ESS Group, Inc. (ESS) was contracted by Cape Wind Associates LLC to conduct an investigation of several areas of Horseshoe Shoal in Nantucket Sound, where previous side-scan sonar observations indicated potential submerged aquatic vegetation (SAV) beds. This investigation was conducted on July 25, 2006, to ground-truth these potential SAV signatures in the general area of the proposed Cape Wind Energy Project. The major goal of this study was to determine the presence or absence, and to qualitatively assess the composition of, SAV in these areas of variable side-scan sonar returns.
The vegetative composition within the study area was found to consist primarily of attached red (Grinnellia americana, Dasya pedicellata and Gracillaria tikvahiae), and green (Codium fragile, Ulva lactuca) macro-algae. Of the 20 observation points, only one location included patches of eelgrass (Zostera marina). Of the species identified above, only C. fragile is not native to New England waters; however its presence since its introduction has rapidly expanded to range from emergent tidal pools to depths of -12 meters (Villard-Bohnsack 2003).
The data collected during this investigation indicates that while there is significant SAV present on Horseshoe Shoal, it is primarily macro-algae and not seagrass. Many of the macro-algae observed are considered seasonal, beginning its growth in early to mid-summer and disappearing by late August (Hillson 1982, Kingsburry and Sze 1997, Villard-Bohnsac 2003). Of the species observed, G. americana is potentially the most likely responsible for the variable side-scan sonar readings collected during previous geophysical studies conducted in 2003 and 2005. G. americana is a fast growing red alga, with a two- to four-inch-wide blade capable of growing to 50 centimeters in length within a single summer growth season (Hillson 1982). These algae would potentially show as an irregularity on side-scan sonar surveys during the summer growth season, and is likely the reason for the original variable sonar returns.