Chlorophyll bloom development and the subtropical front in the North Pacific

Authors

  • Cara Wilson,

    Corresponding author
    1. Environmental Research Division, Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA, Pacific Grove, California, USA
    • Corresponding author: C. Wilson, Environmental Research Division, Southwest Fisheries Science Center, National Marine Fisheries Service, NOAA, 1352 Lighthouse Ave., Pacific Grove, CA 93950, USA. (Cara.Wilson@noaa.gov)

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  • Tracy A. Villareal,

    1. Marine Science Institute and Department of Marine Science, The University of Texas at Austin, Port Aransas, Texas, USA
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  • Mark A. Brzezinski,

    1. Marine Science Institute, University of California, Santa Barbara, California, USA
    2. Department of Ecology Evolution and Marine Biology, University of California, Santa Barbara, California, USA
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  • Jeffrey W. Krause,

    1. Marine Science Institute, University of California, Santa Barbara, California, USA
    2. Dauphin Island Sea Lab, Dauphin Island, Alabama, USA
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  • Andrey Y. Shcherbina

    1. Applied Physics Laboratory, University of Washington, Seattle, Washington, USA
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Abstract

[1] In late summer, satellite ocean color data consistently show localized chlorophyll blooms in the oligotrophic NE Pacific. Based on historical data and the results from recent cruises, these blooms are associated with elevated diatom abundance. However, the physical dynamics that stimulate the blooms remain unknown. Mechanisms suggested to be driving the blooms include mixing at the subtropical front, breaking of internal waves at the critical latitude, shoaling of the mixed layer depth, eddy interactions, and winter mixing of nutrients. To examine these hypotheses, we use data from four summer cruises (2002, 2007, 2008, and 2009) in this region that sampled near a bloom temporally and/or spatially. Conditions associated with five blooms (two blooms were sampled in 2009) are examined. Each area was sampled at a different stage in bloom development, including prebloom, initiation, full bloom, decline, and postbloom conditions. No one variable is found which can explain unequivocally the development of a chlorophyll bloom at a certain location. We describe a set of conditions that could result in the injection of nutrients into the surface water to stimulate a bloom. This “perfect storm” of conditions requires a subsurface stratification minimum layer that intersects the nutricline and that this minimum is close to the base of the mixed layer. These conditions are not predictable in the sense of an annual climatology; however, they do occur often enough to create a reasonably certain, if spatially variable, summer NE Pacific bloom.

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