Oceanography: CBNMS

On a broad scale, the oceanography of Cordell Bank National Marine Sanctuary is influenced by the California Current, an eastern-boundary surface current that is 1,200 kilometers (745 miles) broad and 300 meters deep and transports water of subarctic origin southward along the North American coast at 15 to 30 centimeters (6 to 12 inches) per second (cm/s). Beneath this surface current and within about 100 kilometers of the coast, the California Undercurrent transports waters of subtropical origin northward at about 4 cm/s. In winter, this undercurrent surfaces, where it mixes with California Current waters and is called the Inshore Countercurrent, or Davidson Current.

Together these currents are termed the California Current System, and the sanctuary lies wholly within this system. Thus, the surface and intermediate-depth water masses in the sanctuary are a mixture of subarctic Pacific water with low salinity and cool temperatures together with warmer, saltier Pacific Equatorial water.

A characteristic feature of eastern-boundary currents is an oceanographic process called upwelling. Coastal upwelling in the California Current initiates an annual productivity cycle that supports a rich biological community and attracts migratory animals from around the globe.

The calendar year at Cordell Bank can be separated into three oceanographic seasons:

• the upwelling period from early spring to late summer (March-July), characterized by strong southward wind forcing and currents along with cool surface waters
• the relaxation period (or oceanic period) from late summer to early fall (August-early November), characterized by wind relaxation, warmer water temperatures and mainly northward surface currents
• the winter storm period (or Davidson Current period) from late fall to late winter (mid-November-February), characterized by rough seas and greater mixing of ocean water.

While the seasonal changes in the coastal ocean are important, longer-term climatic phenomena including El Niño-Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO) and global climate change also affect local physical and biological systems. ENSO refers to periodic cycling between anomalously warm (El Niño) and cool (La Niña) ocean water temperatures that spread across the equatorial Pacific Ocean. These temperature anomalies indicate perturbations in the ocean and atmosphere that are manifested over broad scales, including the California Current ecosystem. Off the California coast, El Niño events are characterized by increases in ocean temperature and sea level, enhanced onshore and northward flow, and reduced productivity. In addition, the increase of water temperature during an El Niño could introduce exotic species of southerly or offshore origin that are not usually found in the sanctuary.

Like ENSO, the PDO comprises a warm and a cool interval (associated with changes in surface water temperatures of several degrees), but over a longer period of time. PDOs are periods of sustained climate conditions that are associated with shifts in ecosystem production regimes in cycles of about 50 years.

During the spring and summer, upwelling occurs along much of the central California coast; upwelled waters within the sanctuary originate from an upwelling center at Point Arena, approximately 100 kilometers north of Cordell Bank. Such upwelling centers are readily observed in satellite images as cool zones, typically 3° to 5°C (37.4° to 41° F) cooler than waters 100 kilometers offshore. Satellite images often show a tongue of cool water originating at the Point Arena center and flowing southwards towards Point Reyes and west across Cordell Bank.

Productivity within the nearshore region is a balance between the positive influence of upwelling (which brings enhanced nutrients to the surface, stimulating phytoplankton growth) and the negative influence of mixing and advection (which transports phytoplankton below surface-lit layers as well as offshore). Thus, higher productivity within this region results from a combination of upwelling and relaxation events.

Recent years have illustrated how changes in the intensity and timing of upwelling conditions in the spring/summer have affected reproductive success, recruitment levels and habitat use patterns of seabirds, fishes and marine mammals within the central California ecosystem.

Recent studies have demonstrated complex structure in alongshore flow patterns, which varies with distance from shore in the sanctuary region. Under upwelling conditions, water over the entire continental shelf flows towards the south, while during relaxation events, nearshore (within 15 kilometers of the shoreline) water flow is towards the north and inner-shelf (within 15 to 25 kilometers of the shoreline) water flow is towards the south. This cross-shore variation in flow can cause spatial variation in primary production as well as transport and retention of planktonic larvae.

