Submerged aquatic vegetation and bedforms in eastern Cape Cod Bay: spatial distributions, attributes, and associations

Trembanis, Arthur C.In this study, eelgrass beds and bedforms were mapped in nearshore areas of eastern Cape Cod Bay using phase-measuring bathymetric sonar. Eelgrass beds make up vital fish and invertebrate habitat by providing structure in this dynamic portion of the bay, which is otherwise dominated by unconsolidated sandy environments. Additionally, eelgrass is an important indicator of water quality,which interests resource managers and scientists in monitoring the health and abundance of this important habitat. The advantage of a phase-measuring bathymetric sonar system is that it collects collocated and coincident high-resolution bathymetric data and backscatter imagery across wide swaths of the nearshore. This collocated dataset was used to develop a method of mapping eelgrass distribution using an automated backscatter clustering program, and to determine canopy height and canopy volume of eelgrass beds from the bathymetric dataset. This dataset was also used to examine relationships between large-and small-scale bedforms and eelgrass. This included distributions of eelgrass around sandwaves, multiple longshore and transverse sand bars, as well as ripples. The study of bedforms and eelgrass associations stands as a unique example of the type of ecological study of seagrass that could be carried out with this type of data. The distributions of eelgrass beds, and other bottom types, of all three study areas were successfully generated. The canopy height was found to be about half of the measured canopy height, most likely due to the lay-down of the leaves with the currents in the area. Canopy volume was measured by creating digital terrain models of both the bare and vegetateds eafloor, and obtaining the volume between the two surfaces. Eelgrass was found growing on all parts of 4 sandwaves in Provincetown Harbor, but not on longshore or transverse bars in the North Pamet study area. Interesting case studies of changes in ripple morphology in and around eelgrass beds were identified, showing visual evidence of the well-known energy attenuation effects of the eelgrass canopy on waves and currents. Additionally, a possible relationship between wave exposure and bed patchiness was found across all three study areas, with increased patchiness correlating to increased wave exposure. Both studies also have implications for restoration in the area, parts of which have been previously identified as unsuitable for transplants due to high sediment mobility.University of Delaware, Department of GeologyM.S