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U.S. Geological Survey
Open-File Report 2007-1271
Version 2.0

Seafloor Mapping and Benthic Habitat GIS for Southern California, Volume III

RESULTS

Seafloor Features

Substrate

Classification of the area mapped by sonar shows that approximately 98% of the seafloor is covered by unconsolidated sandy mud. The sand content increases in the nearshore waters and video observations suggest mud is lacking in waters less than 20m deep. There are hard and mixed areas with bedrock outcrop along the ridge of an east west trending bathymetric high in waters ranging in depth from 35 to 50m. This area is of interest as rockfish habitat in an area near oil platforms that provide artificial habitat. Another rocky area was found off Rincon Point. This rock is an extension of the point into the marine environment and appears to consist of a wide range of angular clasts boulder size and up.

Common Biota

As described in Sea Floor Video methods, the presence of benthic organisms and demersal fish were visually observed and digitally recorded in real time. Twenty-five types of subtidal biota were observed. It is important to note that the towed camera is not adept at observing pelagic species, or demersal species that tend to move upward, move quickly, or move away from a location in response to the camera sleds approach. Of the types of biota that could be successfully recorded, the most common included gorgonians, hydrozoan, sea stars, sea whips, sponges, and white urchins. Gorgonians and sponges were predominantly found in rocky habitats although they were both found in smaller numbers in all other habitat types. Hydrozoans were almost solely seen in mud, cobble, and sand habitat. White urchins were found primarily in sandy habitat. Sea stars were observed in large numbers in all habitat types but this may be due to the fact that we did not distinguish between sea star varieties. The video observation locations and substrate types are shown in Figure 2.

Link to Figure 3

Figure 4. Map of video observations for entire study area. To view a large version of Figure 4 in a new window, click on the image above.

Fish biomass

Observable in both the bathymetry and backscatter data are isolated spots of high reflectivity, which are generally small (about 5 – 25 square meters), due to dense areas of fish called fish biomass. These anomalies appear as real bottom features. We have reclassified the polygons derived from spots of high backscatter, or fish biomass, based on the surrounding habitat type. We have also added a new modifier to the Deep Water Marine Benthic Habitat Classification Scheme (Greene et al., 1999), “_fb”, to indicate “any mass of fish, squid or aggregating species that reflects sonar.”

Sand Ripples

Video data were collected in the innershelf, north-east section of the study area to determine at what depth the transition from soft, muddy, bioturbated sediment to sand ripples occurs. Figure 5 shows the location of the three lines as well as the points where sand ripples were observed. In the figure, sand ripple observations appear bright blue over the yellow observation point markers. Video observations of these lines confirmed that at 20m there is degradation of ripples by bioturbation. Figure 6 shows a bioturbed view at the deepest point on camara line 15. Figure 7 shows an image in the zone of transition between bioturbated and sand ripples in the center on camara line 15. Figure 8 shows the shallowest point on camara line 15 where the seafloor is predominately sand ripples. For more images of the seafloor along these three video observation lines, please see the hyperlinks in the project available for download included with this OFR (instructions on viewing hyperlinks in ArcGIS9.x).

Link to Figure 5

Figure 5. Location map of sand ripple features in the nearshore region between Carpenteria and Rincon point. Blue dots represent locations where ripples were observed, yellow dots represent locations where ripples were not observed. To view a large version of Figure 5 in a separate window, click on the image above.

Image of the deepest point of camera line 15. Here the ripples are more degraded by bioturbation.

Figure 6. Figure 6 is at the deepest point of camera line 15. Here the ripples are more degraded by bioturbation.

>image of The zone of transition between boturbated and sand ripples in the center of camera line 15.

Figure 7. The zone of transition between boturbated and sand ripples in the center of camera line 15.

Image of the shallowest point on camera line 15 where the seafloor is predominately sand ripples.

Figure 8. The shallowest point on camera line 15 where the seafloor is predominately sand ripples.

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Habitat Classification

The 77 sq km study area is located entirely on the continental shelf. Within this area 95% of the seafloor is composed of flat and soft mud or sand. 1.19% of the area is hard bottom; the remainder is a mix of low relief hard bottom and sand or coarser sediment bottom.

The hard seafloor is divided into two main rocky regions. One area of rocks is located at the crest of an east-west trending bathymetric ridge in the western and central part of the study area above the 50 m contour and north of the oil platforms. There are 0.373 sq km of exposed bedrock along the ridge of Tertiary age. The only other significant rocky area is a 0.392 sq km area in the east, above the 30m contour line, and directly offshore of Rincon Point. Video over the area shows boulders and larger rocks that are likely a submerged extension of the large delta visible at Rincon Point. Locally, onshore outcrops exist of Sedimentary rocks of Miocene and Pliocene age.

Other hard bottom in the survey, 0.132 sq km, is due to anthropogenic features such as oil platforms and pipelines. These man made structures occur in the southern half or the western and central area of the survey below the 50 m contour.

