A regional analysis of the distribution of Rippled Scour Depressions along the California coast

Rippled scour depressions (RSDs) are prominent sediment features found on continental shelves worldwide. They are characterized by coarse grain sand in comparison to the surrounding sediment plateau and by long period sand waves inside of the depressions (0.4m-1m depth). The coarse grain sand composition of RSDs in relation to the surrounding finer sediment adds heterogeneity to the habitat available to benthic fauna. Bathymetric and acoustic backscatter data from the California Seafloor Mapping Project reveal that RSDs are common features of the California continental shelf including areas within marine protected areas (MPAs). While many studies have described RSDs at specific locations, the study presented here is the first to address the spatial distribution of RSDs at the regional scale. The goals of this study were to: 1) quantify the abundance and patterns of distribution of RSDs along the entire California coast, and 2) determine the percentage of rock reef, sedimentary and RSD habitats within state waters, both inside and outside of the State's MPA network. Our general approach was to develop and use a landscape analysis algorithm-based Topographic Position Index (TPI) to identify the distinct edges of RSDs in order to differentiate the features from other soft sediment and rocky reef habitat. GIS spatial analysis was then used to quantify the distribution and abundance of RSDs along the entire coast and test predicted relationships with proximity to rocky reef, depth, and latitude. RSDs were found to make up 3.4% of the continental shelf in California compared to 8.0% for rocky reef. We also determined that RSD percent cover varied significantly with depth and increased with proximity to rocky reef habitat. Because RSDs are a unique habitat and are found throughout California's MPAs their distribution likely affects the composition and abundance of benthic communities. For this reason, determining the patterns of distribution and abundance for RSDs on the California continental shelf will provide information valuable to the design, monitoring and performance assessment of California's newly designed MPA network mandated by the State's Marine Life Protection Act.