Climate-driven restructuring of phytoplankton productivity and community composition in the south-eastern black sea: Insights from seasonal CO2-Temperature manipulation experiments.

Semi-enclosed marine systems with low buffering capacity, such as the Black Sea, are expected to experience amplified impacts of ocean acidification and warming, yet experimental evidence on their combined short-term effects on natural phytoplankton assemblages remains limited. Here, we present a seasonally resolved one-year study (four experiments conducted between 2022 and 2023) based on 48 h short-term microcosm incubation experiments using natural phytoplankton communities collected from coastal and offshore stations in the south-eastern Black Sea. CO2 concentrations (360, 600, and 760 ppm) and temperature (ambient and +3 °C) were manipulated to examine short-term physiological and compositional responses under projected climate scenarios. We hypothesised that CO2 and warming would exert both independent and interactive effects on short-term particulate organic carbon production (14C uptake rates) and relative community composition, with responses varying seasonally and being most pronounced during summer stratification. Short-term particulate primary production increased by ∼22% and ∼36% at 600 and 760 ppm CO2, respectively (p<0.05), while warming provided an additional 14-22% enhancement depending on season, with significant CO2 × temperature interaction terms detected for total production (two-way ANOVA, p<0.05), indicating synergistic CO2-temperature effects. Warming and moderate CO2 enrichment were associated with increased relative contributions of nano- and picophytoplankton (by ∼6-10%), whereas high CO2 reduced the warming-driven shift toward smaller cells by maintaining microphytoplankton contributions ∼10-15% higher than in the warming-only treatment. Carbonate chemistry responded strongly to CO2 manipulation, with pH declining from in-situ values of 8.09-8.21 to 7.06-7.52 during incubations and minor reductions in total alkalinity, reflecting the weak buffering capacity of the system. Pigment composition and microscopy indicated short-term increases in dinoflagellate relative abundance (∼12-18%) and concurrent declines in diatom markers, accompanied by accelerated nitrate depletion and reduced nitrogen-to-phosphorus (N:P) ratios, consistent with enhanced nitrogen limitation. Overall, these findings demonstrate pronounced short-term sensitivity of natural phytoplankton assemblages in the south-eastern Black Sea to combined CO2 and warming under controlled incubation conditions. Because these results derive from 48 h microcosm experiments, they represent short-term physiological and compositional responses rather than direct evidence of long-term ecosystem restructuring, yet the observed patterns suggest potential implications for trophic efficiency, harmful algal bloom development, and carbon cycling in this low-buffer, stratified basin under future climate forcing.