By contrast, seamounts are mainly volcanic in origin, rise a considerable height from great depths on the continental rise and are limited in length across the summit.
Despite their differences in scale, both banks and seamounts are isolated in space and have higher elevation and a different substrate than the surrounding seabed. The vertical structure, habitat complexity and rocky substrate of banks and seamounts support a very different biological assemblage than the surrounding soft bottom typical of the continental shelf and slope. Because these rocky features extend up into the water column, they provide ideal habitat for attached sessile invertebrates that depend on currents to deliver their food. The hard substrate is also favorable for settlement of larvae from the water column.
Cordell Bank is the centerpiece of Cordell Bank National Marine Sanctuary and is located about 80 kilometers (50 miles) northwest of San Francisco and 32 kilometers west of the Point Reyes lighthouse. It is approximately 7 kilometers wide and 15 kilometers long and sits on the edge of the continental shelf. Shelf depths at its base are about 91 meters (300 feet), with upper pinnacles reaching to within 35 meters of the ocean surface. The bank makes up approximately 2 percent of the total sanctuary area.
Cordell Bank was created as part of the southern Sierra Nevada mountain range nearly 100 million years ago. About 30 million years ago, the north-moving Pacific plate sheared off a block of the North American plate containing Cordell Bank, and since that time, the bank has been migrating slowly north. Its granite structure is a link to its geologic history as well as an indication of its common origin with the Farallon Islands to the south.
Recent discoveries of sedimentary rock on Cordell Bank have intrigued geologists and suggest that the bank may be more geologically complex than previously thought. Between 20,000 and 15,000 years ago, when sea level was about 110 meters lower than it is today, Cordell Bank would have been entirely exposed. Boulder fields at its base on the western side may be indicative of an ancient shoreline.
A percentage of Cordell Bank is made up of consolidated granite reef, but much of the bank is a heterogeneous mixture of rock reef, boulders, cobbles, sand and mud. Its perimeter includes large expanses of white sand waves on the eastern side and relatively flat mud on the northern and western sides. The central portion has extensive reef systems and the shallowest pinnacles (as shallow as 35 meters), while the northern half is deeper, with mixed bolder and reef habitats. Throughout the bank, rocky reefs are cut by sand channels as well as mixed habitats of sand, boulders and cobbles.
Mystery mollusk (Order NUDIBRANCHIA sp. nov.) above the Davidson Seamount at 1498 meters depth. Credit: NOAA/MBARI 2002
<em>Poromitra crassiceps</em> (Crested Bigscale; Melamphaidae) collected during mesopelagic fishes survey aboard NOAA SHIP Bell M. Shimada, at Davidson Seamount (SESA 16), 0-699 meters, May 2015. Identified by Robert N. Lea (CAS) and Erica J. Burton (MBNMS). Scale: Centimeter ruler.
The bank is located on the edge of an underwater peninsula and is surrounded by deep water on three sides. Within 11 kilometers of its western edge, the seafloor drops to 1,829 meters at the sanctuary's western boundary. Data suggest that, as deeper water encounters the base of the bank on the northern edge, the water mass moves up and over the bank. This localized upwelling would be one more mechanism delivering food and nutrients to organisms living on or around Cordell Bank.
Sponges, hydrocorals, tunicates, anemones, gorgonians, solitary corals and hydroids compete for space on the upper-reef areas. More mobile animals like decorator crabs, sea stars, sea cucumbers, snails and demersal fishes move over the invertebrate carpet. Cover on the upper reef areas exceeds 100 percent, as animals are layered one on top of the other.
This living reef also provides critical habitat for first-year juvenile rockfishes (Sebastes spp.) that settle out of the water column as they transition from a pelagic to benthic life stage. Adult rockfishes, lingcod and other benthic fishes also inhabit the complex habitats within the structure of Cordell Bank. The most conspicuous invertebrates occurring in the deeper reef areas include octopus, crinoids, white plumed anemones, sea stars and tube polychaetes.
Habitat loss and fragmentation are perhaps the most serious threats confronting all species of wildlife today. Many of the activities and conditions that indirectly affect marine life are first experienced as an alteration or disturbance to their habitat.
Activities that currently have the greatest potential impact on Cordell Bank are the use of bottom-tending fishing gears, the deposition of lost fishing gear and other marine debris, the introduction of invasive species, and the construction and placement of cables and pipelines on the bank.
Gear restrictions and area closures associated with the Rockfish Conservation Areas (RCAs) and Essential Fish Habitat (EFH) designations have reduced potential habitat impacts from fishing. Regulations protecting the bank from invasive species introductions and seabed disturbance form part of the sanctuary's revised management plan.
MonitoringBenthic Community Characterization and Monitoring
The Cordell Bank National Marine Sanctuary, in partnership with the National Oceanic and Atmospheric Administration (NOAA) Fisheries Laboratory in Santa Cruz, the U.S. Geologic Survey (USGS), California Academy of Sciences and the California Department of Fish and Game, conducted a study to classify habitats and monitor fishes and macro-invertebrates on and around Cordell Bank. Benthic transects were monitored annually from 2002 to 2005. The sanctuary plans to continue the monitoring program in the future; however, this is dependent upon funding levels.
Underwater surveys of fishes, invertebrates and their habitats were conducted on and around Cordell Bank using direct observation and video-transect methods from an occupied research submersible (Delta). Transects were distributed over the bank to sample all bank habitats and associated biota.
The data provided the sanctuary staff with a species inventory of fishes and benthic invertebrates. Long-term data may be used to identify habitat affinities for species complexes, develop habitat suitability models and provide a foundation to evaluate change over time.