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Science 26 April 2002:
Vol. 296. no. 5568, pp. 730 - 733
DOI: 10.1126/science.1069174
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Reports

The North Atlantic Spring Phytoplankton Bloom and Sverdrup's Critical Depth Hypothesis

D. A. Siegel,1* S. C. Doney,2 J. A. Yoder3

More than 50 years ago, Harald Sverdrup developed a simple model for the necessary conditions leading to the spring bloom of phytoplankton. Although this model has been used extensively across a variety of aquatic ecosystems, its application requires knowledge of community compensation irradiance (IC), the light level where photosynthetic and ecosystem community loss processes balance. However, reported IC values have varied by an order of magnitude. Here, IC estimates are determined using satellite and hydrographic data sets consistent with the assumptions in Sverdrup's 1953 critical depth hypothesis. Retrieved values of IC are approximately uniform throughout much of the North Atlantic with a mean value of 1.3 mol photons meter-2 day-1. These community-based IC determinations are roughly twice typical values found for phytoplankton alone indicating that phytoplankton account for approximately one-half of community ecosystem losses. This work also suggests that important aspects of heterotrophic community dynamics can be assessed using satellite observations.

1 Institute for Computational Earth System Science and Department of Geography, University of California Santa Barbara, Santa Barbara, CA 93106-3060, USA.
2 National Center for Atmospheric Research, 1850 Table Mesa Drive, Boulder, CO 80305, USA.
3 Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882-1197, USA.
*   To whom correspondence should be addressed. E-mail: davey{at}icess.ucsb.edu

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