Abstract
An integrated regional understanding of geophysics and biology are critical for comprehensive marine planning. Yet as of 2017, the US Federal seafloor off central California was unmapped using modern methods and largely unsampled. A partnership called the Expanding Pacific Research and Exploration of Submerged Systems focused on a biological and geological characterization of the area offshore Morro Bay, California from 2017 to 2019. Subsequent reports by the US Geological Survey will describe the geophysical, geotechnical, and habitat mapping results. This report describes the biological characterization on the seafloor lead by the Monterrey Bay Aquarium Research Institute, conducting 40 remotely operated vehicle dives from 371 to 1173 meters. Seafloor habitats and megafauna (fish and invertebrates) were observed across 46.8 km of the seafloor. Biological communities were compared using 18.52 km2 of subsampled linear benthic transect video. From 185 hours of observational and quantitative transects at 25 sites, nearly 120,000 annotations of organisms and their habitat were created. The primary habitat observed was soft substratum (80 %) and bedrock constituted 3.2 % of the area surveyed. Within the soft substrata is a ~1300 km2 area containing several thousands of seafloor depressions called pockmarks. Two pockmarks were selected for additional sampling to determine if biological communities (megafauna and infauna) inside individual pockmarks significantly differ from those outside the pockmarks. Substrata were binned into soft, pockmark fields, and mixed/hard habitat for analysis across a range of depths. Over 101,000 megafaunal organisms were observed in video representing an equal split of 35 % predator/scavengers, 34 % surface deposit feeders, and 30 % suspension/filter feeders. Abundant biological detritus, in the form of dead and dying pyrosomes and salps, represented a large flux of carbon to the seafloor. We conducted 97 quantitative transect video surveys at 13 sites finding 173 taxa with a species richness ranging from 8–55 taxa. Densities ranging from 0.07 to 5.2 m-2 decreased with depth and among substrate groups. Seventy percent of the transects occurred inside the oxygen minimum zone and we conclude the presence of hard substratum was a better predictor of species richness and density than oxygen concentration. Overall, transects within the depth vs. substratum categories were less than 40 % similar in multivariate analyses based on cluster analysis. We used the distinct (dissimilar) biotic clusters to create and describe 18 biotopes. Biotope analysis allowed us to explain habitat associations using finer depth and substratum categories. For instance, mud containing coarse sand occurred only in very low oxygen areas and supported unique biologic assemblages, while hummocky mud supported somewhat different species than flat mud plains. We hypothesized the soft substrate pockmark fields would be a distinct biotope, however, 33 megafauna and 29 infauna taxa were observed in and around pockmarks with no significant differences of density nor species richness at either of the two sites. The biotic associations to seafloor features and substrates can inform future marine planning decisions. If wind turbine development continues for this area, these data can inform the selection of appropriate reference areas and survey designs for impact-related studies.