1. Department of Botany and Plant Pathology, 2082 Cordley Hall, and,
2. College of Oceanographic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
3. Institute for Computational Earth System Science and Department of Geography, University of California, Santa Barbara, Santa Barbara, California 93106-3060, USA
4. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
5. Atmospheric and Oceanic Sciences Program, Princeton University, PO Box CN710, Princeton, New Jersey 08544, USA
6. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University 71 Dudley Rd, New Brunswick, New Jersey 08901, USA
7. School of Marine Sciences, 209 Libby Hall, University of Maine, Orono, Maine 04469-5741, USA

Correspondence to: Michael J. Behrenfeld¹ Correspondence and requests for materials should be addressed to M.J.B. (Email: mjb@science.oregonstate.edu).

Abstract

Contributing roughly half of the biosphere's net primary production (NPP)^{1, 2}, photosynthesis by oceanic phytoplankton is a vital link in the cycling of carbon between living and inorganic stocks. Each day, more than a hundred million tons of carbon in the form of CO₂ are fixed into organic material by these ubiquitous, microscopic plants of the upper ocean, and each day a similar amount of organic carbon is transferred into marine ecosystems by sinking and grazing. The distribution of phytoplankton biomass and NPP is defined by the availability of light and nutrients (nitrogen, phosphate, iron). These growth-limiting factors are in turn regulated by physical processes of ocean circulation, mixed-layer dynamics, upwelling, atmospheric dust deposition, and the solar cycle. Satellite measurements of ocean colour provide a means of quantifying ocean productivity on a global scale and linking its variability to environmental factors. Here we describe global ocean NPP changes detected from space over the past decade. The period is dominated by an initial increase in NPP of 1,930 teragrams of carbon a year (Tg C yr^-1), followed by a prolonged decrease averaging 190 Tg C yr^-1. These trends are driven by changes occurring in the expansive stratified low-latitude oceans and are tightly coupled to coincident climate variability. This link between the physical environment and ocean biology functions through changes in upper-ocean temperature and stratification, which influence the availability of nutrients for phytoplankton growth. The observed reductions in ocean productivity during the recent post-1999 warming period provide insight on how future climate change can alter marine food webs.

1. Department of Botany and Plant Pathology, 2082 Cordley Hall, and,
2. College of Oceanographic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA
3. Institute for Computational Earth System Science and Department of Geography, University of California, Santa Barbara, Santa Barbara, California 93106-3060, USA
4. NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
5. Atmospheric and Oceanic Sciences Program, Princeton University, PO Box CN710, Princeton, New Jersey 08544, USA
6. Environmental Biophysics and Molecular Ecology Program, Institute of Marine and Coastal Sciences and Department of Geological Sciences, Rutgers University 71 Dudley Rd, New Brunswick, New Jersey 08901, USA
7. School of Marine Sciences, 209 Libby Hall, University of Maine, Orono, Maine 04469-5741, USA

Correspondence to: Michael J. Behrenfeld¹ Correspondence and requests for materials should be addressed to M.J.B. (Email: mjb@science.oregonstate.edu).

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