of the intertidal reefs, and 3) biodiversity and abundances of sessile and motile species associated with reefs. As a starting substrate in all treatments, I used restoration mats, which were created by affixing 36 drilled oyster shells to 0.4 x 0.4 m pieces of black mesh (Vexar). Five mats were deployed on the fore-reef, midreef, and backreef areas of each reef. In my experiment, I manipulated two habitat conditions: 1) leveling of existing dead margins to bring the top of the dead margin below mean high water to facilitate settling of larvae, and 2) deploying artificial seagrass seaward of the mats to act as a wake buffer. All combinations of these variables and all appropriate controls were replicated on six oyster reefs each, for a total of thirty reefs. Reefs that were leveled were significantly reduced in height and this difference was maintained throughout the 1 year study. Unleveled reefs actually increased in mean height over the 12 months. Tracking loose shells covering our restoration mats over time likewise documented that shell movement was minimal on control reefs lacking dead margins and significantly greater on reefs with dead margins. Midreef areas on reefs with dead margins were almost completely buried by loose shells. Quarterly monitoring of the number of spat settling on all restoration mats allowed for comparisons between treatments and locations on oyster reefs. After determining that overall water flow on the fore reef areas of all treatments was similar, I tested the null hypothesis that all treatments had similar recruitment of oyster larvae. My first alternative hypothesis was that artificial seagrass would increase oyster recruitment if the grass was a successful wake buffer and minimized sediment resuspension known to be lethal to newly settled oyster or prevented disarticulated oyster shell from moving and reforming mounds. My second alternative hypothesis was that the leveling of the dead margins would increase the total reef surface area available to larval oyster recruitment and thus lead to an increase in the number of recruits and eventually 3-dimensional reef structures (when oysters grow in close proximity and affix themselves together). Statistical analyses showed the artificial seagrass did not decrease the negative impacts caused by recreational boat wakes. Hence, it is not a recommended method for reef restoration. Recruitment of oysters significantly increased over time and significantly differed on various regions of the reefs. Recruitment was always highest on the fore-reef regions and lowest on back-reef regions. Although overall recruitment did not differ among treatments, it was significantly lower on midreef regions of the impacted reefs. This suggests that the leveling of the oyster reefs would increase the surface area available for future oyster recruitment. To look at biodiversity, I tested the hypothesis that all treatments would have similar biodiversity on a month by month basis. Alternatively, biodiversity should always be greatest on leveled reef with artificial seagrass due to increased 3-dimensional structure nearby and longer submersion times. To enumerate biodiversity, two lift nets were placed on each reef, one contained a restoration mat and the other contained only mesh (control). In most months, the four experimental treatments were similar according to the biodiversity measures analyzed. However, biodiversity was always higher in lift nets with restoration mats when compared to lift nets with mesh only. This result again suggests that the mats as designed are important restoration tools. Overall, my results show that placing seagrass in front of oyster reefs may not help to better restoration efforts. However, leveling dead margins on reefs and using the restoration mats is beneficial to oyster reef habitat restoration efforts. As a result of my research, restoration mats, in combination with leveling dead margins, are currently being used in a large-scale, community-based oyster reef restoration project within Canaveral National Seashore boundaries.
Walters, Linda J.
Master of Science (M.S.)
College of Sciences
Length of Campus-only Access
Masters Thesis (Open Access)
Barber, Andrea, "Restoration Of Intertidal Oyster Reefs Affected By Intense Recreational Boating Activity In Mosquito Lagoon, Florida" (2007). Electronic Theses and Dissertations. 3072.