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SPI images illustrating two extremes in benthic habitat quality. The image at left shows black, highly anoxic sediments that lack visible benthic organisms, with a layer of white sulfur bacteria at the sediment surface and methane gas being produced at depth. At the other extreme, the right image shows oxygenated sediment with evidence of extensive biological activity at depth (feeding voids and burrows). |
In addition to the observation of sediment-organism relationships, overall quality of the benthic habitat can also be characterized by analysis of SPI images. This analysis consists of calculating an Organism-Sediment Index (OSI; Rhoads and Germano 1982, 1986), which is based on many parameters, including the community successional stage, the depth of oxygen penetration, and a variety of chemical parameters (such as the presence of methane).
Using the OSI, habitat quality is defined relative to an index scale of -10 to +11. The lowest value (-10) is for highly disturbed and degraded benthic habitats. These areas consist of extremely low oxygen penetration, no large organisms, and the presence of methane gas bubbles. Areas having an OSI of +11 have very deep oxygen penetration, mature Stage III communities, and no methane gas bubbles at depth.
In general, OSI values of +6 or greater indicate undisturbed or nondegraded benthic habitats. The calculation of the OSI is extremely useful for mapping gradients of disturbance in a particular area and documenting recovery of the benthic habitat after a disturbance.
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| ** Note: This is based on the imaged evidence of reduced, low reflectance (i.e., high oxygen demand) sediment at the sediment-water interface. The apparent Redox Potential Discontinuity (aRPD) is a measure of the depth of oxygen penetration into the sediments. |
More recently, Nilsson and Rosenberg (1997) developed an index of Benthic Habitat Quality (BHQ) based on the same SPI infaunal successional model but using a more quantitative determination of the relative densities of surface and subsurface organisms. They closely tied the results of that index to traditional biological and geological sampling. As a result, researchers and resource 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.
| Sediment Level | Features | Score |
|---|---|---|
| Surface | Fecal pellets Small diameter tubes (≤ 2 mm) Large diameter tubes (> 2 mm) Feeding pit or mound |
1 1 2 2 |
| Subsurface | Infauna Few burrows (1 to 3) Many burrows (> 3) Shallow oxic voids (at ≤ 5 cm depth) Deep oxic voids (at > 5 cm depth) RPD depth 0.1 to 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 |
References
Nilsson, H.C., and R. Rosenberg. 1997. "Benthic Habitat Quality Assessment of an Oxygen Stressed Fjord by Surface and Sediment Profile Images." Journal of Marine Systems. Volume 11. Pages 249 to 264.
Rhoads, D.C., and J.D. Germano. 1982. "Characterization of Benthic Processes Using Sediment Profile Imaging: An Efficient Method of Remote Ecological Monitoring of the Seafloor (REMOTSTM System)." Marine Ecology Progress Series. Volume 8. Pages 115 to 128.
Rhoads, D.C., and J.D. Germano. 1986. "Interpreting Long-Term Changes in Benthic Community Structure: A New Protocol." Hydrobiologia. Volume 142. Pages 291 to 308.
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