On the assessment of sinking particle fluxes from in situ particle size distributions

The biological carbon pump is a vital component of the global carbon cycle, particularly through sinking of particulate organic carbon (POC) to the ocean interior. Particle size distribution (PSD) observations from the Underwater Vision Profiler (UVP) have been widely used to quantify sinking POC fluxes. This approach assumes that the sinking POC flux is a function of the PSD multiplied by a power law relating particle size to sinking rates and carbon content. The coefficients of the power law are quantified by regressing UVP data against sediment trap flux observations. Here, we systematically assess the performance of this approach using a large UVP dataset of co-located and coincident sediment trap and thorium-234 flux observations from the North Pacific (50°N, 145°W, August 2018) and the North Atlantic (49°N, 16.5°W, May 2021) sampled during the EXPORTS (EXport Processes in the Ocean from RemoTe Sensing) field campaign, which span both diverse environmental conditions and sinking flux values. Globally, when power law coefficients are evaluated over all sites and depths, the UVP flux method explains 80% of the variance in POC flux. However, when coefficients are determined using regional subsets of the EXPORTS dataset, the method performs poorly. Reasons include lack of knowledge of particle characteristics beyond PSD; undersampling of rare large particles; spatial and temporal scales mismatches between UVPs and flux observations; and difficulties arising from non-steady state conditions. To improve UVP-based sinking POC flux estimates regionally, additional data on particle characteristics such as transparency and morphology are needed.