Sand ripples were observed in the nearshore region between Carpentaria and Rincon Point. As depth increases offshore there is an increase in the number of burrowing and bioturbation which decreases the continuity of the bedforms (see figures 6 - 8). We assume that the bedforms suggest stronger bottom currents, and based on the video we have separated the nearshore rippled zone from the more bioturbated zone by dividing polygons using the 20 m depth contour.

Figure 9 illustrates the distribution of benthic habitat types. Table 1 provides a translation of the benthic habitat codes used in this study. Details of the classification methodology can be found in Greene et al. (1999).

Habitat Classification Map

Link to Figure 2

Figure 9. Map showing the habitat types for the study area. To view a large version of Figure 9 in a separate window, click on the image above.

Habitat Classification Table

Habitat Code Habitat
description
Area (m2) Area (%)
Sh(b)d2B{2} Shelf hard boulder deformed bedrock Sloping Low Complexity Intertidal-30m 248 0.000321963
Sh(b)d2B{3} Shelf hard boulder deformed bedrock Sloping Low Complexity 30-100m 52 0.000000001
Sh(b)d2D{3} Shelf hard boulder deformed bedrock Sloping High Complexity 30-100m 1000 0.001298237
Sh(b/c/r)d2B{2} Shelf hard boulder cobble rock deformed bedrock Sloping Low Complexity Intertidal-30m 15,417 0.020014913
Sh(b/c/r)d2C{2} Shelf hard boulder cobble rock deformed bedrock Sloping Moderate Complexity Intertidal-30m 88,218 0.114527835
Sh(c)d2B{2} Shelf hard cobble deformed bedrock Sloping Low Complexity Intertidal-30m 7,205 0.009353795
Sh(c)d2C{2} Shelf hard cobble deformed bedrock Sloping Moderate Complexity Intertidal-30m 373 0.000484242
Sh(m)d2C{2} Shelf hard mud deformed bedrock Sloping Moderate Complexity Intertidal-30m 5,878 0.007631035
Sh(r)d2B{3} Shelf hard rock deformed bedrock Sloping Low Complexity 30-100m 7,204 0.009352496
Sh(r)d2C{3} Shelf hard rock deformed bedrock Sloping Moderate Complexity 30-100m 20,188 0.0262088
Sh(r)d2E{3} Shelf hard rock deformed bedrock Sloping Very High Complexity 30-100m 30,176 0.039175587
Sh(r/b)d2B{2} Shelf hard rock boulder deformed bedrock Sloping Low Complexity Intertidal-30m 1,422 0.001846092
Sh(r/b)d2B{3} Shelf hard rock boulder deformed bedrock Sloping Low Complexity 30-100m 100 0.000129824
Sh(r/b)d2C{3} Shelf hard rock boulder deformed bedrock Sloping Moderate Complexity 30-100m 3,512 0.004559407
Sh(r/c)d2B{3} Shelf hard rock cobble deformed bedrock Sloping Low Complexity 30-100m 500 0.000649118
Sh_a2B{3} Shelf hard anthropogenic Sloping Low Complexity 30-100m 99,907 0.129702922
Sh_a2C{3} Shelf hard anthropogenic Sloping Moderate Complexity 30-100m 4119 0.005347436
Sh_a2D{3} Shelf hard anthropogenic Sloping High Complexity 30-100m 9,848 0.012785034
Sh_a2E{3} Shelf hard anthropogenic Sloping Very High Complexity 30-100m 17,907 0.023247522
Shd2B{2} Shelf hard deformed bedrock Sloping Low Complexity Intertidal-30m 266,646 0.346169591
Shd2B{3} Shelf hard deformed bedrock Sloping Low Complexity 30-100m 274,232 0.356018014
Shd2C{2} Shelf hard deformed bedrock Sloping Moderate Complexity Intertidal-30m 6,763 0.008779974
Shd2C{3} Shelf hard deformed bedrock Sloping Moderate Complexity 30-100m 26,104 0.033889168
Shd2D{3} Shelf hard deformed bedrock Sloping High Complexity 30-100m 5,572 0.007233774
Shd2E{3} Shelf hard deformed bedrock Sloping Very High Complexity 30-100m 4,156 0.005395471
Sm(b/c/m)2C{3} Shelf mixed boulder cobble mud Sloping Moderate Complexity 30-100m 265,618 0.344835004
Sm(b/c/s)2B{2} Shelf mixed boulder cobble sand Sloping Low Complexity Intertidal-30m 370,423 0.480896689
Sm(b/c/s)2D{3} Shelf mixed boulder cobble sand Sloping High Complexity 30-100m 13,702 0.017788438
Sm(b/c/s/m)2B{2} Shelf mixed boulder cobble sand mud Sloping Low Complexity Intertidal-30m 517,130 0.671357083
Sm(b/r)2B{2} Shelf mixed boulder rock Sloping Low Complexity Intertidal-30m 94 0.000122034
