Bio-Argo float data suggest that disaggregation is a major driver of flux attenuation during large phytoplankton blooms in the North Atlantic

Approximately 5-12 Pg of organic carbon is exported from the ocean’s euphotic zone each year. The rate at which this organic carbon flux decreases with depth has critical consequences for both the functioning of deep ocean ecosystems and the timescale of biological CO₂ sequestration. However, it has been difficult to constrain the mechanisms behind this attenuation of organic carbon flux and the variability of these mechanisms in time and space. In situ measurements of one or more individual processes that attenuate flux, therefore, are highly desirable, especially when accompanied by estimates of total flux attenuation at the same temporal and spatial scales. Substantial effort has been made to connect vertical flux attenuation with bacterial and/or zooplankton metabolism. However, the disaggregation of fast-sinking aggregates, another potential mechanism of flux attenuation, has not, to our knowledge, been quantified in the ocean. Here we simultaneously estimate both the loss of sinking phytoplankton aggregates with depth and the production of small particles at depth (possibly a signature disaggregation) using three years of chlorophyll a fluorescence data from 21 autonomous “bio-Argo” profiling floats in the sub-polar North Atlantic. Fluorescence data were divided into “spikes” caused by phytoplankton aggregates and a “baseline” due to smaller particles. During several large spring blooms, we found a clear matchup in depth and time between the attenuation of phytoplankton aggregate flux, calculated from fluorescence spikes, and a rapid increase in small phytoplankton particles well below the productive layer. This pattern is best explained by disaggregation, which appears to cause at least 30-50% of phytoplankton aggregate flux attenuation at the peak of these blooms. These results reveal the importance of disaggregation to oceanic carbon cycling and highlight the potential of bio-Argo to advance understanding of disaggregation globally.