The effect of physical and biological site characteristics on the survival and expansion of transplanted eelgrass (Zostera marina L)

Eelgrass (Zostera marina L.) was transplanted at seven sites along the New Hampshire side of the Piscataqua River in 1993 and 1994. The eelgrass transplanting was one component of the New Hampshire Port Authority Mitigation Project, designed to mitigate for impacts to natural resources associated with the expansion of the port facility. Over 2.5 hectares of eelgrass were transplanted using a newly developed transplanting technique, the horizontal rhizome method, and ultimately created eelgrass habitat at several sites. However, transplants did not survive at any of the intertidal areas planted and were greatly reduced at several subtidal sites. The intertidal transplants were lost due to severe ice scouring during the 1993/1994 winter. This dissertation focused on determining which factor(s) caused the loss of subtidally transplanted eelgrass. I investigated the relationship between subtidal transplant survival and various physical and biological site characteristics by analyzing field data and conducting field and mesocosm experiments. The results of my research show that light, current, and sediment characteristics were not limiting at the transplant sites and that transplant growth rates were not significantly different among the range of sediment types found at the transplant sites. My research also showed that belowground growth rates for eelgrass transplanted using the horizontal rhizome method were significantly higher than for eelgrass transplanted with the most commonly used transplanting method, the bundle technique. These results indicate that the variability in the survival of subtidally transplanted eelgrass was not the result of the transplanting technique or physical site characteristics, but was due to some other factor such as bioturbation. I then conducted experiments to quantify the effect of bioturbation on transplant survival. The results of a mesocosm experiment showed that green crabs (Carcinus maenas) can significantly decrease transplant survival when they occur in densities of greater than 4.0 per square meter. Similarly, the results of a field experiment showed that the clam worm ( Neanthes virens) can significantly decrease transplant survival by pulling the tips of the eelgrass blades into the sediment. These results demonstrated that bioturbation by N. virens and C. maenas significantly decreased survival rates of transplanted eelgrass, and that protecting transplants from the bioturbating activity of these organisms significantly increased transplant survival. The final aspect of my research was to create a transplant site selection model by combining all factors that influence transplant survival (as demonstrated by the research conducted for this dissertation and as documented in the literature). The result of this effort was the development of the Preliminary Transplant Suitability Index (PTSI) and Transplant Suitability Index (TSI). The PTSI/TSI methodology provides a framework and quantitative approach for selecting potential transplanting sites. The indices were applied retroactively to the New Hampshire Port Authority eelgrass mitigation sites and correctly differentiated between the successful and unsuccessful sites. The model is now being used as a site selection tool in other estuaries on the East Coast of the United States and will provide natural resource managers with an effective tool for identifying and prioritizing potential seagrass restoration sites