Nearshore Benthic Habitat GIS for the Channel Islands National Marine Sanctuary and Southern California State Fisheries Reserves Volume II

GIS Description
  Abstract
  Introduction
  Updates
  File formats
  Acknowledgments
  References
  Contacts

Regional Data
  Bathymetry
  Navigation
  Samples

Mapped areas
  N. Anacapa Passage
  S. Anacapa
  S. Anacapa Passage
  SE. Santa Cruz

Data Catalog

Revision History

GIS DATA FOR STUDY AREAS

Sidescan Sonar Images

Sidescan sonar imaging provides high-resolution images of the sea floor by recording the intensity of sound reflected off the sea floor (acoustic backscatter). The sidescan data have been processed and mosaicked into continuous images of four study areas. The sidescan GIS data layer is an image built from data with a horizontal resolution of 1 m with grayscale values proportional to the acoustic backscatter of the sea floor. Sonar images of the sea floor were produced from 100 KHz sidescan sonar records obtained with a Klein 2000 sonar. The sidescan sonar data were processed following the methodology of Chavez (1984), using the USGS Mini Image Processing System (MIPS).

Diver and Video Observation Data

Point-based visual observations of benthic habitat are derived from towed video camera and Deepworker submersible. Postional accuracy is on the order of 10 meters. Towed camera video observations were logged real time using the method of Anderson et al. (in press). Deepworker video observations were logged with the same methodology but not in real-time.

Benthic Habitat Polygon Data

In this project, we use textural image analysis to generate a habitat classification of the sea floor, a method which has been used successfully in other sidescan sonar mapping studies (Blondel, 1996). Textural image analysis involves the calculation of image homogeneity and entropy (as defined by Shokr, 1991). Entropy measures the roughness of acoustic texture, and homogeneity measures the degree of organization of the texture (Blondel and Murton, 1997). These values were computed for approximately 10x10 pixel training regions, called "signatures," for areas identified in the point coverage of visual observations, as was done in an analysis by Cochrane and Lafferty (2002). These signatures were then used to perform a supervised classification of the entire sidescan image. The resulting classified image was edited to remove areas where sonar noise or attenuation resulted in misclassification. The classified images were also low-pass filtered to reduce the number of polygons to less than 100,000 when the images were converted to polygon features. The classified image was then converted to an ArcInfo grid and then a polygon coverage with a bottom type attribute, as defined by Greene and others (1999), which consists of either "h" for hard bottom, "m" for mixed low relief rocky bottom and thin sediment, or "s" for thick sediment bottom type. The polygon coverage was edited to remove areas where noise or other sonar data degradation produced erroneous classification. A low-pass filter was used to fill in unclassified pixels and generalize the classification to reduce the number of polygons. A "megahabitat" attribute was added to all the polygons at this point, most polygons are located on the Shelf (S) while a few are located on basin or canyon flanks (F).

The submersible- and bottom-camera-video observations were combined in an ArcGIS point shapefile, which can be overlain on the sonar images to aid in interpretation. The bottom observations were used to select areas of the image that represented rocky sea floor for textural image analysis (discussed above). The observations were also used to identify macrohabitat, the third polygon habitat attribute of Greene and others (1999). The macrohabitat classification could be improved significantly with additional video surveying. Macrohabitats shown in this report include rock pinnacles (hp), rock with kelp (h_k), differentially eroded rock (h_d), rock with landslide (hl), rock with landslide and kelp (hl_k), mixed sand and rock areas with sandwaves (mw), mixed sand and rock areas with landslide (ml), and sand areas with sand waves (sw). Area data for habitat classifications were generated using XTools Pro Extension for ArcGIS 9.X and the ArcMap Field Calculator Tool.

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North Anacapa Passage

Geology and Habitat

Table 1. (nanphab.html)

Rocks of the Miocene Monterey Formation, exposed about 5 km west of the study area on Santa Cruz Island (Dibblee, 2001b) probably underlie the southern part of the North Anacapa Passage study area. The Monterey Formation consists of marine siliceous shale and mudstone with layers of hard porcellaneous shale and chert that outcrop as raised resistant ridges in the study area. Blocky outcrops in the southwestern part of the study area may be Miocene basaltic volcanic rocks that underlie the Monterey Formation (Dibblee, 2001b). These rocks are deformed into broad open folds trending approximately east-west that have tilted the strata with dips generally less than ~25°, as observed on underwater video. The resistant bedrock layers form bouldery outcrops and overhangs approximately 0.5-2 meters high and 3-5 meters wide that are up to several hundred meters long. The seafloor is composed of primarily mud and sand in the northern part of the study area, covering slightly deeper parts of the shelf edge and upper slope. Rockfish are abundant near the bouldery outcrops; also common are lingcod, flatfish, red and white urchins, anemone, sponges, sea stars, algae, and kelp. Sandy areas are usually covered with prodigious numbers of brittle stars.

