Imagery Analyses

All images (plan-views and profiles) were visually analyzed by recording observed features into a standardized spreadsheet file. Easily identifiable features in plan-view images included amphipod tubes, bacteria mats, clam siphons, and epifauna. The following sections briefly describe the data collected from the SPI images. Further details of how these data were obtained can be found in Kiley (1989) and Viles and Diaz (1991).

Prism penetration, which provides a geotechnical estimate of sediment compaction, is measured as the distance the sediment moves up the length of the faceplate (25 centimeters). If the weight on the prism is kept constant, this measure provides a means for assessing the relative compaction between stations or different habitat types. Deep prism penetration usually indicates recent, rapid accumulation of sediments that have not had time to de-water (Don Rhoads, personal communication).

Surface relief, the difference between the maximum and minimum distances the prism penetrates on a single deployment, estimates small-scale bed roughness across the width of the prism (15 centimeters). The causes of roughness can include small sand waves (bedforms), fecal mounds, feeding pits or tubes, hydroid colonies, or submerged aquatic vegetation (SAV). Surface relief provides qualitative and quantitative data on habitat characteristics that can be used to evaluate present and past habitat quality.

Depth of the apparent Redox Potential Discontinuity (RPD), the depth to which sediments are oxidized, is an important estimator of benthic habitat quality. Reddish-brown sediment color tones are assumed to indicate oxidizing sediments, or at least not intensely reducing sediments (Diaz and Schaffner 1988), in accordance with the classical association of RPD depth with sediment color (Fenchel 1969). RPD depth was defined as the area of all pixels in the image discerned as being oxidized divided by the width of the digitized image.

Sediment grain size, a geotechnical measure of sediment texture, is used to infer the nature of the physical forces acting on the benthic habitat. For example, a predominance of fine sediments may indicate lower-energy water flow conditions resulting in sedimentation. Sediment texture descriptions (such as sandy silt or gravelly sand) followed the Wentworth classification as described in Folk (1974) and represented the major modal class for each layer identified in an image.

Surface features, both physical and biological in origin, can be seen at or on the sediment surface and include seagrass, worm tubes, fecal pellets, benthic animals, bacteria mats, algae, shells, sediment ripples, feeding pits, and mounds.

Subsurface features include burrows, water-filled voids, rhizomes, infaunal organisms, gas voids, shell debris, detrital layers and sediment lenses of different grain size. Subsurface features reveal information about the balance of physical and biological controls within the habitat. For example, the presence of gas voids with a mixture of nitrogen and methane from bacterial metabolism (Reineck and Singh 1975) indicate anaerobic metabolism (Rhoads and Germano 1986) and are associated with high rates of bacterial activity. Muddy habitats with large amounts of methane gas often indicate areas of oxygen stress or high organic loading (SAIC 1987, Day et al. 1988). Conversely, habitats with burrows, infaunal feeding voids and/or visible infauna are generally more biologically accommodated and considered "healthy."

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