Diverse but uncertain responses of picophytoplankton lineages to future climate change

Climate change is projected to modify the physical and chemical environment of the ocean, but the quantitative impact on the distribution of phytoplankton groups is unclear. Most Earth System Models (ESMs) predict future declines of phytoplankton in low latitude waters, contradicting observations showing that picophytoplankton can reach high abundance in warm waters. Here, we used a historic and three climate scenarios along with quantitative niche models to project Prochlorococcus, Synechococcus, and picoeukaryotic phytoplankton distributions for the year 2100. First, we found global responses with up to 50% and 9% increase for Prochlorococcus and Synechococcus abundances, respectively, and 8% decrease for picoeukaryotic phytoplankton. All groups increased in abundance at low latitude, and Synechococcus and picoeukaryotic phytoplankton showed bands of decreases and increases in mid- and high-latitudes, respectively. Prochlorococcus temporal trends were consistent among ESMs and increased with the strength of the scenario, while Synechococcus and picoeukaryotic phytoplankton showed mixed results. Second, we evaluated sources of uncertainty associated to future projections. The anthropogenic uncertainty, associated to climate scenarios, increased with time and was relevant for Prochlorococcus. The environmental and biological uncertainty, associated to ESMs and niche models, respectively, represented the largest fraction but differed among lineages. Quantifying uncertainties is key because the predicted differences in the future distribution and abundance can have large-scale consequences on ocean ecosystem functioning.