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Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean


Biological productivity in most of the world's oceans is controlled by the supply of nutrients to surface waters. The relative balance between supply and removal of nutrients—including nitrogen, iron and phosphorus—determines which nutrient limits phytoplankton growth. Although nitrogen limits productivity in much of the ocean1,2, large portions of the tropics and subtropics are defined by extreme nitrogen depletion. In these regions, microbial denitrification removes biologically available forms of nitrogen from the water column, producing substantial deficits relative to other nutrients3,4,5. Here we demonstrate that nitrogen-deficient areas of the tropical and subtropical oceans are acutely vulnerable to nitrogen pollution. Despite naturally high nutrient concentrations and productivity6,7,8, nitrogen-rich agricultural runoff fuels large (54–577 km2) phytoplankton blooms in the Gulf of California. Runoff exerts a strong and consistent influence on biological processes, in 80% of cases stimulating blooms within days of fertilization and irrigation of agricultural fields. We project that by the year 2050, 27–59% of all nitrogen fertilizer will be applied in developing regions located upstream of nitrogen-deficient marine ecosystems. Our findings highlight the present and future vulnerability of these ecosystems to agricultural runoff.

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Figure 1: SeaWiFS image of chlorophyll a in the GOC from 6 April 1998, one day after peak irrigation.
Figure 2: Five-year time series.

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We are grateful for funding from the David and Lucile Packard Foundation and the US NSF Graduate Research Fellowship Program. Thanks to G. van Djiken and R. Labiosa for remote sensing support, I. Ortiz-Monasterio for irrigation data, E. Cruz-Colin for sharing unpublished data, and K. Beman for editorial advice. We thank P. Vitousek and R. Dunbar for comments on an earlier draft of this manuscript.

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Correspondence to J. Michael Beman.

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Supplementary information

Supplementary Methods

Background on remote sensing data and processing. (RTF 9 kb)

Supplementary Figure S1

Identification of peaks in unfiltered and filtered chlorophyll time series. (PDF 420 kb)

Supplementary Video 1

SeaWiFS ocean colour images from the years 2000 and 2001. (QT 25739 kb)

Supplementary Legends

This file contains the Supplementary Legends for Supplementary Figure S1 and Supplementary Video 1. (DOC 20 kb)

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Michael Beman, J., Arrigo, K. & Matson, P. Agricultural runoff fuels large phytoplankton blooms in vulnerable areas of the ocean. Nature 434, 211–214 (2005).

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