The NBST carried 4 collection tubes with a diameter of 12 cm (Valdes and Price 2000). The STST included 5 frames (KC Denmark) clipped onto a surface-tethered, free drifting array line at increasing depths and each frame carried 4, 7 cm diameter collection tubes. The WW trap consisted of one, 4-tube trap frame (KC Denmark) tethered by a bungee below the profiling component of the WW array.  To prepare tubes for deployment, seawater was collected from a depth of 150 m using a CTD rosette and pumped through a 1 μm filter cartridge. Trap tubes were filled with filtered water overlying a jar containing a polyacrylamide gel layer (Durkin et al. 2015). Trap platforms were deployed for between 1 day and 3.5 days (see Trap Deployment Log).  Identically prepared tubes were incubated in parallel onboard the ship to serve as process blanks.

Upon recovery, collection tubes were allowed to settle for at least 1 hour before the overlying water was siphoned off.  Jars containing polyacrylamide gel were removed from trap tubes and the remaining overlying water was carefully pipetted off the gel.  Gels were stored at 4 degrees C and imaged within the following 2 days before being stored at -80 degrees C. 

Polyacrylamide gel layers were imaged on a dissecting microscope (Olympus SZX16) with either a Luminera Infinity 2 (FK170124) or an Allied Vision Technologies StingRay (EN572 and EN581) camera attachment.  Particles collected in gel layers during EN572 and EN581 were imaged under brightfield illumination.  Particles collected in gel layers during FK170124 were imaged under both brightfield and oblique illumination, producing two separate sets of images for each sample.  EN572 gel layers were imaged with a transparent grid to assist in tracking gel location during imaging. The grid was not used when imaging samples collected during subsequent cruises because the pronounced grid lines complicated image analysis. All gel layers were imaged at 4 increasing magnifications, though the combination of magnifications varied by cruise: at 7x, 20x, 40x, and 115x for EN572 samples, at 7x, 20x, 40x, and 80x for EN581 samples, and at 7x, 20x, 50x, and 115x for FK170124 samples. At magnifications greater than 7x, multiple focal planes within a field of view were imaged to capture particles embedded in different depths of the gel layer.  The number of focal planes imaged was consistent across all fields of view for a given magnification but varied across cruises due to variation in gel thickness and particle types present. To determine whether measured particle properties changed if gel layers are frozen, samples collected during FK170124 were thawed after being stored for approximately 1 year at -80  degrees C and imaged again under both brightfield and oblique illumination.