For size fractionated suspended particulate organic matter, on R/V Atlantic cruise AT42-09 in April 2019, samples were obtained from Niskin bottles but were collected by opening the bottom of the Niskin bottle into an acid cleaned bucket. This ensured that particles that had sunk below the spigot of the Niskin bottle were included. Between 100 and 120 L of water was gravity filtered, in sequence, through nylon mesh (142 mm diameter) of decreasing pore size (500, 180, 53, 20 µm) and a subset of this 20 µm filtered water (~20 L) was then filtered through a 5 µm mesh. For mesh sizes 20 µm and above, the large diameter of the mesh and abundance of functional pore-space prevented clogging, and water flowed through the mesh quickly, indicating that clogging did not occur. Water filtered more slowly through the 5 µm mesh (on the order of 30 minutes). If flow decreased substantially, the mesh was back-rinsed (see below) before additional water was passed through the mesh. After filtration, each nylon mesh was back rinsed with ~500 ml of prefiltered “rinse water” to produce a resuspension of particulate matter from particles from each size class. The “rinse water” had been generated during transit by pumping surface water in sequence through water filters of size 10, 5, 1 µm to remove particles, followed by a 0.2 µm filter (Pall AcroPak 1500 Capsule with a Supor Polyethersulfone membrane) capsule which removes bacteria. After back-rinsing, the resuspended particles were split with one half used for particulate matter measurements. In all cases the actual volumes were carefully recorded and used for normalization during analysis. The resuspended particulate matter from each sample and size class was collected by vacuum filtration through a 1.2 µm nominal pore size, 25 mm diameter, GF/C glass fiber filter (Whatman WHA1822025). These filters had been previously pre-combusted for at least two hours at 400°C.  Two depths were sampled per day and multiple days were combined to represent each station with seven depths sampled total with 92 m sampled twice on different days (April 6 and April 9). Samples were frozen after collection and shipped frozen. 

At Horn Point Laboratory, samples were wafted with HCl overnight to remove carbonate, dried at 40°C, packed in both silver and tin capsules, and sent to the UC Davis Stable Isotope Facility for C and N analysis utilizing an Elemental Analyzer (Elementar Vario EL Cube) attached to an Isotope Ratio Mass Spectrometer (Isoprim VisION). Blank combusted GF/C filters were included in analyses and did not show measurable material.