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 m 2 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 m −2 respectively) than outside (0.38, 3.06, and 4.48 ind. 10 m ). 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 undescribed level of ecological patchiness to soft sediment communities on the continental shelf. KEY WORDS: Rippled scour depressions · Continental shelf · Marine spatial planning · ROV · Soft sediment · Benthic ecology · Sorted bedforms · Sebastes Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 468: [119][120][121][122][123][124][125][126][127][128][129][130][131][132][133] 2012 1978, Auster et al. 1991, Zajac et al. 1998, Auster et al. 2003b, Diaz et al. 2003, Lindholm et al. 2007, Stoner et al. 2007.Rippled scour depressions (RSDs; Fig. 1) have been identified and physically described in small-scale studies on many of the world's continental shelves (Garnaud et al. 2005, Gutierrez et al. 2005, Lo Iacono & Guillen 2008, Bellec et al. 2010. These elongate shallow depressions (0.3 to 1 m in depth), oriented normal to isobathic contours, are characterized by long crested bedforms (0.5 to 1 m wavelength), coarse sediment (0.3 to 1 mm), and abundant shell hash. RSDs have been found in water depths of 15 to 160 m with reported dimensions of 30 to 250 m wide and 100 to 3000 m long (Cacchione et al. 1984, Bellec et al. 2010. The sediments surrounding RSDs are typically characterized by finer grain size (0.05 to 0.3 mm), shorter period bedforms, and a sharp edge delineating the coarser-grained RSDs (Green et al. 2004, Goff et al. 2005, Diesing et al. 2006. Further, RSDs can be persistent, with their spatial extent and boundaries remaining stable fo...

Commentary: Variability in Shelf Sedimentation in Response to Fluvial Sediment Supply and Coastal Erosion Over the Past 1,000 Years in Monterey Bay, CA, United States

“…First, sands with "littoral" grain sizes (i.e., >180 microns) are found in a diversity of settings throughout the inner and outer Monterey Bay. For example, sandy settings include offshore bedrock reef aprons, rippled-scour depressions, sand waves, and the majority of the outer shelf of the bay (Eittreim et al, 2002;Storlazzi and Reid, 2010;Hallenbeck et al, 2012;Golden, 2013;Rosenberger et al, 2019). All of these sand deposits are potential sources to the four coring sites, in contrast with the first Carlin et al (2019) assumption.…”

Character, distribution, and ecological significance of storm wave-induced scour in Rhode Island Sound, USA

“…This textural heterogeneity reflects a complex patchwork of contrasting sedimentary environments that, in turn, creates contrasting epifaunal-and infaunal-dominated habitats (Hallenbeck et al 2012). The coarse lag deposits of Pleistocene till and glacial outwash that floor some of the scour depressions reflect higher-energy sedimentary environments characterized by processes associated with erosion and non-deposition.…”

Effect of hypoxia on rockfish movements: implications for understanding the roles of temperature, toxins and site fidelity

“…Prey transitioning into or out of the sand, such as sand lance Ammodytes hexapterus (Love 2011), or vertically migrating pelagic invertebrates (Murie 1995) may also be potential prey. Juvenile fish are another known food item for quillback rockfish (Murie 1995), and have been shown to reside within rippled scour depression habitat, defined in part as depressions in the seabed comprised of sediments coarser than the surrounding area (Hallenbeck et al 2012). Multibeam and sidescan sonar data from the foray areas show gravel beds recessed up to 0.5 m below the surrounding sand, which may similarly provide juvenile fish habitat and prey for quillback rockfish in an otherwise featureless area (Goldfinger et al 2012).…”