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MONTEREY BAY SANCTUARY: OCEANOGRAPHY
MONITORING PROJECT INFORMATION
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MBARI Time Series (MBTS) Program
Principal Investigator(s)
Francisco Chavez (Monterey Bay Aquarium Research Institute)
Timothy Pennington (Monterey Bay Aquarium Research Institute)
 
Start Date
1992-01-01
Funding
David and Lucile Packard Foundation
NOAA
 
Overview
The Monterey Bay Aquarium Research Institute (MBARI) Time Series program (MBTS) focuses on the relations between oceanic carbon and nitrogen cycles and climate variability, with an emphasis on primary production. The approach is multidisciplinary, multiplatform, and both observational and experimental. Shipboard measurements of fundamental physical, chemical and biological parameters are made during time series cruises in Monterey Bay (since 1989) and offshore into the California Current (since 1997). These data sets have been augmented by a sensor and platform development drive to automate data collections, first by producing high-frequency time series data from moorings in Monterey Bay (since 1992), and recently with high spatial resolution data sets collected by Autonomous Underwater Vehicles (AUVs; in Monterey Bay since 2003) and drifters.

This time series work is an example of MBARI’s ability to support programs that would be exceedingly difficult to sustain with government grants (19 years since 1989). The data now span many time and space scales and permit construction of synthetic views of system dynamics. The work has documented seasonal cycles, El Niños and La Niñas and longer decade-scale cycles (e.g., Pacific Decadal Oscillation, aka PDO) --- all of which must be characterized before they can be separated from the effects of global warming.

Below we highlight recent results of our time series work. A linked report introduces basic California Current System oceanography with summary graphs, and also relates the CalCOFI (California Cooperative Oceanic Fisheries Investigations) and MBTS programs to one another. A second link provides access to the detailed results of the MBTS program.

 
Summary to Date
Time series data are used to assess change (Figure 1). We believe that the northeast Pacific changed from a warm ‘El Viejo’ to a cool ‘La Vieja’ regime following the strong 1997-98 El Niño. However, exploration of this ‘regime shift’ has been confounded by the occurrence of one strong La Niña and two weak El Niño’s during the intervening years. Niño’s are individually complex; when they co-occur with seasonal cycles and regime shifts, the thread of causation is easily lost. Nevertheless, due to the potential effects of global warming, it remains imperative that we learn -- as quickly as possible -- as much as we can about the earth’s climate and ocean system.

In spite of the multiple sources of variability listed above, a number of changes occurred in Monterey Bay after 1998 and have persisted as part of the regime shift to La Vieja. Both sea surface and 60 m temperatures have been below normal for most of the past 7 years (Figure 1A-B), and 60 m nitrate has been above normal (Figure 1C). As an important local consequence of the increased nitrate, overall phytoplankton biomass has been high (Figure 1D). We believe these changes are indicative of stronger upwelling and a faster California Current, which in turn may imply that the entire north Pacific subtropical gyre is rotating more quickly during La Vieja.

Contrary to upwelling system dogma, however, centric diatoms have not been favored throughout the recent cool regime. Centric diatoms are often thought of as signature or ‘keystone’ phytoplankton species within upwelling systems, where they flourish and support the base of the food chain. Centrics were abundant from 1998-2002, but in spring 2003 became less so --- as during the 1997-1998 El Niño (Figure 1E). Several months following the centric decrease, dinoflagellates bloomed instead and their numbers remain high (Figure 1F).

This shift in the dominant phytoplankton taxa may be related to their differing ecological niches. Centric diatoms have high maximum growth rates and thus outcompete other phytoplankton under the high nutrient conditions characteristic of upwelling. Dinoflagellates, on the other hand, have slower maximum growth rates, but can swim and so undertake vertical migrations down into higher-nutrient subsurface waters if surface levels fall. Thus dinoflagellates are favored when near-surface nitrate is low. Such a switch seems to have occurred in 2004, when the shallow water column became unusually stratified (Figure 1G) due to a reduction in local upwelling-favorable winds (Figure 1H). Stratification restricted mixing of nutrients to the surface from below so that centric diatoms did poorly while dinoflagellates bloomed. Thus overall phytoplankton biomass remained high, but the dominant taxa changed. This change in phytoplankton must have affected fish and other higher-trophic level organisms in as yet unexplored ways. The post-2003 reduction in wind also remains unexplored.

 
Monitoring Trends
  • Following the 1997-98 El Niño, the NE Pacific switched to the ‘La Vieja’ phase of the Pacific Decadal Oscillation.
  • Ocean temperatures have been cool.
  • In 2003, dinoflagellates replaced diatoms as the dominant phytoplankton taxon in Monterey Bay.
  • This shift in ‘keystone’ phytoplankon is associated with an increase in near surface stratification and a decrease in upwelling-favorable winds.
 
Discussion
It appears that we are currently experiencing a cool La Vieja ocean characterized by generally high nutrient levels and high overall phytoplankton abundance, but that increased water column stratification after 2003 reduced near-surface nitrate, which in turn caused a switch in keystone phytoplankton taxa. The cool La Vieja is at least in part a basin-scale phenomenon, but the relationship between basin- and local-scale dynamics remains an important yet difficult part of the story. For example, we are currently (winter 2007) experiencing a mild El Niño yet our local ocean -- contrary to expectation and experience -- remains very cold. Is this a local- or remotely-driven effect? We are uncertain. Is it important? You bet. Why? Because we are certain that (1) conclusions about global climate change begin with local observations, and (2) unusual conditions are often highly informative. With 19 years’ experience we are also certain that the interplay between seasonal, interannual and decadal change is difficult to unravel. Nevertheless, given that our civilization’s continued success will depend on understanding and hopefully mitigating climate change, we must continue to explore the complex relations within the ocean and climate system.
 
Study Parameters
Click here to view the parameters measured in this study.
Study Methods
Click here to view the methods and materials used in this study.
 
Images and Documents

Figure 1. Time series of anomalies, with higher [or lower] than normal values in red [or blue]. (A) 0 m and (B) 60 m temperatures have in general remained cool since 1998, resulting in high (C) 60 m nitrate and (D) 0 m chlorophyll (overall phytoplankton biomass) values. However, (E) centric diatoms decreased sharply in 2003 and were apparently replaced by (F) dinoflagellates in 2004. This phytoplankton switch may have been caused by increased (G) near-surface stratification (0-20 m difference in the water density parameter, sigma-t) which in turn resulted from decreased (H) wind-driven upwelling after 2003 (upwelling indices from http://www.pfeg.noaa.gov).



Figure 2. Sea surface partial pressure of carbon dioxide (pCO2) has been measured during Monterey Bay Time Series cruises since 1993 (blue dots, upper panel). In spite of considerable seasonal and coastal variability (red lines), pCO2 and acidity (blue dots, lower panel) both increase over the decades (green lines, greater acidity, decreasing pH) as a result of the ocean’s absorption of CO2 from the atmosphere. These increases are in the same direction but stronger than increases observed in the atmosphere and in the open ocean near Hawaii (orange lines); the stronger increases are presumably driven by recent multi-decadal cooling. Courtesy MBARI and Dr. F. Chavez.



Pennington et al. (2007)
A Report to the Sanctuary Integrated Monitoring Network (SIMoN), Ocean observing in the Monterey Bay National Marine Sanctuary: CalCOFI and the MBARI time series
516 KB PDF
 
 
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