Enhanced Acoustic Backscatter Due to High Abundance of Sand Dollars, Dendraster excentricus

Abstract

Detailed acoustic surveys of benthic sediments were conducted in July 1995 and September 1998 in the vicinity of Humboldt Bay, California. During these surveys, a band of enhanced acoustic backscatter was observed oVshore from the bay entrance, approximately parallel to the isobaths, in water depths ranging from 16± 24 m. In order to assess the cause of the increase in backscatter levels, a more comprehensive study was conducted in August and September 1999 using 100 kHz side-scan sonar, bottom grab sampling and underwater video recording. New observations indicated that a dense population of sand dollars (Dendraster excentricus) coincided with the enhanced backscatter band. Compared to the two previous acoustic studies, the central section of the band expanded westward by 180 m and the southern section of the band shifted eastward by 160 m, possibly resulting from a change in the biological or physical factors which inXuence the location and breadth of sand dollars. 135We wish to thank our colleagues, P. Ratta, M. Forguson, and J. T. Walsh, who helped to develop and carry out this project. We are very grateful to Dr. J. E. Pequegnat for his guidance at every stage, Dr. M. A. de Angelis for the invaluable use of the video equipment, and the captain and crew of the R/V Coral Sea for their assistance during the cruises. Lastly, we are indebted to Humboldt State University for providing the facilities and support necessary to complete this project.The relationship between high sand dollar abundance and enhanced acoustic backscatter was further veriWed in the nearshore region oV Samoa Beach California, where a dense, banded population of sand dollars was previously observed. Video footage conWrmed the presence of a band of sand dollars, also nominally parallel to the isobaths, in water depths of 8± 15 m. A band of enhanced backscatter coincided with the dense sand dollar population. The identiWcation of dense aggregations of sand dollars through enhanced acoustic backscatter could lead to the use of acoustic techniques to study sand dollar distributions and abundance .

The effects of fine-scale surface roughness and grain size on 300 kHz multibeam backscatter intensity in sandy marine sedimentary environments

“…No synoptic observations were made of seabed surface roughness, but many studies have demonstrated the importance of roughness at this site (e.g. Cacchione et al, 1999;Fenstermacher et al, 2001;Ferrini and Flood, 2003). Backscatter intensity that we observe at this site ranges from À 32.5 to À 15 dB, and sediment type ranges from coarse sand to medium silt (median grain size: 0f to 5.5f).…”

“…In addition to ripples, roughness in the form of shells (e.g. sand dollars; Fenstermacher et al, 2001) or biogenic structures (Urgeles et al, 2002) can result in increased backscatter intensity.…”

Combining three-dimensional acoustic coring and a convolutional neural network to quantify species contributions to benthic ecosystems

“…Nevertheless, the detection of buried objects using acoustics is impeded by the high attenuation of acoustic waves due to scattering and absorption in the sediment [15,16]. Consequently, there are only a few examples of benthic surveys that identify details below broad habitat types using acoustics [32] and, to date, it has not been possible to reliably identify invertebrate species directly from acoustic data. A logical next step would be to incorporate how biological communities modify seafloor sediment in models of seafloor processes [7], but progress in this area has been slow due to a lack of detailed mechanistic information about specific organism–sediment interactions.…”

Multibeam backscatter as a tool for sea-floor characterization and identification of oil spills in the Galicia Bank

“…Typically the differences between muddy and rocky seafloor are on the order of 10-20 dB . However, recent regional studies have revealed that backscatter intensity may be influenced by other factors such as the occurrence of benthic fauna (Fenstermacher et al, 2001;Edwards et al, 2003;Kostylev et al, 2003;Valentine et al, A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 3 2003), sediment compaction (Nitsche et al, 2004), the rate of the sediment reworking by organisms (Borgeld et al, 1999;Urgeles et al, 2002), the presence of anthropogenic organic residues (Lewis et al, 2000), the content of natural hydrocarbons in fluidventing structures such as mud volcanoes (Fonseca et al, 2002;Sager et al, 2003;Somoza et al, 2003) or the presence of authigenic carbonates related to the bacterial oxidation of methane (Orange et al, 2002;Díaz-del-Río et al, 2003).…”