Dataset: Zooplankton abundance and size data from Bongo plankton net tows conducted in the subtropical Southern California Bight during February and April 2021

This study was carried out within the temperate to subtropical Southern California Bight and surrounding coastal areas between 32° and 35° N, and 117° and 120° W. Specific locations sampled fall within the CalCOFI and CCE-LTER programs' quarterly survey grid from lines 76.7 to 93.4 and stations 26 to 55.

Sampling took place during February 2021 and April 2021 on R/V Bob and Betty Beyster cruises BBB2102 and BBB2104 and on R/V Shearwater cruise SW2104. Each cruise was composed of one or more day trips using one of the two research vessels. The two-three letters in the cruise ID number indicate the research vessel and the four digits indicate YYMM.

Zooplankton were collected using a 71-centimeter (cm) diameter dual Bongo plankton net system equipped with a small CTD to record depth and physical variables during horizontal net tows at a ship speed of 1.5-2.5 knots down to 30-33 meters (m) depth. The Bongo was equipped with two nets, one with a 53-micrometer (µm) and the other with a 202 µm mesh size, and a flowmeter mounted in the center of each of the two nets.

Once the nets were retrieved, they were gently rinsed with saltwater. The content of the cod-ends was then concentrated onto a sieve of 50 µm mesh size, transferred into glass jars, and preserved in buffered 3.7% formaldehyde. The 53 µm samples were split using a 10-milliliter (mL) Hensen Stempel Pipette and sorted under a dissecting microscope. Zooplankton were identified based on morphological characteristics and measured using an eyepiece micrometer. The 202 µm samples were size fractioned into two replicate small (202 - 999 µm), one large (1000 - 4999 µm), and one extra large (> 5000 µm) size fractions, split into appropriately sized subsamples using a Folsom splitter, and analyzed by ZooScan. The organism's images were then identified manually and sized using ImageJ. In most identified taxonomic groups, up to 30 individuals were size-measured and a target of 50 individuals were counted per sample. Length measurements represent the total body length for all organisms except copepodites where prosome length was measured instead. Width measurements were taken at the widest part of the body.

Known problems or issues: Unicellular organisms such as dinoflagellates, diatoms, and tintinnids are underrepresented in the samples due to their small size and fragile structure.