CO₂ effects on taxonomic composition and nutrient utilization in an Equatorial Pacific phytoplankton assemblage

We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO₂ levels of 150 and 750 ppm (dissolved CO₂ ∼3 to 25 μM). By the end of the experiment, the phytoplankton community in all samples was dominated by diatoms and Phaeocystis sp. However, the relative abundance of these phytoplankton taxa differed significantly between CO₂ treatments. Taxonomic pigment analysis and direct microscopic examination of samples revealed that the abundance of diatoms decreased by ∼50% at low CO₂ relative to high CO₂, while the abundance of Phaeocystis increased by ∼60% at low CO₂. This CO₂-dependent shift was associated with a significant change in nutrient utilization, with higher ratios of nitrate:silicate (N:Si) and nitrate:phosphate (N:P) consumption by phytoplankton in the low CO₂ treatment. Despite the significant changes in taxonomic composition and nutrient consumption ratios, total biomass and primary productivity did not differ significantly between the CO₂ treatments. Our results suggest that CO₂ concentrations could potentially influence competition among marine phytoplankton taxa and affect oceanic nutrient cycling.