Applications of Sediment Profiling Imagery (SPI)

Ecological Interpretations of SPI

Rhoads and Cande (1971) developed sediment profiling cameras 30 years ago to examine the effects of infauna interacting with sediments at the sediment-water interface. Their work has led to a better understanding of sediment dynamics from both biological and geological perspectives. Rhoads and Germano (1986) based their analyses of SPI images on a model of community succession where life-history attributes and functional relationships of infauna change along a predictable trajectory, although the species composition may vary geographically, temporally, or episodically. This model of community succession began with surface-dwelling "pioneering" species, which were replaced by deeper dwelling late-succession species. Physical and biological characteristics distinguished each community. For example, small polychaete tubes at the sediment-water interface often characterize pioneer communities, while late successional communities often are associated with subsurface feeding pockets and deep burrows. This paradigm for interpreting SPI images focused on four issues important to coastal managers, which could be addressed by examining benthic communities:

• secondary production, particularly with regard to commercially important species,
• pollutant transfer within food webs,
• accumulation of organic material that could fuel hypoxic events, and
• nutrient cycling related to primary production.

Uses of SPI for Benthic Assessments

Even though coastal managers have used SPI for 30 years, most studies and assessments of benthic communities do not use this technology. SPI was never promoted as a replacement for traditional benthic sampling (e.g., grabs and cores). Species lists and abundance, parameters not measurable with SPI, remain a cornerstone of many benthic assessments. So while SPI may considerably augment interpretations of benthic community data, there is not often a compelling argument for adding SPI to a monitoring program. Most exceptions have focused on coastal management issues that involve dredged material and hypoxia (see examples below).

A logical extension of this work has been the use of SPI to map habitats (e.g., Great Lakes, Boyer and Shen 1988; Narragansett Bay, Valente et al. 1992 and Diaz 1995; offshore areas, Cutter and Diaz 1998; and New York/New Jersey Harbor, this project). Palermo et al. (1998) used SPI-based habitat maps to examine alternative locations for island CDFs (confined disposal facilities) and pits for NY/NJ Harbor.

Tracking Dredged Material
During the 1980s, the U.S. Army Corps of Engineers (USACE) and resource agencies heightened their examination of potential impacts to benthic communities from the disposal of dredged material. These examinations often require tracking thin layers of dredged material that would not be detected in standard USACE surveys with a vertical accuracy of 15 centimeters. SPI is often the most cost-effective means for tracking dredged material in these situations.

Surveys are typically done in a radial pattern, emanating from the center of a dredged material disposal area with stations initially placed at short intervals and then increased to longer intervals near the end of the transect. Freshly placed dredged material has a distinctive appearance relative to the undisturbed bottom, and the thickness of this layer is measured to the nearest centimeter across the SPI camera faceplate. The thickness of this layer coupled with the coordinates for stations can be plotted to show the footprint of the disposal area and allow inferences about how the material was placed and any post-placement transport that has occurred. This type of monitoring is especially important in thin-layer disposal of dredged material (Wilber 1992), a practice where dredged material is spread into layers believed to be thin enough to allow many benthic organisms to survive. Such practices typically have a goal of placing dredged material in layers no thicker than 15 centimeters, which corresponds with the accuracy limit of industry-standard bathymetric survey practices.

Monitoring Disposed Dredged Material
Material dredged from harbors and navigation channels is often placed in underwater disposal sites by ports and the USACE. These groups, as well as resource agencies and the public, are concerned about the fate of that material, the benthic communities impacted by the disposal, and the fish and crustaceans that feed upon those benthos. In response to this concern, monitoring occurs to collect information critical to future management decisions. The Disposal Area Monitoring System (DAMOS) is one such monitoring program, and SPI plays a critical role in this program (Germano et al. 1994).

The USACE initiated DAMOS in 1977 to monitor disposal sites in New England. Over the years, DAMOS developed a paradigm that described the succession of benthic communities on dredged material. Early succession stages are characterized by small infauna that live at the sediment-water interface, often in small tubes and in high densities. Worms that feed on detritus accumulating at the sediment-water interface dominate this successional stage. Bioturbation is limited because the organisms are small and the food is at the sediment surface. A late succession community is characterized by deeply burrowing species that Rhoads (1967) called "conveyor belt species." These animals move particles over several centimeters, creating water-filled pockets (feeding voids). The presence of these organisms also is associated with well-oxygenated sediments. In both cases, the organisms that exhibit these characteristics leave distinctive signatures in the sediment that can be quantified using SPI.

In the DAMOS program, SPI is used as a cost-effective means to rapidly survey disposal sites to test whether they are assimilating into the environment according to the expectations of coastal managers. Deviations from expectations result in more detailed monitoring that would be too costly to apply on a routine basis and too slow to provide timely feedback within typical dredging schedules.

Benthic Habitat Quality in Oxygen-Stressed Systems
Nilsson and Rosenberg (1997) used SPI to develop an index of benthic habitat quality in oxygen-stressed fjord systems, and they closely tied the results of that index to traditional biological and geological sampling. As a result, managers can more rapidly and accurately map the spatial extent of hypoxic conditions within the sediments. The index assigns points to an image based on the type and extent of signatures that animals leave in the sediments. High scores are assigned to features that correlate with considerable bioturbation, and the overall score for an image is the sum of the feature scores. Calibrating the index to a particular estuary provides a cost-effective assessment tool that can aid management decisions or direct follow-up studies.

Benthic Habitat Quality Index developed by Nilsson and Rosenberg (1997)
Sediment Level	Features	Score
Surface	Fecal pellets Small diameter tubes Large diameter tubes Feeding pit or mound	1 1 2 2
Subsurface	Infauna in image Few burrows (1 to 3) Many burrows (more than 3) Shallow oxic voids (< 5 cm) Deep oxic voids (> 5 cm) RPD depth < 1.0 cm RPD depth 1.1 to 2.0 cm RPD depth 2.1 to 3.5 cm RPD depth 3.6 to 5.0 cm RPD depth > 5.0 cm	1 1 2 1 2 1 2 3 4 5

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