Particle cycling rates at Station P as estimated from the inversion of POC concentration data

© The Author(s), 2022. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Amaral, V., Lam, P., Marchal, O., Roca-Martí, M., Fox, J., & Nelson, N. Particle cycling rates at Station P as estimated from the inversion of POC concentration data. Elementa: Science of the Anthropocene, 10(1), (2022): 00018, https://doi.org/10.1525/elementa.2021.00018.
Particle cycling rates in marine systems are difficult to measure directly, but of great interest in understanding how carbon and other elements are distributed throughout the ocean. Here, rates of particle production, aggregation, disaggregation, sinking, remineralization, and transport mediated by zooplankton diel vertical migration were estimated from size-fractionated measurements of particulate organic carbon (POC) concentration collected during the NASA EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise at Station P in summer 2018. POC data were combined with a particle cycling model using an inverse method. Our estimates of the total POC settling flux throughout the water column are consistent with those derived from thorium-234 disequilibrium and sediment traps. A budget for POC in two size fractions, small (1–51 µm) and large (> 51 µm), was produced for both the euphotic zone (0–100 m) and the upper mesopelagic zone (100–500 m). We estimated that POC export at the base of the euphotic zone was 2.2 ± 0.8 mmol m−2 d−1, and that both small and large particles contributed considerably to the total export flux along the water column. The model results indicated that throughout the upper 500 m, remineralization leads to a larger loss of small POC than does aggregation, whereas disaggregation results in a larger loss of large POC than does remineralization. Of the processes explicitly represented in the model, zooplankton diel vertical migration is a larger source of large POC to the upper mesopelagic zone than the convergence of large POC due to particle sinking. Positive model residuals reveal an even larger unidentified source of large POC in the upper mesopelagic zone. Overall, our posterior estimates of particle cycling rate constants do not deviate much from values reported in the literature, i.e., size-fractionated POC concentration data collected at Station P are largely consistent with prior estimates given their uncertainties. Our budget
This study was supported by the National Aeronautics and Space Administration (NASA) program award 80NSSC17K0555, NSF-OCE 1829614 to PJL, and NSF-OCE-1829790 to OM. VJA was supported by the NSF Graduate Research Fellowship Program and the UC Eugene Cota-Robles Fellowship. MRM was supported by the Ocean Frontier Institute International Postdoctoral Fellowship Program and the Woods Hole Oceanographic Institution’s Ocean Twilight Zone study.