Biogeography

Cordell Bank National Marine Sanctuary

Introduction

caption This 3-D image is the seafloor extending west of Point Reyes to the edge of the continental shelf. The vertical relief of the continental slope is exaggerated due to the spatial scale of the image. (Photo: USGS)

The Cordell Bank National Marine Sanctuary protects an area of 397 square nautical miles (526 square miles) off the northern California coast. Cordell Bank is located on the continental shelf, about 43 nautical miles (nm) northwest of the Golden Gate Bridge and 18 nm west of the Point Reyes lighthouse. The main feature of the sanctuary is an offshore granitic bank 4.5 miles wide by 9.5 miles long, although the sanctuary boundary starts six miles offshore of Point Reyes.

This rocky submerged island emerges from the soft sediments of the continental shelf, with the upper pinnacles reaching to within 120 feet of the ocean's surface. The continental shelf depth at the base of the Bank is roughly 400 feet.

Cordell Bank National Marine Sanctuary protects a very productive offshore area. The combination of ocean conditions and undersea topography creates a rich and diverse marine community. The prevailing California current flows southward along the coast, and the upwelling of nutrient-rich deep ocean water provides the foundation for a flourishing marine ecosystem. The Sanctuary supports healthy resident populations and is a destination feeding ground for many migratory marine mammals, seabirds, and fishes. Invertebrates proliferate on the Bank and in the surrounding water column.

Flora and Fauna

Marine Mammals

caption Northern Elephant Seal (Mirounga angustirostris). Photo by GFNMS files

Cordell Bank attracts many diverse species of marine mammals. A total of twenty-six species of whales, dolphins, seals, and sea lions are known to frequent the surrounding waters. The sanctuary is one of the most important feeding grounds in the world for the endangered blue and humpback whales. These whales travel from their breeding areas in Mexico and Central America to feed on the abundant krill and schooling fish that aggregate near the bank. In late summer, breaching humpbacks are frequently seen around the bank. Pacific white-sided dolphins are attracted by plentiful food resources and can be seen in large numbers. California sea lions, elephant seals, northern fur seals, and Steller sea lions frequent sanctuary waters to feed on krill, squid, and juvenile fishes.

Fish

caption Blue Shark (Prionace glauca). Photo by Howard Hall

This diverse group includes bony fish, sharks, rays, and some unique fish that do not fit within any of these sub-categories. They live within a variety of habitats including sandy bottoms, open water, and rocky reefs. The area around Cordell Bank supports more than 246 species of fish. These include 44 species of rockfish, ranging in size from the 8-inch pygmy rockfish to the 3-foot yellow-eye rockfish. The sanctuary provides critical habitat for many recreational and commercially important fish species, which play an important role in the ecosystem and our economy. Status index estimates are only approximations of local fish populations. For quantitative stock assessments for the fish, please contact the California Department of Fish and Game or the National Marine Fisheries Service.

Marine Birds

caption Black-footed Albatross (Phoebastria nigripes). Photo by Rich Stallcup

Seabirds do not comprise a taxonomic or evolutionary group of birds; they are simply defined as birds that spend most of their life feeding and living on the open ocean, coming to land only to breed. Cordell Bank's productive waters make it a major foraging locality for thousands of seabirds. This includes resident species that nest on the nearby Farallon Islands as well as highly migratory and vagabond pelagic birds. The sanctuary has been called the “albatross capital of the northern hemisphere,” because numerous species visit these waters.

Invertebrates

caption Comb Jelly (Beroe sp.). Photo by Jamie Hall

Marine invertebrates are the most diverse and abundant group of organisms in the ocean. This grouping is not based on a single taxon, but is defined as a group of animals found in the marine environment which lack a vertebral column. This incredible array of animals is found within all the habitats of the sanctuary including the rocky shelves, sand flats, and open ocean. There are hundreds of species of invertebrates in the Cordell Bank National Marine Sanctuary, such as sponges, urchins, anemones, lobsters, crabs, snails, squid, jellies, and sea stars. Some of these species are of recreational or commercial importance, and most of them play an important role in the ecosystem.

Plants and Algae

caption Dinoflagellate (Not Identified). Photo by CBNMS

Both macroalgaes and phytoplankton are primary producers, deriving energy from the sun through photosynthesis to form the base of the food web in the ocean. Phytoplankton is general term describing very small plants and algae that drift with the ocean’s currents. Macroalgae is larger algae that typically attaches to a substrate to anchor it down. Both of these groups play a very important biological role in Cordell Bank National Marine Sanctuary.

