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Dredging Effects on Eelgrass (Zostera marina) Distribution in a New England Small Boat Harbor

Abstract : While speculation on effects of dredging on seagrass beds is plentiful, actual empirical data documenting these effects are not. In this study, acoustic-based seagrass mapping techniques were used to generate detailed maps of seagrass distribution before and after dredging operations. Eelgrass (Zostera marina) within Scituate Harbor, MA, was monitored during mid-summer in 2001, 2003, and 2004; navigation maintenance dredging of the harbor was performed during fall 2002. Similar surveys were also performed during the same timeframe at an undredged harbor near Wood Island, ME. Two types of potential impacts were examined. Direct impacts involved physical removal of vegetation along with the dredged sediments. Indirect impacts in adjacent undredged areas may occur as a result of increased turbidity and/or siltation associated with dredging activities. Using hydroacoustic techniques, the authors were able to easily map and quantify direct impacts to eelgrass resources. Assessment of indirect impacts, however, was more complex. In the first post-dredging survey, a substantial reduction in coverage occurred in adjoining undredged areas, suggesting possible indirect impacts. This was followed by a modest recovery between the first and second post-dredging years. However, monitoring of other undredged sites within the region showed natural year-to-year variations in eelgrass coverage to be almost as large as those occurring at the dredged site. Results emphasize the need for long-term data to discern any potential effects of dredging on seagrass dynamics as opposed to a host of other factors contributing to high variability in measured parameters.