Particle size distribution (PSD) measurements were made in June 2019 at Station ALOHA (22.75°N, 158°W) in the North Pacific Subtropical Gyre aboard the R/V Kilo Moana (KM1910). PDSs were measured using the Laser In Situ Scatterometer and Transmissometer 100X (Type B, Sequoia Scientific Inc., hereafter LISST) which uses laser diffraction in an approximately 100 ml sample volume to estimate the PSD via inversion into 32 logarithmically spaced particle size classes.
Sinking PSDs were measured from material captured in sediment traps. The standard Hawaii Ocean Time-series (HOT, https://hahana.soest.hawaii.edu/hot/) traps and sampling protocol were used, which includes a 335 µm prefilter to remove zooplankton swimmers. Two sediment trap deployments were sampled: one 81 hr deployment beginning 16-June-2019 and one 73 hr deployment beginning 21-June-2019. For each, trap arrays were deployed at 75, 150, and 300 meter depths. Each array had 12 traps at each depth. On the second deployment one trap per depth was capped and used as a blank so that there were a total of 69 trap measurements (two deployments and three depths, 11 or 12 traps each). After recovery, from each trap a 125 ml split of the trap solution was taken for LISST and IFCB processing. Sinking PSDs were then measured from the LISST in discrete chamber mode. Twenty diffraction measurements were made from each sample before draining each sample back into the 125 ml split bottle; this process was repeated in triplicate, totaling 60 LISST measurements per sediment trap. Trap solution was used as a blank for all LISST measurements.
Addtional details on sampling strategy and procedures can be found in Cael & White (2020).
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