U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
ACTIVITIES AND PRELIMINARY RESULTS OF NEARSHORE BENTHIC HABITAT MAPPING IN SOUTHERN CALIFORNIA, 1998
Guy R. Cochrane1 and Kevin D. Lafferty2
Open-File Report 00-321
This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government.
1U.S. Geological Survey, MS 999, 345 Middlefield Road, Menlo Park, CA 94025
2U.S. Geological Survey, Western Ecological Research Center, c/o Marine Science Institute, University of California, Santa Barbara, CA 93106
Regional Geologic Setting
Sidescan Surveying System
The nearshore benthic habitat of the Santa Barbara coast and Channel Islands supports a diversity of marine life that are commercially, recreationally, and intrinsically valuable. Some of these resources are known to be endangered including a variety of rockfish and the White Abalone. State and National agencies have been mandated to preserve and enhance these resources and require detailed habitat characterization in order to do so. This project will characterize and map the benthic habitat in areas that have been selected because they have been set aside as National Sanctuaries or State Preserves, or are areas of ongoing or planned fish population studies.
Various management strategies are being developed to protect marine resources in the Santa Barbara Channel Islands Region. One approach under investigation is to implement no-take marine reserves (Agardy, T., 1997; Bohnsack, 1998; Roberts, 1997). One small reserve presently exists on Anacapa Island and there is a growing momentum to add additional reserves to form a reserve network (Lafferty et al., 2000). Reserves may provide relatively pristine marine communities in a wild state for study and appreciation. In addition, they may buffer some species from over-fishing. A key feature of marine reserve design is to protect a representation of the existing habitats in a region (Roberts, 1997). Unfortunately, the distribution of habitats is not well known in this area since the underwater equivalent of soils and vegetation maps that are widely available for terrestrial systems do not yet exist. Managers need habitat maps to help determine the most appropriate boundaries for reserves in a network in order to meet various criteria and goals (such as habitat representation, reserve size, habitat heterogeneity, reserve spacing, inclusion of sensitive habitats, etc.). Another use for habitat mapping is to better understand the distribution of those habitats that are particularly important to fished species or sensitive species. Combining habitat mapping with ongoing studies of egg and larval fish counts by the National Marine Fisheries Service (Russell Vetter), rockfish population studies by the California Department of Fish and Game (Dave VenTresca), and white abalone (Kevin Lafferty and others, USGS) will extend the ability to predict the distribution of these species and identify areas with appropriate habitat that might be suitable for restoration. Additional uses for habitat mapping include managing visitor use, kelp distribution, and archeological resources.
This report discusses geophysical data collected within the National Marine Sanctuary/National Park in 1998 on cruise B-1-98-SC of the USGS Western Region Coastal and Marine Geology Team.
The R/V Ballena, owned by the Channel Islands National Marine Sanctuary, was used for the geophysical surveying. Combined sidescan-sonar imaging (sidescan), seismic reflection profiling (profiling), and towed bottom camera work began on June 15, and ended June 26. Data were collected in nearshore waters, here defined as 0 to 100 meters in depth. 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.
Regional Geologic Setting
The area of study is the Northern Channel Islands and adjacent areas of the California coast. The Northern Channel Island chain runs from east to west, forming the southern margin of the Santa Barbara Channel and the southernmost range of the western Transverse Ranges. The regional structure of the Northern Channel Islands forms a broad anticlinorium, mapped eastward as a continuous structure over at least 220 km to the Santa Monica Mountains (Seeber and Sorlien, in review). Present-day folding and uplift of the Northern Channel Islands may be a result of slip on the north-dipping Channel Islands Thrust, the regional thrust fault inferred to underlie the Northern Channel Islands (Shaw and Suppe, 1994). The thrust faulting has uplifted and exposed volcanic and sedimentary rocks of Miocene age on the islands and adjacent seafloor, and small local exposures of older rocks in the study area (Vedder et al., 1986).
The 1998 survey was navigated with a Leica Differential Global Positioning System (DGPS) which provided a ship position with accuracy of 1-5 m in DGPS mode. At times during the cruise differential signal was interrupted. In non-differential mode, the receiver provided a position with 30-50 m accuracy. A KVH Industries Inc. azimuth digital gyro-compass provided ship headings with 0.5 degree accuracy. Navigation data were recorded using Yo-Nav version 1.19 (Gann, 1992). The sidescan fish was towed approximately 30 m above the seafloor. The distance of the fish behind the ship was not known during this survey and must be estimated when the data are processed in order to produce the sidescan image mosaics.
Sidescan Surveying System
A Klein 2000 sidescan system was used for geophysical surveying. The unit features 8 channels of processed data, 7 subsurface from the towfish (5 sonar and 2 instrumentation) and 1 surface (external analog input). Two sonar channels each were devoted to 100 KHz and 500 KHz sidescan data and a fifth sonar channel was used for 4 KHz subbottom profiling. The resolution of the profiler data is approximately 0.15 m of sub-bottom thickness (penetration of tens of meters is typical in unconsolidated sediment).
A Triton Elics Isis brand side-scan data recording system was used on the cruise. The Isis system simultaneously records 5 channels of data; port and starboard 100 KHz side-scan data, port and starboard 500 KHz side-scan data, and profiler data. Side-scan data shown in this report are 100 KHz data. Typically, 2048 samples were recorded per channel over a swath width of 400 m yielding a pixel size of 0.1 m of seafloor area for the side-scan data.
The sidescan-sonar data were processed following the methodology of Chavez (1984), using The USGS Mini Image Processing System (MIPS). The slant-range, destripe, and beam pattern-processing routines, executed within MIPS, correct geometric and radiometric distortions inherent in the sonar data. The slant-range algorithm removes the water column artifact from the sonograph and corrects slant-range distance to true ground distance; the destripe routine corrects fluctuations in adjacent ping values within the sonar record; the beam pattern routine corrects variations in beam intensity with range. The processed data files were mosaiced to form a composite image. A linear stretch was applied to the final mosaics to enhance the contrast between low and high backscatter areas. The final mosaics were exported from MIPS in TIFF format. The TIFF images were imported into an arc/info GIS database that contained coastline, geology, and bathymetry coverages. The processed and georeferenced sidescan data will be released in a future report.
Preliminary Analysis of 1998 Data
|General location of 1998 operations in the Channel Islands|
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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