Smaller-scale features such as fronts, which delineate the boundary between different water masses, can influence the distribution of various organisms that either accumulate passively at these zones or actively aggregate here due to the abundance of prey items. Such fronts have been identified in the vicinity of Cordell Bank. These fronts could indicate the boundary between cold, newly upwelled water and warmer water masses, as a result of 1) regional upwelling; and 2) localized upwelling due to the dramatic change in bathymetry in the vicinity of the bank.

Monitoring

Applied California Current Ecosystem Studies (ACCESS)
ACCESS is a research partnership to support integrated ocean management in northern and central California. Point Blue, Cordell Bank and Greater Farallones National Marine Sanctuaries have been investigating the spatial and temporal relationships between oceanographic processes, zooplankton, and marine birds and mammals in the region surrounding Cordell Bank and Greater Farallones national marine sanctuaries. This project has several objectives, including: 1) Understand how the timing, intensity, and duration of upwelling influences the distribution and abundance of euphausiids (better known as krill) thus affecting the distribution and abundance of krill predators in the region; 2) Identify persistent locations of predator and prey aggregations and potential areas of high trophic transfer in the Greater Farallones region that may be associated with bathymetric and hydrographic features; 3) Monitor physical and biological characteristics of the pelagic ecosystem, with the goal of developing indicators of ecosystem health, to understand change on a variety of scales and detect natural and anthropogenic impacts.

Research cruises have been conducted in spring, summer and fall (three to five cruises per year) from 2004 to the present. This study has shown large inter-seasonal and inter-annual differences in lower trophic level abundance as well as predator presence in the sanctuaries. This assessment of the pelagic system specifically meets the sanctuary’s mandate to conduct long-term monitoring of the resources within the sanctuary and provides important information for resource protection and management.

Cordell Bank Ocean Monitoring Program
The Cordell Bank Ocean Monitoring Program (CBOMP) collected information on the spatial and temporal variability in the oceanographic system of the Cordell Bank region from 2004 to 2010. Data on the abundance of seabirds, marine mammals, other vertebrates and marine debris were collected by trained observers along six 12-kilometer east-west transects centered on Cordell Bank. Physical and biological characteristics of the pelagic system were measured along transects using a CTD (vertical profiles of salinity, temperature, chlorophyll-a, and light levels at set stations), TSG (continuous surface values of salinity, temperature, chlorophyll-a) and echo sounder (continuous measurements of relative abundance of zooplankton).

Starting in 2010, CBOMP was replaced by the Applied California Current Ecosystem Studies (ACCESS) program.

Cordell Bank Oceanographic Buoy
The Cordell Bank buoy was deployed in the spring of 2007, through a collaboration between University of California-Bodega Marine Laboratory and Cordell Bank National Marine Sanctuary. The system is comprised of sensors for water velocity, water temperature, salinity, turbidity, chlorophyll fluorescence, and wind velocity.

The mooring is located at a depth of 85 meters on the northern part of Cordell Bank, about 20 nautical miles west of Point Reyes. This buoy provides near-real-time data that are linked with regional coastal ocean observing systems and are used by sanctuary staff, research oceanographers and local communities to understand offshore ocean conditions better. Further, over time, this mooring will provide an invaluable record of fluctuations and change in the ocean environment that supports the highly productive marine ecosystem in this region.

Wind Events and Shelf Transport (WEST)
The WEST program was an interdisciplinary study of coastal upwelling off northern California from 2000 to 2003 conducted by a group of researchers from various universities. Using modeling and field observations, WEST strived to understand the competing influences of wind forcing on planktonic productivity, concentrating on the region offshore of Bodega Bay, including the waters of the Greater Farallones and Cordell Bank national marine sanctuaries.

Field observations of winds, ocean circulation, nutrients, phytoplankton and zooplankton were combined with model studies of winds, circulation and productivity. Results from the WEST study have revealed new insights on the temporal and spatial structure of wind-driven upwelling and the response of plankton communities.