Time‐Varying Cycling Rates of Size‐fractionated Particulate Organic Carbon During the Decay of a North Atlantic Spring Bloom

Abstract The concentrations of particulate organic carbon (POC) in the ocean are the result of a dynamic balance between multiple sources and sinks. Here, measurements of POC concentration in the small size fraction (SSF; 1–5 ) and the large size fraction (LSF; ) from the North Atlantic EXport Processes in the Ocean from RemoTe Sensing (EXPORTS) cruise in the Porcupine Basin are combined with a non‐steady state POC cycling model to estimate the temporal variations in rates of POC production, (dis)aggregation, sinking, remineralization, and POC ingestion/egestion by migrating zooplankton over the decline of a phytoplankton bloom. We infer coherent variations in POC cycling rates throughout the water column, with distinct temporal variability. Throughout the upper 500 m, disaggregation exerts a major control on both size fractions. Near the surface, disaggregation rates increase with time and are the major source of POC in the SSF. Aggregation strongly influences surface POC in both size fractions, although it becomes negligible with depth. POC is supplied to the LSF between 175 and 500 m by migrating zooplankton, which contribute between 15% and 100% of the total LSF source in this depth interval. Our results highlight a need to improve estimates of POC sinking fluxes, stressing differences in multiple approaches. In general, we quantify the intensity of particle cycling processes that are hard to observe and identify key mechanisms that lead to the observed temporal and vertical variations in POC concentrations.