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

Abstract

We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO 2 levels of 150 and 750 ppm (dissolved CO 2 ~3 to 25 M). 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 CO 2 treatments. Taxonomic pigment analysis and direct microscopic examination of samples revealed that the abundance of diatoms decreased by ~50% at low CO 2 relative to high CO 2 , while the abundance of Phaeocystis increased by ~60% at low CO 2 . This CO 2 -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 CO 2 treatment. Despite the significant changes in taxonomic composition and nutrient consumption ratios, total biomass and primary productivity did not differ significantly between the CO 2 treatments. Our results suggest that CO 2 concentrations could potentially influence competition among marine phytoplankton taxa and affect oceanic nutrient cycling.