Seamounts & Banks: MBNMS

Banks and seamounts rise above the seafloor and typically consist of a different substrate as compared to the surrounding seabed. Their vertical nature and rocky substrate create habitat complexity and support a very different biological assemblage than that which surrounds them.

A bank is located on the continental shelf and the water depth above it is relatively shallow. Banks have a continental origin and can cover extensive surface area but do not extend thousands of meters into the water column.

In contrast, seamounts are mainly volcanic in origin, rising to considerable height from great depths along the continental rise and are limited in length across the summit.

Seamounts, though common in the world’s oceans, often have very different biological assemblages than the surrounding seafloor sediments, due likely to the complex, rocky and current-swept habitats. 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.

In November of 2008, NOAA designated the Davidson Seamount Management Zone (DSMZ), 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 this sanctuary.

The Davidson Seamount Management Zone is located 75 miles (121 kilometers) due west of San Simeon. The shallowest point is 4,101 ft (1,250 m) below the ocean’s surface and the deepest part of the DMSZ is 12,713 ft (3,875 m). Davidson Seamount itself is 7,480 ft (2280 m) tall, as measured from the west-side base to the summit. The seamount is also 26 miles long and 8 miles wide. In total, the DSMZ covers an area of 775 square miles and increases the MBNMS to 6,094 square miles.

The geological structure and origin of Davidson, Guide, Pioneer, and Gumdrop Seamounts have only recently been described, by scientists at the Monterey Bay Aquarium Research Institute, as an atypical type of oceanic volcanism, having northeast-trending ridges that reflect the ridge-parallel structure of the underlying crust. Unlike most intra-plate ocean island volcanoes, the seamounts are built on top of spreading center segments that were abandoned at the continental margin when the tectonic regime changed from subduction to a transform margin. The Davidson Seamount consists of about six subparallel linear volcanic ridges separated by narrow valleys that contain sediment. These ridges are aligned parallel to magnetic anomalies in the underlying ocean crust. The seamount is 12.2 ± 0.4 million years old and formed about 8 million years after the underlying mid-ocean ridge was abandoned.

Assemblages of large corals and sponges, along with many associated animals such as sea stars, anemones, crustaceans, octopus and fishes, are common on the seamount. Expeditions in 2002 and 2006 observed 18 species new to science. Recent studies at Davidson Seamount indicate a significant shift in species composition and relative abundances of species occurs with depth. Whereas, species diversity and density at Davidson Seamount do not significantly change with depth, and can vary greatly on a single isobath. Ecological processes influencing the distribution, abundance and dynamics of seamount fauna are less well known than other charismatic ecosystems such as kelp beds and corals reefs, making it difficult to develop management criteria.

Seamount environments may represent optimal habitats for particular faunal groups resulting in thriving and dense populations encountered only rarely in other habitats. Based on research at Davidson Seamount and nearby Monterey Canyon, preliminary evidence suggests seamount communities may serve as a source of larvae for non-seamount habitats.

Due to proximity to the coast, Davidson Seamount faces 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.

Though relatively close to shore and one of the largest seamounts on the West Coast, Davidson Seamount appears to be relatively pristine, based on observations of biological communities during submersible explorations in the past decade. This contrasts with observations of various other seamounts worldwide, which have been subject to severe damage by trawling – a fishing practice that can decimate coral assemblages, which may then require decades or centuries to recover.

Sanctuary staff developed a management plan for the Davidson Seamount Management Zone. It contains background information on the Davidson Seamount, and the activities necessary for effective understanding and protection of this unique area. Goals are to develop and implement a resource protection plan for Davidson Seamount, increase understanding of the seamount through characterization and ecological studies, and develop education programs for this and other seamounts throughout the nation. The plan can be downloaded here.

Monitoring

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.

Davidson Seamount Expedition 2002
In May 2002, scientists from the sanctuary, Monterey Bay Aquarium Research Institute (MBARI), Monterey Bay Aquarium, Moss Landing Marine Laboratories (MLML) and NOAA Fisheries embarked on an expedition to explore and characterize the Davidson Seamount. Depth-stratified species assemblages were found at the crest, slope and base habitats of the seamount. The crest of Davidson Seamount had the highest diversity of species, including large gorgonian corals and sponges. The majority of corals were observed almost exclusively on high-relief, ridge areas. Faunal assemblages were arranged in large, contiguous patches that are susceptible to physical disturbance.

Davidson Seamount 2006: Exploring Ancient Coral Gardens
In January 2006, scientists returned to Davidson Seamount with the following objectives:

• to investigate why deep-sea corals live where they do
• to determine age and growth patterns of these corals
• to improve species identifications
• to share the exploration with the general public

They used a simple model, derived from results of the 2002 cruise, to test their understanding of coral distribution and to guide exploration in other regions of the seamount. (More than 99.98 percent of the seamount remains unexplored.) Specific regions were targeted based on a topographic index, substratum type and coral species depth records. Complementary studies of the biodiversity and population dynamics of seamount fauna included collections of corals for taxonomic studies as well as age and growth studies of corals using innovative radiometric techniques.

Marine Mammal and Seabird Surveys
The Davidson Seamount is typically regarded as having a greater abundance and diversity of marine mammals and seabirds than the surrounding area. In an effort to characterize marine mammal abundance above, and moderate distances away from, the seamount, several aerial surveys have been conducted (April 2009, January 2010, April 2010, July 2011). In addition, the first dedicated ship-based survey to record marine mammal and seabird observations above and surrounding the Davidson Seamount was conducted during July 2010. Zooplankton and CTD data were also collected above the seamount. Fin whales (Balaenoptera physalus) were the most commonly encountered marine mammal. The majority of fin whale sightings were above and to the west of the seamount where, based upon zooplankton net tows, krill abundance was greatest; and foraging behavior was noted by observers.

Pioneer Seamount Ocean Acoustic Observatory
A vertical array of four hydrophones was installed at Pioneer Seamount in August 2001 to passively monitor the Pacific Ocean in the region south of San Francisco. The hydrophones were connected to shore via a telephone cable that came ashore at Pillar Point.

Data from the hydrophones were relayed to NOAA Pacific Marine Environmental Laboratories and to San Francisco State University, where they were made available for public access. In this data set, the loudest and most obvious signals were created by passing ships. Ship traffic and relative ship size can be inferred from the data set. Scientists have also identified a method for analyzing the ship signals to determine the speed of each ship and its distance of closest approach to the array.

Blue, fin, humpback and sperm whales were heard on Pioneer Seamount. Humpback and blue whale calls appeared prominent. Seasonality was evident, with most calls appearing in winter and fall months. Sounds from blue whales were quite prominent over the eight months of monitoring. The cable was damaged and had not functioned since September 2002. The cable was removed in 2011.