Sm(b/s)2B{2} Shelf mixed boulder sand Sloping Low Complexity Intertidal-30m 201 0.000260946
Sm(c)2B{2} Shelf mixed cobble Sloping Low Complexity Intertidal-30m 538 0.000698451
Sm(c/b)2B{2} Shelf mixed cobble boulder Sloping Low Complexity Intertidal-30m 190 0.000246665
Sm(c/b)_a2B{3} Shelf mixed cobble boulder anthropogenic Sloping Low Complexity 30-100m 698 0.000906169
Sm(m/r)2B{3} Shelf mixed mud rock Sloping Low Complexity 30-100m 712 0.000924344
Sm(m/r)_a2B{3} Shelf mixed mud rock anthropogenic Sloping Low Complexity 30-100m 6,178 0.008020506
Sm(m/r/s)2C{2} Shelf mixed mud rock sand Sloping Moderate Complexity Intertidal-30m 1,710 0.002219985
Sm(m/r/s)2C{3} Shelf mixed mud rock sand Sloping Moderate Complexity 30-100m 55,160 0.07161073
Sm(m/s)2B{3} Shelf mixed mud sand Sloping Low Complexity 30-100m 2,472 0.003209241
Sm(m/s)_a2B{3} Shelf mixed mud sand anthropogenic Sloping Low Complexity 30-100m 4,081 0.005298103
Sm(r)2B{3} Shelf mixed rock Sloping Low Complexity 30-100m 880 0.001142448
Sm(r/c)2B{2} Shelf mixed rock cobble Sloping Low Complexity Intertidal-30m 33 0.000000001
Sm(r/s/b)2C{3} Shelf mixed rock sand boulder Sloping Moderate Complexity 30-100m 8,034 0.010430033
Sm(r/s/m)2B{3} Shelf mixed rock sand mud Sloping Low Complexity 30-100m 832 0.001080133
Sm(s)2B{2} Shelf mixed sand Sloping Low Complexity Intertidal-30m 333 0.000432313
Sm(s)2B{3} Shelf mixed sand Sloping Low Complexity 30-100m 348 0.000451786
Sm(s/c)2B{2} Shelf mixed sand cobble Sloping Low Complexity Intertidal-30m 3,461 0.004493197
Sm(s/m)2B{2} Shelf mixed sand mud Sloping Low Complexity Intertidal-30m 143 0.000185648
Sm(s/m)2B{3} Shelf mixed sand mud Sloping Low Complexity 30-100m 3,024 0.003925867
Sm(s/r)2C{3} Shelf mixed sand rock Sloping Moderate Complexity 30-100m 724 0.000939923
Sm2B{2} Shelf mixed Sloping Low Complexity Intertidal-30m 127,572 0.165618637
Sm2B{3} Shelf mixed Sloping Low Complexity 30-100m 466,420 0.605523506
Sm2C{3} Shelf mixed Sloping Moderate Complexity 30-100m 27,038 0.035101721
Sm2D{3} Shelf mixed Sloping High Complexity 30-100m 8,368 0.010863644
Sm2E{3} Shelf mixed Sloping Very High Complexity 30-100m 1,568 0.002035635
Sm_a2B{3} Shelf mixed anthropogenic Sloping Low Complexity 30-100m 200,188 0.259891385
Sm_a2C{3} Shelf mixed anthropogenic Sloping Moderate Complexity 30-100m 56 7.27012E-05
Sm_a2E{3} Shelf mixed anthropogenic Sloping Very High Complexity 30-100m 29,068 0.037737141
Ss(m)2C{3} Shelf soft mud Sloping Moderate Complexity 30-100m 2,666 0.003461099
Ss(m/s)2B{2} Shelf soft mud sand Sloping Low Complexity Intertidal-30m 7,948,483.32 10.31901181
Ss(m/s)2B{3} Shelf soft mud sand Sloping Low Complexity 30-100m 64,949,984.57 84.32044587
Ss(m/s)_fb2B{2} Shelf soft sand Sloping Low Complexity Intertidal-30m 71,611 0.09296802
Ss(m/s)_fb2B{3} Shelf soft mud sand Sloping Low Complexity 30-100m 722,770 0.938326453
Ss(s)2B{2} Shelf soft sand Sloping Low Complexity Intertidal-30m 471,58 0.061222241
Ss(s)2B{3} Shelf soft sand Sloping Low Complexity 30-100m 1,696 0.002201809
Ss(s)2C{3} Shelf soft sand Sloping Moderate Complexity 30-100m 1,664 0.002160266
Ss2B{2} Shelf soft Sloping Low Complexity Intertidal-30m 158,102 0.2052538
Ss2B{3} Shelf soft Sloping Low Complexity 30-100m 99,378 0.129016155
Ss2C{2} Shelf soft Sloping Moderate Complexity Intertidal-30m 5,101 0.006622305
Ss2C{3} Shelf soft Sloping Moderate Complexity 30-100m 4,684 0.00608094
Ss2D{3} Shelf soft Sloping High Complexity 30-100m 520 0.000675083
Ss_a2B{3} Shelf soft anthropogenic Sloping Low Complexity 30-100m 2 0.000000001

Table 1. Column one is the Greene et al (1999) Habitat Code. Column two is the definition of the habitat code. Column three is the total area of the habitat type in square meters. Column four is the percent total of the habitat type for the total study area.

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