Dataset: Upper-pelagic particle numbers from imagery in the Sargasso Sea and in the Gulf of Trieste

This dataset represents Log10-particle numbers per volume versus Log10-particle size bins at various threshold levels of the image analysis program taken between 4 and 7-meter depth in the Sargasso Sea and the Gulf of Trieste on July 18, 2021.

In-situ imaging
The basic configuration of our system is the same as in previous shadowgraph cameras (Arnold and Nuttall-Smith, 1974; Cowen and Guigand, 2008; Ohman et al., 2019), except for the direct inline configuration without mirrors and smaller spatial scales of our system [image field: 15.36 millimeter (mm) x 11.52 mm, 1280 x 960 pixels, image volume: 5.3 milliliters (ml)]. The light source was a red LED (625 nanometers, Cree XLamp) collimated by a 150 mm focal-length plano-convex lens. The light then passes in sequence, through a 25.4 mm sapphire window, 30 mm of seawater, and another 25.4 mm thick sapphire window, a 100 mm plano-convex lens, before being collected by a 1/3” monochrome CMOS chip with a global shutter (Imaging Source, LLC) and equipped with a 25 mm board camera lens (f/2.5, V-4325, Marshall Electronics). In this telecentric setup, blur at the far edges of the image path is symmetric, and the center of mass is retained so that the edge of the particle is rendered relatively accurately even if it is slightly out of focus (Watanabe and Nayar, 1997; Lange, 2022).  Images were recorded by a mini-PC on a 1 TB micro-SD card. For the conductivity, temperature, and depth (CTD) rosette casts in the Sargasso Sea, the optical setup and the electronics were enclosed in a stainless-steel housing rated to 6000 meters. For the shallow deployments in the Gulf of Trieste, optics and electronics were enclosed in a lighter polyvinyl chloride (PVC) housing and still equipped with 25.4 mm sapphire windows to retain the same optical configuration as the deep-sea version. The lower practical particle size cut-off in this analysis was 43 mm, which is equivalent to approximately 4 pixels of linear dimension.  

For images from the Sargasso Sea, the camera was mounted on the lower ring of the CTD rosette deployed during the Oceanic Flux Program (Conte et al., 2001). Images (n = 45,512) of the surface layer (0-100 m) were taken at 1-second intervals during 11 casts (both down- and upcasts) from April 14 to April 21, at 31.0 - 32.5 N Latitude and 63.0 - 64.3 W Longitude.

Images in the Gulf of Trieste (n = 2,125; 45° 31.56’ N, 13° 35.41 E) were recorded by a SCUBA diver on July 18, 2021, below the first thermocline at depths between 4 and 7 meters for 36 minutes with a frame rate of 1 image per second. A stage micrometer (1 mm total, 0.01 mm increments) and stepped neutral density filters on a microscope slide (11 discrete density steps from OD = 0.04 to 1.0, design wavelengths 400 to 700 nanometers, Edmund Optics) were recorded in pure water for calibration.