Oceanography: MBNMS

The oceanography of the Monterey Bay National Marine Sanctuary is influenced by the California Current, a 1,200-kilometer (745-mile) broad and 300-meter (985-feet) deep surface current that transports water of subarctic origin southward along the North American coast at 15 to 30 centimeters (5.9 to 11.8 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 four cm/s. In winter, this current 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. The pelagic organisms of the sanctuary originate and travel north- or southwards in these different water masses.

Three oceanographic seasons were originally described for Monterey Bay in the 1940s and are still in common use today:

• the upwelling period from early spring to late summer (February to July), when surface waters are cool
• the oceanic, or California Current, period from late summer to early fall (August to October), characterized by wind relaxation
• the Davidson Current period from late fall to late winter (November to January) characterized by winter storm conditions

During the spring and summer, upwelling occurs along much of the coast within the sanctuary, but Point Año Nuevo and Point Sur “anchor” areas of especially strong upwelling. These upwelling centers are readily observed in satellite images as cool zones, typically 3 to 5 ºC (37.4 to 41 ºF) cooler than waters 100 kilometers offshore.

Satellite images often show a tongue of cool water originating at the Año Nuevo center and flowing southwards across the mouth of Monterey Bay. This upwelling plume spins off eddies circulating both offshore and within the bay.

In the north bay, northward flow and lack of wind (the Santa Cruz mountains block upwelling-favorable winds) create an upwelling shadow characterized by strong thermal stratification and warm surface waters. In summer, this area flushes slowly and is often brown with very high phytoplankton biomass. (It has been called the “armpit” or “incubator” region of the bay.)

The upwelling center near Point Sur is generally cooler and more extensive than that near Año Nuevo. During strong upwelling conditions (persistent northwesterly winds), these cool waters can merge, forming a continuous band of upwelled water from north of Monterey Bay to south of Point Sur.

Varying degrees of iron limitation have been measured in the coastal waters of central California during the summer upwelling seasons. Iron limitation is most common and severe in waters removed from continental shelf sources of iron. Thus, areas with a wide continental shelf (e.g., Monterey Bay and the shelf to the north) tend to have waters that are iron-replete compared to areas with a narrow shelf (south of Monterey Bay). Iron-replete and iron-limited waters tend to differ in the associated phytoplankton communities and relative biomass at many trophic levels.

While the seasonal changes in the coastal ocean and Monterey Bay are important, longer-term interannual variations, principally El Niño events, also affect local physical and biological systems. During the 1982-1983 El Niño (one of the strongest in the last century), for example, dramatic changes were observed in the phytoplankton composition of Monterey Bay, such as an increased relative abundance of dinoflagellates and tropical species and a decreased abundance of diatoms.

During the 1992-1993 El Niño, total phytoplankton abundance and primary production rates measured in March 1992 were reduced by factors of three to four compared to March 1990. In addition, the increase in water temperature during an El Niño brings exotic species of southerly or offshore origin that are usually not found in the sanctuary.

Sanctuary offshore waters are in relatively good condition, but nearshore coastal areas, harbors, lagoons, estuaries and tributaries show a number of problems, including elevated levels of coliform bacteria, detergents, oils, nitrates, sediments and persistent pesticides such as DDT and toxaphene. These contaminants can have a variety of biological impacts – including bioaccumulation, reduced recruitment of anadramous species and transfer of human pathogens – as well as interference with recreational uses due to beach closures.

Phytoplankton blooms, including harmful algal blooms, have increased in frequency and distribution worldwide since 1980. The frequency of such blooms may be increasing with nutrient enrichment from agricultural and urban storm runoff as well as sewage effluent.

Monitoring

Spatial and Temporal Variability in Oceanographic and Meteorological Forcing along Central California
Researchers with the United States Geological Survey (USGS) analyzed more than 20 years (1980-2002) of hourly deep-water buoy data from off central California to investigate long-term trends and the behavior of the measured oceanographic and meteorological variables during different climatic regimes. Statistically significant differences were observed in the monthly mean significant wave height, dominant wave period, sea-level barometric pressures, sea-surface water temperature, and wind speed and direction during normal, La Niña and El Niño months. In addition to these monthly differences, statistically significant long-term trends in monthly mean significant wave height, dominant wave period, sea-level barometric pressures, sea-surface water temperature and wind speed were observed.

Monterey Bay Ocean Time Series Observations
Researchers at the Monterey Bay Aquarium Research Institute (MBARI) are studying the biogeochemical response of the central California ecosystem to climate and ocean variability. They are exploring the dynamic connections among physical, chemical and biological processes. This program has found that since 1998, ocean temperatures have fallen; the California Current has strengthened; and subsurface nitrate, surface chlorophyll and primary production have all increased due to a change in the Pacific Decadal Oscillation (PDO). This PDO-associated cooling makes it difficult to detect global warming in Monterey Bay.

Monterey Bay Aquarium Incoming Seawater Monitoring
As part of the Monterey Bay Aquarium’s ongoing water quality program, incoming seawater is monitored with both spot measurements and continuously on a five-minute interval using in-situ sensor technology. Data on water temperature, dissolved oxygen and nutrients have detected both seasonal events, such as upwelling, and periodic events, such as El Niños.

Center for Integrated Marine Technologies (CIMT)
CIMT is an interdisciplinary coastal research consortium that integrates data collected via remote sensing, moorings and ship-board surveys in the Monterey Bay region. CIMT uses these technologies to investigate linkages among coastal upwelling, nutrient delivery, phytoplankton and organisms at higher trophic levels (squid, fishes, seabirds, sea turtles, seals and whales).

Monterey Bay Microbial Observatory
Small, single-celled planktonic microbes represent the most abundant organisms in the world’s oceans. The goal of this study is to describe the microbes of Monterey Bay and correlate the presence and absence of particular groups with other physical, chemical and biotic variables.

Characterization of Geologic and Oceanographic Conditions at Pleasure Point
The USGS is compiling baseline geologic and oceanographic information on the coast and inner shelf off Pleasure Point in Santa Cruz County. This study will provide high-resolution topography of the coastal bluffs and bathymetry of the inner shelf out to water depths of 20 meters. In addition, the spatial and temporal variation in waves will be documented. These data will provide the baseline data needed for future studies directed toward predicting the impacts of stabilization on the sea cliffs, beach and nearshore sediment profiles; natural rock reef structures; and offshore habitats and resources.