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

• Published in: Biogeosciences
• Main Authors: N. Briggs, K. Guðmundsson, I. Cetinić, E. D'Asaro, E. Rehm, C. Lee, M. J. Perry
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2018
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; North Atlantic
• Online Access: https://doi.org/10.5194/bg-15-4515-2018; https://doaj.org/article/2619727894f045879baf3ca991f03e9c

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 ( r 2 = 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 SiO 4 limitation, which is not included in the model. In addition, we estimate gross oxygen productivity from O 2 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 SiO 4 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 ...