Marine Reptiles

caption Leatherback Turtle (Dermochelys coriacea). Photo by Nancy Black

Reptiles are relatively uncommon residents of the marine environment. Of the small group of reptiles that inhabit the sea, perhaps the most easily recognized in the Eastern Pacific are the sea turtles. There are only seven species of sea turtles worldwide, and all but one are endangered. Though not considered residents of the sanctuary, leatherback turtles can sometimes be seen in Cordell Bank National Marine Sanctuary.

History of Cordell Bank

Coastal California has a rich history of native Americans and early settlers utilizing the marine environment. For millions of years, Cordell Bank was an unknown mystery hidden beneath the waves. Nearshore food resources were so plentiful that there was no reason for the native Miwok Indians to venture miles offshore.

In the late 1800s there was a strong incentive to survey the coast of California to promote safer maritime commerce. Cordell Bank was discovered in 1853 by George Davidson of the U.S. Coast Survey while returning from a mapping expedition on California's north coast.

Edward Cordell, an accomplished surveyor, conducted additional surveys in 1869 when he was sent to relocate a "shoal west of Point Reyes." Cordell was attracted to the location by the numerous birds and marine mammals. To measure the depth, Cordell lowered a lead weight over the edge until it hit bottom and then measured the line on its return to the surface. Always considered a productive fishing area, not much was known about what life existed on the bank until 1977.

Cordell Bank was first explored underwater in 1977 by a non-profit research association, Cordell Expeditions. Over the next 10 years, divers documented the organisms living on and above the bank. Through these efforts, images of the biological diversity of Cordell Bank were available to the public for the first time. This effort was instrumental in creating Cordell Bank National Marine Sanctuary.

Natural Environment: Geologic History

Cordell Bank was originally created 93 million years ago as part of the southern Sierra Nevada mountains. As the Pacific Plate moved north, it sheared off part of the North American plate and carried Cordell Bank to its present location west of Point Reyes. The Bank continues to move north at a rate of about three centimeters per year. Between 20,000 and 15,000 years ago when sea level was about 360 feet below current sea level, most of Cordell Bank was exposed, making it a true island.

Today Cordell Bank emerges from the soft sediments of the continental shelf deposited more recently by coastal erosion. Within seven miles of the bank's western edge, the seafloor drops to over a mile deep.

Oceanographic Seasons

Spring and Summer

caption Winter: the storm season

The productive ecosystem off the coast of California has three oceanographic seasons. Just as spring and summer are growing seasons in our backyard gardens, there are growing seasons in the water column around Cordell Bank. During springtime, strong northwest winds push the water southward along the California coast. Gale winds and ocean currents, combined with Earth's rotation, drive surface waters away from the shore. These surface waters are replaced by upwelling of deeper and colder nutrient-rich waters from offshore. The nutrients become available for surface dwelling phytoplankton, or marine algae. Phytoplankton are the foundation of the food web and the infusion of nutrients and increased sunlight in spring initiates a bloom of life that radiates up the food web. An abundance of phytoplankton, zooplankton, and young fish are food for animals at higher levels of the marine food web.

Late Summer and Fall

caption Fall: the relaxation period

During the late summer and fall, coastal winds die down and the sea surface becomes calmer. The northward-flowing Davidson Current moves over the continental shelf bringing warm, nutrient-poor waters from the south.

Winter Storm Season

caption Winter: the storm season

The winter storm season (mid-November through early spring) is dominated by rough seas and mixing of deeper ocean water. The water on top of the continental shelf becomes mixed between the seafloor and the surface. The temperature, salinity, and concentration of nutrients are more similar throughout the water column.

Further Reading

Cordell Bank National Marine Sanctuary Official Site



Disclaimer: This article is taken wholly from, or contains information that was originally published by, the National Marine Sanctuary. Topic editors and authors for the Encyclopedia of Earth may have edited its content or added new information. The use of information from the National Marine Sanctuary should not be construed as support for or endorsement by that organization for any new information added by EoE personnel, or for any editing of the original content.

Glossary

Citation

(2010). Cordell Bank National Marine Sanctuary. Retrieved from http://www.eoearth.org/view/article/51cbed577896bb431f69189c

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