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|Title:||Comparison of acoustic and aerial photographic methods for quantifying the distribution of submersed aquatic vegetation in Sagamore Creek, NH|
|Authors:||Sabol, Bruce M.|
Reine, Kevin J.
Shafer, Deborah J.
|Keywords:||Underwater acoustics--New Hampshire|
Aquatic plants--New Hampshire
|Publisher:||Environmental Laboratory (U.S.)|
Engineer Research and Development Center (U.S.)
|Series/Report no.:||ERDC; TN-DOER-E23|
|Abstract:||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. Manual sampling, including diver surveys and physical sample collection, provides the highest level of certainty of species composition and density, but is labor-intensive and results in limited spatial coverage per unit of sampling effort. Aerial photography (Finkbeiner et al. 2001) 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 (Sabol et al. 2002) 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 recent years numerous acoustic surveys of SAV have been conducted at Corps small boat harbors in New England (Sabol et al. 2005; Sabol and Johnston 2002; Sabol and Berry 2001). The primary purpose of these surveys has been to determine the density and distribution of ecologically valuable eelgrass (Zostera marina). Eelgrass in the New England region is a robust bladed seagrass with stems frequently exceeding a meter in length during the period of peak biomass (typically June-August). While eelgrass is not the only SAV species present, during the summer it is far greater in stature than other species such as the marine macroalgae Fucus. During these surveys (Sabol and Berry 2001) a limited amount of physical sampling was conducted to determine how to distinguish areas containing eelgrass from areas colonized with other species. It was noted that vegetated areas containing some eelgrass typically exhibited vegetation heights, measured acoustically, exceeding 1 ft (0.3 m) in height, while vegetated areas without eelgrass were typically less than this height. This height-based rule was subsequently applied to several eelgrass surveys (Sabol et al. 2005). 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.|
|Appears in Collections:||Technical Note|