Oceanography: GFNMS

The oceanography of the Greater Farallones National Marine Sanctuary region is complex and the result of numerous factors. Regional currents such as the California and Davidson Currents, local outflow from San Francisco Bay, tidal currents, and submarine and terrestrial physical features all influence the area’s oceanographic environment.

The sanctuary is comprised of offshore waters that extend out to the continental slope around the Farallon Islands, nearshore waters (up to the mean high tide line) from Bodega Head in Sonoma County to Rocky Point in Marin, and estuaries and lagoons. Profound seasonal changes in ocean currents, winds and rainfall characterize the sanctuary and are integral in sustaining the sanctuary’s biological diversity.

Tides and Waves
Tidal currents and waves are important in the coastal ocean. They mix the water column, allowing nutrients near the seabed to reach plankton growing in the lighted surface regions. Tides move nutrients and other suspended materials vertically and back and forth, but they generally do not transport these materials long distances.

Sanctuary estuaries such as Bolinas Lagoon, Tomales Bay, Estero Americano and Estero de San Antonio exchange water and water-borne materials with the coastal ocean through tidal currents, although inner waters may take a long time to flush. On a larger scale, outflow from San Francisco Bay enters the shelf of the sanctuary.

The mouths of Tomales Bay and Bolinas Lagoon remain open year-round, however the mouths of the two Esteros are typically closed during summer and fall by seasonally-formed sand bars, isolating the Esteros from the ocean. Tidal currents are strong near the coastline and diminish offshore, becoming overwhelmed by ocean currents over the continental shelf and slope.

Sanctuary Seasons
Offshore, currents are dominated by seasonal winds. In the spring and early summer (typically March-July), coastal winds blow from north to south, driving the offshore California Current and the process of upwelling.

The California Current is part of the clockwise-flowing North Pacific gyre. During the spring-summer upwelling season, the strongly southward currents are deflected offshore due to the Coriolis effect, a phenomenon caused because the earth is a rotating sphere. As surface water is pushed offshore, cold, deep waters from the California Undercurrent are upwelled to the surface over the continental shelf.

Plants require sunlight and nutrients for growth, and they thrive in the presence of nutrients brought to the surface during upwelling. Single-celled plants like diatoms, in particular, increase in abundance dramatically during the upwelling season. The resultant phytoplankton blooms are the foundation of the rich sanctuary food webs, which involve zooplankton, benthic and pelagic invertebrates, fishes, birds and mammals.

Spring-summer currents over the middle and outer shelf move strongly southward during upwelling, but nearshore flow patterns are mixed. San Francisco Bay and other nearshore outflows are carried south by prevailing coastal currents. During brief periods of weak winds (relaxation periods), much of the inner-shelf and mid-shelf waters in the gulf reverse direction and flow north.

Phytoplankton levels peak during these relaxation periods. The offshore water flows back towards shore, carrying a fertilizing soup of nutrients, algae and zooplankton. Diatoms and other phytoplankton bloom in the sunlit water, and pelagic larvae of benthic animals settle on the continental shelf.

In the fall, upwelling winds weaken and water temperatures increase. Sometimes known as the oceanic or transition season, this period (typically August-November) is characterized by the onshore flow of oceanic surface waters (warmer and with lower salinity).

Periods of upwelling winds and phytoplankton blooms do still occur during the fall but are generally of short duration and infrequent. This is generally the “best” weather along our coast – sunny, clear and warm. Warmer-water species are often spotted offshore – such as leatherback turtles, Dermochelys coriacea, and ocean sunfish, Mola mola, as they travel north following warm-water currents.

Winter in the sanctuary is characterized by the passage of rain-bearing cold fronts, accompanied by westerly and southerly winds that drive northward flow and downwelling over the shelf. Although fronts are typical during the months of December through March, upwelling winds are equally common, and many upwelling events are also observed at this time of year (although lower levels of light in winter produce only weak phytoplankton blooms).

During the downwelling events, warm oceanic surface waters move onshore and land runoff is held nearshore. Large plumes of terrestrial runoff are also subject to the Coriolis effect, hence San Francisco Bay outflow typically remains close to shore and flows north around Point Reyes following major rain and runoff events. The lowest surface seawater salinities are observed in the sanctuary during the winter runoff season

Eddies
Eddies are found both offshore, in the core of the California Current, and in the waters over the shelf. In the sanctuary’s coastal waters, fast flow past headlands like Point Reyes and Bodega Head may create eddies that move through the region.

Eddies and open embayments partly retain nutrient-rich, upwelled waters and help explain the high levels of plankton, fishes, mammals and birds observed in this region year-round. As a result, the sanctuary is one of the most complex and productive areas along the biologically-rich California coast.

Monitoring

The following list includes some of the projects underway in the sanctuary. Please click on the Projects tab at the top of this page for more information.

Sanctuary Ecosystem Assessment Surveys (SEA Surveys)
SEA Surveys are designed to investigate the relationship among hydrographic conditions, physical features and the distribution and abundance of marine organisms in the Greater Farallones. These surveys include counts of marine turtles, birds and mammals along set transect lines.

One component of the Farallon SEA Surveys is to assess biological productivity (chlorophyll-a; phytoplankton species inventory; euphausiid abundance and distribution; distribution and abundance of jellyfish; assessment of drift algae). SEA’s plankton tows and harmful algal bloom assessments will be used to sample for introduced species as well as native populations.

Distribution and Abundance of Marine Birds, Mammals and Zooplankton Relative to the Physical Oceanography of the Greater Farallones and Cordell Bank
PRBO Conservation Science scientists, in partnership with University of California-Bodega Marine Laboratory and the Cordell Bank and Greater Farallones National Marine Sanctuaries, have been investigating the spatial and temporal relationships among krill, krill predators and oceanographic processes in the Greater Farallones and the region surrounding Cordell Bank. This project aims to 1) understand the effects of varying oceanographic regimes on predator-prey relationships and food-web dynamics in the central California region and 2) provide a scientific basis for the design and implementation of a marine protected area (MPA).

Research cruises were conducted winter, spring-summer and fall (three to five cruises/year) from 2004-2007. This study has shown large inter-seasonal and inter-annual differences in lower trophic level abundance as well as predator presence in the sanctuaries. Data have allowed scientists to begin to develop a picture of how mobile marine organisms may benefit from a pelagic marine reserve within the highly productive areas of the California Current marine ecosystem.

Wind Events and Shelf Transport (WEST)
The WEST program was an interdisciplinary study of coastal upwelling off northern California from 2000 to 2003. A number of research institutions were involved in this effort, including San Francisco State University, Scripps Institution for Oceanography, Monterey Bay Aquarium Research Institute, University of California-Davis and the Desert Research Institute.

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 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.

Wind to Whales
This project, through the Center for Integrated Marine Technologies (CIMT) at the University of California Santa Cruz, uses emerging technology to assess the processes underlying the dynamics of the coastal upwelling ecosystems along the California coast. The project includes study of primary production, nutrient flux, harmful algal blooms and the effects of these on the distribution, abundance and productivity of organisms at higher trophic levels, including squid, fishes, seabirds, sea turtles, pinnipeds and whales.