Monthly flow cytometry measurements in a sub-Arctic fjord, Ramfjord in Tromsø, Norway between September 2020 and August 2021

Fjord water was collected on a monthly basis between September 2020 and August 2021 from 5m and 30m depth in a sub-Arctic fjord (Ramfjorden, Tromso, Norway) with a GoFlo (General Oceanics). Flow cytometry was used for the determination of bacteria, virus, pico- and nano-sized phytoplankton abundances. Unfiltered samples of 5 mL were fixed in duplicates with glutaraldehyde (0.5% final concentration) and frozen at -80C until analysis within 3 months. The samples were thawed and pico- and nanophytoplankton were analyzed directly on an Attune Acoustic Focusing Flow Cytometer (Applied Biosystems by Life Technologies). The populations of phytoplankton were grouped based on their pigmentation on biplots of green vs. red fluorescence. Before counting bacteria and viruses, the DNA was stained with SYBR-green I and groups were discriminated on biplots of side scatter vs. green fluorescence. Actively dividing bacterial cells contain more DNA, therefore to determine the ratio of High Nucleic Acid (HNA) bacteria to Low Nucleic Acid (LNA) bacteria as an indicator of the relative activity of the bacterial community, HNA and LNA bacteria were counted separately.