Open-File Report 00-321: Activities and Preliminary Results of Nearshore Benthic Habitat Mapping in Southern California, 1998

Activities and Preliminary Results of Nearshore Benthic Habitat Mapping in Southern California, 1998

Preliminary Analysis of 1998 Data

General location of 1998 operations in the Channel Islands

Two areas were surveyed in 1998, a 59 square km area north of Anacapa Island (Figure 1, 66K), and a 113 square km area surrounding the Big Sycamore Canyon Reserve, on the California coast, northwest of Pt. Dume (Figure 2, 76K). Approximately 6 hours of bottom video was recorded in each of the two areas that will assist in habitat classification of the sidescan data.

Figure 3 (208K) is an example of the sidescan and profiler data collected east of the Anacapa reserve. The sidescan data shows the seafloor geology whereas the profiler reveals the thickness of sediment beneath the seafloor and structure of rocks beneath the sediment. The Figure shows a circular pattern of outcropping rock rills interspersed with sediment fill. Our interpretation of the profiler data is that the outcrop pattern is the product of folding of layered sedimentary rocks and subsequent exposure of the dipping layers by erosion of the top of the fold, probably the result of wave action during a low sea level stand. A geologic map (Vedder et al., 1986) shows that bedrock in the area of Anacapa Island is either undifferentiated sedimentary rocks of Miocene age (Tm, Figure 4 (325K)), or volcanic rocks of Miocene age (Tmv, Figure 4 (325K)). The layering of the rocks in the data (Figure 3 (208K)) identify them as sedimentary rocks, probably of the Monterey Formation of Pliocene and Miocene age (Dibblee and Ehrenspeck, 1998).

Modern unconsolidated sediments are not shown north of Anacapa on the Vedder et al. (1986) geologic map (Figure 4 (325K)). Scholl (1960) did sample sediment covering some of the bedrock in this area. This points out the problem of using low resolution geophysical data designed for regional geologic mapping, and oil exploration, to map surficial geology for benthic habitat. The data set being collected in this project is designed for surficial geologic mapping. The low reflectivity (dark pixels) and uniformity (little difference in adjacent pixels) of much of the area in the sidescan data (Figure 3 (208K)) suggest that unconsolidated sediment is present in the depressions between the rocky rills. Bottom camera video (23M) taken over the rills and intervening depressions confirm that much of the seafloor is covered by sand. The profiler data can resolve seafloor parallel reflections thicker than approximately 15 cm. The absence of seafloor parallel reflections in the depressions between the rock outcrops indicates that the existing sand layers are thinner than 15 cm.

In the example area bottom video (23M), the most abundant macroinvertebrate was the small, white sea urchin (Lytechinus pictus). A bat star (Asterina miniata) is also seen. A small rocky outcrop with a slight layer of sediment supported gorgonians (they appear to be Lophogorgia chilensis).

By combining the surficial geology with oceanographic data, and biologic data from this and other projects, we will infer the benthic habitat in a future report. For example, using the habitat characterization scheme of Greene et al. (1999), the "mesohabitat" of the example area (Figure 3 (208K)) is classified as outer continental shelf (water depth 30-200m), thick sand (?) (any sand > 5 cm thick), flat bottom, probable winnowing by tidal currents, with occasional bedding outcrop (less than 10% of the area), with a dusting of sand (less than 1 cm thick), and patchy cover of encrusting organisms (20-70% cover).

The habitat classification scheme (Greene et al., 1999) must be applied to areas surveyed in addition to the example area in this report. Now that some baseline data area available, one may be able to monitor change in these habitats. For example, 5 cm of sediment cover could easily be removed by ocean currents or storms in some of the study areas and subsequently deposited atop rocky habitat nearby, greatly altering the habitat's suitability for different species. The geophysical surveying done in this and future years will be combined with existing population studies, sediment sampling, ROV, submersible, and bottom video camera observations to better understand benthic habitat - faunal relationships.

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Suggested citation:

Cochrane, Guy R., and Lafferty, Kevin D., 2000, Activities and Preliminary Results of Nearshore Benthic Habitat Mapping in Southern California, 1998: U.S. Geological Survey Open-File Report 00-321, https://pubs.usgs.gov/of/2000/0321/.

U.S. Department of the Interior
SALLY JEWELL, Secretary

U.S. Geological Survey
Suzette M. Kimball, Acting Director