Seasonal dynamics and ecology of the Pseudo-nitzschia delicatissima group in the Black Sea

Abstract

Seasonal dynamics, including the Pseudo-nitzschia delicatissima (PDG) group's vertical and spatial distribution, were analysed based on field data collected in the Black Sea from 1948 to 2020. The main annual bloom occurred in February-March when the abundance in the upper mixed layer could reach 5 × 106 cells m−3. Bloom was most intense in the centre of the sea and on the outer shelf waters. In April, the bloom subsided, and in summer, only small peaks could be found in the deep-sea waters. On the contrary, high cell abundance was observed in the shelf areas in summer and autumn. Three periods with different levels of biomass were distinguished in the centre of the deep-water basin: highest (February-March), moderate (April-June), and lowest (July-December). Nutrient levels varied during these three periods, contributing to different growth-promoting phases. Six experiments with natural phytoplankton conducted in April-June showed that dissolved nitrate (N) and phosphate (P) were growth-limiting factors for PDG. Very low dissolved silicate (DSi) in October likely limited growth, as evidenced by the algae's lack of response to N and P additions. In February-March, dissolved inorganic nutrient concentrations were highest, with dissolved inorganic nitrogen (DIN), P, and DSi above 2.0 µM, 0.2 µM, and 2.5 µM, respectively. During the moderate growth period, DIN (˂ 1.2 µM) and P (˂ 0.15 µM) were expected to significantly limit the growth of PDG, while DSi was too high (> 3.5 µM) to have such an effect. In the second half of the year, DSi fell below 2.0 µM, which could severely inhibit cell growth in addition to DIN and P. Thus, after June, low DIN, P, and DSi combined resulted in the lowest PDG biomass.