Dataset: Lab measurements of the maximum lengths of different phytoplankton chains collected by phytonet tows from BIOSWOT-Med study in the Northwest Mediterranean Sea during May 2023

During BioSWOT-Med, vertical phytonet hauls were performed from the R/V L’Atalante at three locations near water parcels sampled by Zooglider, two south of Menorca (Stas. M, M2) and one in the eddy (Sta. B3, Fig. 1a, Suppl. Table S1). Each haul was from 0-150 meters (m) with a 20-micrometer (µm) mesh net. Net collector contents were diluted to 1 liter (L) with filtered seawater. Two samples (60 milliliters (mL)) were fixed with acidified Lugols solution and buffered formaldehyde for light microscopy observations. One mL of the fresh net material was filtered onboard without fixative at low vacuum onto a 25-millimeter (mm) polycarbonate membrane and rinsed with MilliQ water, dried for 3 hours at 50 degrees Celsius (°C), then kept at room temperature for subsequent SEM analyses. Phytoplankton diversity was assessed by cell counts in light microscopy (Nikon TE200) while SEM samples were gold coated and analyzed on a Phenom ProX benchtop SEM. From the samples, diatom identification and measurements were made of the longest chains visible for each genus or species that was relatively abundant.