Rippled scour depressions add ecologically significant heterogeneity to soft sediment habitats on the continental shelf

Todd Russell Hallenbeck, California State University, Monterey Bay

Thesis (M.S.) Division of Science and Environmental Policy


Comprehensive high-resolution seafloor mapping of California's state waters have revealed rippled scour depressions (RSD) to be one of the most abundant and widespread habitats of the inner continental shelf. These sharply delineated elongate features range in size from 100's to 1000's of m² in aerial extent and are characterized by 30 to 50 cm deep depressions of coarser sediments and longer period bedforms than found on the surrounding seabed. Although RSDs have been identified on many of the world's continental margins, previous studies focused on their geomorphology and dynamics, leaving the ecological influence and associated biological communities of RSDs unexplored. Here we test the hypothesis that there are ecologically important differences in the distribution and abundance of benthic fish and invertebrate groups inside and outside RSDs. A small remotely operated vehicle (ROV) was used to survey twenty RSDs in three depth zones (shallow [<15 >m], intermediate [15 to 30 m], deep [>30 m]) within Monterey Bay, California. Density and richness of benthic communities and habitat characteristics including substrate, bedform type, and depth were determined from the recorded video imagery. Sediment grab samples taken inside and outside of RSDs confirmed the mean grain size was significantly larger inside RSDs (0.5 to 0.9 mm) than outside (0.15 to 0.4 mm). As predicted from known species/grain size relationships, mean density of combined trophic groups was lower inside RSDs in the shallow, intermediate, and deep zones (0.03, 0.16, 0.21 ind m⁻² respectively) than outside (0.04, 0.31, 0.45 ind m⁻² respectively). Richness of trophic groups was also lower inside RSDs in each depth zone (0.03, 0.10, 0.14 taxa m⁻² respectively) than outside RSDs (0.03, 0.17, 0.25 taxa m⁻² respectively). Surprisingly, RSDs did contain significantly more young of the year (YOY) rockfish (especially ESA threatened canary rockfish, Sebastes pinniger) and small flatfish than adjacent fine sediments, suggesting a possible nursery function for these otherwise depauperate coarse-grained habitats. These results indicate RSDs can add a significant and previously undescribed level of ecological patchiness to nearshore soft sediment communities, which may be further amplified by the dynamic nature of RSD bedforms. Moreover, this study illustrates the utility of high-resolution habitat mapping data in refining our understanding of seafloor landscape variability and species habitat relationships specifically in the context of adaptive management in marine spatial planning.