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South Anacapa

Geology and Habitat

Table 2. (sanahab.html)

Miocene basaltic volcanic rocks (Dibblee, 2001a) underlie most of the southern part of the shelf south of Anacapa Island. The volcanic rock outcrops are highly fractured and blocky. The portion of the study area west of the midpoint of West Anacapa Island, at depths of 50 to 60 meters, is underlain by the Miocene Monterey Formation (Dibblee, 2001b) consisting of marine siliceous shale and mudstone with layers of hard porcellaneous shale and chert that outcrop as raised resistant ridges in the study area. These rocks are folded into approximately east west trending and decline-syncline pairs that have been eroded leaving a complex outcrop pattern on the seafloor with strata dips varying from nearly flat to 45°. Similar folds of this formation at the western edge of the study area are described in the South Anacapa Passage section of this report. Resistant layers in the Monterey Formation form bouldery outcrops and overhangs approximately 0.5-1 m high and 1-5 m wide that are several hundred meters long. There is a possible landslide deposit approximately 300 meters wide in the near shore of West Anacapa Island. Mud and sand areas covering slightly deeper parts of the shelf edge and upper slope are separated by broad low-relief northeast-to northwest-trending volcanic rock highs. Rockfish were observed near the bouldery outcrops only at depths greater than 40 m; many white urchins were observed on sand-mud bottom on one video transect south of Central Anacapa Island in water depths of approximately 70 m. Sandy areas are often covered with prodigious numbers of brittle stars. There were scattered gorgonians, anemones, sponges, sea stars, and sea whips. Bioturbation mounds and holes in sand and mud were primarily found in depths greater than 40 m in the central section of the area south of Anacapa Island.

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South Anacapa Passage

Table 3. (sanphab.html)

The Miocene Monterey Formation, which is exposed ~5 km west of the study area on Santa Cruz Island (Dibblee, 2001b), probably underlies the South Anacapa Passage study area. The Monterey Formation consists of marine siliceous shale and mudstone with layers of hard porcellaneous shale and chert that outcrop as raised resistant ridges in the study area. These rocks are exposed as broad open folds trending approximately east-west with strata that dips generally less than ~25°, as observed on underwater video. Several NE-trending faults cut the study area but appear to have small offsets of several tens of meters or less. Unconsolidated sand and gravel overlies these rocks in small patches on the order of tens of meters across throughout the study area, and a larger deposit about 1.2 km across covers the northwest corner of the study area. Resistant bedrock layers form bouldery outcrops and overhangs approximately 0.5-2 m high and 3-5 m wide that are several hundred meters long. Brecciated zones along some of the NE-trending faults also form bouldery outcrops. Between these ridges are flat, low-relief areas underlain by rock, often mantled by thin deposits of unconsolidated sand and gravel. Rockfish are abundant near the bouldery outcrops; also common are lingcod, flatfish, red and white urchins, anemones, sponges, sea stars, algae, and kelp. Sandy areas on the western side of the study area are usually carpeted with brittle stars.

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Southeast Santa Cruz

Table 4. (secruhab.html)

Miocene basaltic volcanic rocks (Dibblee, 2001b) underlie most of the shelf southeast of Santa Cruz Island. The eastern nearshore area has outcrops of Monterey Formation (Dibblee, 2001b) sedimentary rocks and recent landslide deposits. The volcanic rock outcrops appear to have some layering, are highly fractured, and blocky. Mud and sand areas covering slightly deeper parts of the shelf edge and upper slope separate northeast to northwest trending volcanic rock highs. Rockfish were observed on all video transects across rocky areas. White urchins were observed in sandy areas on all transects deeper than 40 m. Sandy but not muddy areas are often covered with prodigious numbers of brittle stars. There were scattered observations of gorgonians, anemone, sponges, sea stars, sea whips, and driftweed. Kelp was observed on one video transect that was shallower than 30 m. We surmise that the sand-mud areas are highly distributed based on the high numbers of burrow mounds and holes that were seen on all video transects in sand-mud areas.

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