Seasonal variations of phytoplankton primary production under different CO2 concentration conditions

In the present study, the effects of CO2 enrichment (360 ppm, 600 ppm, 760 ppm) and temperature changes (ambient and above +3ºC) on phytoplankton primary production and community structure were investigated for the first time in natural phytoplankton samples with seasonal incubation experiments between November 2021 and November 2023 in the south-eastern Black Sea ecosystem. In addition, size fractionated primary production (picophytoplankton, nanophytoplankton and microphytoplankton) was determined by C-14 incubation experiments. This revealed the contribution of different size classes to primary production on a seasonal scale, and how the rate of primary production varies with both phytoplankton biomass and changes in CO2 and temperature. Seasonal samplings and incubation experiments were carried out at 2 different stations (coastal and offshore) at 10 m depth. During the project period, total primary production ranged from 41 to 8553 mgCm-2d-1. In terms of phytoplankton size classes, microphytoplankton was the prominent group with the highest primary production (6-6410 mgCm-2d-1), followed by nanophytoplankton (9-3552 mgCm-2d-1) and picophytoplankton (1-946 mgCm-2d-1). The experiments showed that the amount of primary production increased with increasing CO2 concentration, and when the temperature-CO2 relationship was examined, it was found that there was a synergistic effect on primary production rates. Among the phytoplankton size classes calculated based on pigment, microphytoplankton (9-93%) was dominant in the region, followed by nanophytoplankton (1-77%) and picophytoplankton (3-75%). Microscopic cell counts revealed a total of 122 phytoplankton species in the stations. Of the identified species, 66 were Dinophyceae species, 50 were Bacillariophyceae species and 6 were other taxonomic groups. The results of this study showed how the primary production rates of phytoplankton size classes, which form the basis of the food chain, will change in the face of projected climate scenarios and contributed to the production of future projections of the productivity of the Black Sea ecosystem, which has a special place in the world's oceans.