A bank is an elevation of the seafloor located on the continental shelf, over which the water depth is relatively shallow. These features are of continental origin and can cover extensive surface area but do not extend thousands of meters into the water column.
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. Their vertical structure, habitat complexity and rocky substrate support a very different biological assemblage than the soft bottom that typically surrounds them.
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
Cordell Bank is the centerpiece of Cordell Bank National Marine Sanctuary. It is approximately 7 kilometers (4.3 miles) 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 bathymetry and location of Cordell Bank combine to make it a very productive marine environment. Nutrients and productivity emanating from an upwelling center near Point Arena are carried over the bank and sustain a thriving biological community. Localized upwelling may also contribute to productivity.
Davidson Seamount
In November of 2008, NOAA designated the Davidson Seamount Management Zone (DMSZ), increasing the Monterey Bay National Marine Sanctuary (MBNMS) and protecting Davidson Seamount, making it the first seamount within a national marine sanctuary. Several other seamounts – including Gumdrop, Pioneer, and Guide Seamounts – occur just beyond Monterey Bay National Marine Sanctuary.
Davidson Seamount is located 129 kilometers southwest of Monterey. This inactive volcano is approximately 2,300 meters tall and 42 kilometers long, yet its summit is far (1,250 meters) below the ocean surface.
Inhabitants
On 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.
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.
Davidson Seamount’s rocky outcrops, particularly near seamount peaks, are inhabited by a suite of deep-sea corals and sponges that are typically absent or quite rare in more typical ocean settings. Sea stars, anemones, crustaceans, octopus and fishes are common there. Expeditions in 2002 and 2006 observed 18 species new to science.
Conservation and Management Issues
Due to proximity to the coast, Cordell Bank and Davidson Seamount face a number of anthropogenic threats, including but not limited to vessel traffic, sea temperature rise, ocean acidification, commercial harvest, underwater cables, cumulative research collection, bio-prospecting, and military activity. Sanctuary regulations provide important, although not comprehensive, defenses against some of these threats. Activities that currently have the greatest potential impact on Cordell Bank are the use of bottom-tending fishing gear, 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.
Davidson Seamount appears to be relatively pristine, based on observations of biological communities during submersible explorations. This contrasts with observations of various other seamounts worldwide, which have been subject to severe damage by trawling.
Monitoring
Currently there is one main study underway to develop a better understanding of the habitats and communities at and around Cordell Bank:
Several studies have been conducted within the sanctuary at Davidson Seamount, and adjacent to the sanctuary at Pioneer Seamount. These include seamount characterization, coral distribution study, marine mammal and seabird surveys, and passive acoustic monitoring.