Comparison of Acoustic and Aerial Photographic Methods for Quantifying the Distribution of Submersed Aquatic Vegetation in Sagamore Creek, NH

摘要

Maintenance dredging in the Black Channel portion of the Portsmouth Harbor and Piscatiqua River Federal Navigation Project in Portsmouth (commonly referred to as Sagamore Creek) occurs in close proximity to submersed aquatic vegetation (SAV). Species, density, and spatial distribution are of concern to resource agencies given the potential impacts associated with dredging activities including the physical removal of vegetation as well as increases in turbidity and/or siltation. A variety of techniques are available for determining these attributes, including manual sampling, aerial photographic surveys, and acoustic-based surveys. Aerial photography is a standard technique for characterizing SAV distribution and, under some conditions, distinguishing species. It may underestimate SAV coverage if water clarity is low or there is poor contrast between SAV and adjoining bottom material. Acoustic surveys employ the acoustic reflectivity of the SAV for detection and for determining canopy geometric characteristics. Although acoustic techniques are not limited by water clarity, they are typically unable to distinguish species. Both photographic and acoustic techniques require some physical ground-truth sampling to verify interpretation and output. In preparation for scheduled maintenance dredging in Sagamore Creek, near Little Harbor, NH, SAV surveys were scheduled. During coordination meetings to plan these surveys, the validity of the height-based rule for distinguishing eelgrass from marine macroalgae was questioned. Accordingly, a study was planned to compare an acoustic-based estimate of eelgrass distribution with that from aerial photography. In particular, the effect of the acoustic-based plant height discriminant for estimating eelgrass coverage was to be evaluated in this study.