HOE_Legacy2A-KM1513-Diazotroph Abundance_2021-05-01_v1.0

This dataset comes from sampling performed during the Hawaii Ocean Experiment - Legacy 2A cruise aboard the R/V Kilo Moana (cruise KM1513) in the north Pacific subtropical gyre. At select stations and depths (ranging from 5 to 45m), the entire volume of a Niskin bottle was drained directly into clean ~10L carboys fitted with spigots. Carboys were covered in dark cloth and the volume was gravity filtered through a 47-mm diameter, 10 micron pore size, black polycarbonate filter with a polyester drain disk as a backing filter. If the volume was not filtered after 2 hours, filtration was terminated and the remaining volume of the carboy was measured in order to calculate volume filtered.  Following filtration, filter holders were fit with a short section of tubing and a syringe leur fitting on one side and a 2-way valve on the outflow side. For each filter, 5-ml of 2% glutaraldehyde was slowly injected onto the filter and samples were allowed to fix for 30 minutes. Fixative was drained after this time and 60ml of air was used to flush all filters. Polycarbonate filters were then mounted onto 3x2” glass slides with immersion oil, cover slides were added and the edges of each cover slip was sealed with quick dry nail polish. All slides were stored at -20°C and counted within 30 days. Enumeration of diazotrophic taxa was performed using epifluorescence microscopy. Random areas were counted until a minimum of 1000 cells were enumerated, generally this amounted to roughly 20% of the slide area for DDAs. The entire slide was counted for Trichodesimum abundance. Endosymbiont bearing diatoms of the following genus were enumerated: Rhizosolenia, Hemialus, Climacodium and Chaetoceras. Free Richelia intracellularis were also counted. Trichodesmium filaments were counted and the length of each filament was recorded. Trichodesmium cell number was then calculated by dividing the filament length by the mean cell length (12 micron ±2 micron).