Chlorophyll a and nutrients in water during the PHYCOB cruise in the Southwestern Black Sea in 2021

Samples for biological and chemical parameters were collected at the surface (3m), pyrocline, and the deep chlorophyll a maximum using the CTD (Conductivity, Temperature, and Depth) rosette outfitted with Niskin bottles. Nitrite and nitrate (NO3-+NO2-) referred to as Nox, ortho-phosphate as phosphorus (o-PO43-) and silica (SiO2) were analysed using a QuAAtro39 Continuous Segmented Flow Analyzer. Size fractionated chlorophyll a was collected by filtering water through 0.2 µm and 2 µm polycarbonate filters and frozen on board to be analysed later in the lab using standard fluorometric techniques. Duplicate particulate organic carbon and nitrogen (POC and PON) samples were collected on pre- combusted GF/F glass fiber filters, and stored frozen at -20°C until subsequent analysis on anelemental analyser in the lab. Whole water samples were preserved with 10% buffered formalin (1 % v/v final) and analyzed by flow cytometry to assess picoplankton densities. Abundance of heterotrophic bacteria (stained with SYBR Green I), phycoerythrin-containing picocyanobacteria, and photosynthetic picoeukaryotes were quantified using a Beckson Dickson Accuri C6 Flowcytometer using fluorescence patterns and particle size from side angle light scatter.