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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 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 two 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 α 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 under-estimation by the diel cycles method (-19 ± 5 %). At the end of the diatom bloom, this relative difference increases to -58 % for a six-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 cycles-based gross primary productivity over the entire deployment, providing further qualitative support to 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 chlorophyll α 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. [ABSTRACT FROM AUTHOR]

Abstract: Budgets of nitrate, dissolved oxygen, and particulate organic carbon (POC) were constructed from data collected on-board a Lagrangian, profiling float deployed between April 4 and May 25, 2008, as part of the North Atlantic Bloom Experiment. These measurements were used to estimate net community production (NCP) and apparent export of POC along the float trajectory. A storm resulting in deep mixing and temporary suspension of net production separated the bloom into early (April 23–27) and main (May 6–13) periods over which ∼264 and ∼805mmolCm−2 were produced, respectively. Subtraction of the total POC production from the NCP yielded maximum estimates of apparent POC export amounting to ∼92 and 574 mmolCm−2 during the early and main blooms, respectively. The bloom terminated the following day and ∼282 mmolCm−2 were lost due to net respiration (70%) and apparent export (30%). Thus, the majority of the apparent export of POC occurred continuously during the main bloom and a large respiration event occurred during bloom Termination. A comparison of the POC flux during the main bloom period with independent estimates at greater depth suggest a rapid rate of remineralization between 60 and 100m. We suggest the high rates of remineralization in the upper layers could explain the apparent lack of carbon overconsumption (C:N>6.6) in the North Atlantic during the spring bloom. [Copyright &y& Elsevier]