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

Abstract

Comprehensive mapping of California's state waters has revealed rippled scour depressions (RSDs) to be abundant and widespread on the inner continental shelf. Ranging from 100s to 1000s of m2 in areal extent, RSDs are 30 to 50 cm deep depressions of coarser sediments and longer period bedforms than found on the surrounding seabed. Although RSDs have been physically described on many continental margins, previous studies have not addressed the ecological influence and associated biological communities of RSDs. 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 ROV was used to survey 20 RSDs in 3 depth zones (≥15 m, 15 to 30 m, and ≤30 m) within Monterey Bay, California. Density and richness of benthic communities were determined from the recorded video imagery. Sediment grab samples confirmed significantly larger mean grain sizes inside (0.71 mm) than outside (0.22 mm) the RSDs. Overall mean faunal density (fish and invertebrates) was lower inside RSDs in the shallow, intermediate, and deep zones (0.32, 1.61, and 2.17 ind. 10 m0-2 respectively) than outside (0.38, 3.06, and 4.48 ind. 10 m-2). Faunal richness was also lower inside RSDs in each depth zone (0.29, 1.03, and 1.43 taxa 10 m-2) than outside RSDs (0.29, 1.72, and 2.48 taxa 10 m-2). Surprisingly, RSDs did contain significantly more young-of-the-year rockfishes and small flatfishes than adjacent fine sediments, suggesting a possible nursery function for these otherwise depauperate coarsegrained habitats. These results indicate that RSDs can add a significant and previously un - described level of ecological patchiness to soft sediment communities on the continental shelf. © Inter-Research 2012.