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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 15, issue 14 | Copyright
Biogeosciences, 15, 4515-4532, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 25 Jul 2018

Research article | 25 Jul 2018

A multi-method autonomous assessment of primary productivity and export efficiency in the springtime North Atlantic

Nathan Briggs1, Kristinn Guðmundsson2, Ivona Cetinić3,4, Eric D'Asaro5, Eric Rehm6, Craig Lee5, and Mary Jane Perry7 Nathan Briggs et al.
  • 1National Oceanography Centre, Southampton SO14 3ZH, UK
  • 2Marine Research Institute, P.O. Box 1390, 121 Reykjavík, Iceland
  • 3GESTAR/Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA
  • 4Ocean Ecology Laboratory, NASA Goddard Space Flight Center Code 616, Greenbelt, MD 20771, USA
  • 5Applied Physics Laboratory and School of Oceanography, University of Washington, Seattle, WA 98105, USA
  • 6Département de Biologie et Québec-Océan, Université Laval, Québec QC G1V 0A6, Canada
  • 7Darling Marine Center, School of Marine Sciences, University of Maine, Walpole, ME 04573, USA

Abstract. Fixation of organic carbon by phytoplankton is the foundation of nearly all open-ocean ecosystems and a critical part of the global carbon cycle. But the quantification and validation of ocean primary productivity at large scale remains a major challenge due to limited coverage of ship-based measurements and the difficulty of validating diverse measurement techniques. Accurate primary productivity measurements from autonomous platforms would be highly desirable due to much greater potential coverage. In pursuit of this goal we estimate gross primary productivity over 2 months in the springtime North Atlantic from an autonomous Lagrangian float using diel cycles of particulate organic carbon derived from optical beam attenuation. We test method precision and accuracy by comparison against entirely independent estimates from a locally parameterized model based on chlorophyll a and light measurements from the same float. During nutrient-replete conditions (80% of the study period), we obtain strong relative agreement between the independent methods across an order of magnitude of productivities (r2 = 0.97), with slight underestimation by the diel cycle method (−19±5%). At the end of the diatom bloom, this relative difference increases to −58% for a 6-day period, likely a response to SiO4 limitation, which is not included in the model. In addition, we estimate gross oxygen productivity from O2 diel cycles and find strong correlation with diel-cycle-based gross primary productivity over the entire deployment, providing further qualitative support for both methods. Finally, simultaneous estimates of net community productivity, carbon export, and particle size suggest that bloom growth is halted by a combination of reduced productivity due to SiO4 limitation and increased export efficiency due to rapid aggregation. After the diatom bloom, high Chl a-normalized productivity indicates that low net growth during this period is due to increased heterotrophic respiration and not nutrient limitation. These findings represent a significant advance in the accuracy and completeness of upper-ocean carbon cycle measurements from an autonomous platform.

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We rigorously tested emerging methods for measuring the rate of photosynthesis in the ocean using autonomous robotic platforms. We found similar accuracy to traditional, labor-intensive, ship-based measurements across a variety of ocean conditions. Photosynthesis is the basis of nearly all life, both on land and in the ocean. Our results suggest that by scaling up existing technologies, we can greatly improve global monitoring of one of life's key processes.
We rigorously tested emerging methods for measuring the rate of photosynthesis in the ocean...