Climate-driven phytoplankton community shifts in the North Pacific Subtropical Gyre

Natural climate variability influences phytoplankton community both directly and indirectly by altering ocean stratification and availabilities of nutrient and light, or grazing pressure. The world’s largest ecosystem, North Pacific Subtropical Gyre (NPSG), is largely controlled by basin-scale decadal climate variability, such as the North Pacific Gyre Oscillation and the Pacific Decadal Oscillation. These indices have two phases, known as warm phase and cool phase, respectively. Previous studies reported that warm phase was related to the dominance of pico-phytoplankton induced by warm temperature anomaly (i.e., strong stratification) while cool phase was related to the dominance of nano-phytoplankton induced by cold temperature anomaly (i.e., weak stratification). Besides the impact of natural climate variability, anthropogenic global warming has accelerated in recent years and it might have abnormal impact on marine ecosystems. However, there is little information about the responses of phytoplankton community to recent climate change in the NPSG. Here, we present the temporal variations of deseasonalized and normalized NPSG phytoplankton community using phytoplankton pigment concentrations and cell densities, obtained on monthly intervals over the period 1988−2018 at Station ALOHA (22°45’N, 158°W). These variations were compared with the variations of climate indices, physical, and biogeochemical parameters from Station ALOHA. The NPSG climate indices showed five phase transitions; warm (~1997) – cool (1998−2002) – warm (2003−2006) – cool (2007−2013) – warm (2014~). Before 2006 year, the phase transitions of phytoplankton community (pico→nano→pico) were coincident with physical factors (e.g., stratification; strong→weak→strong) and biogeochemical factors (e.g., particle export; low→high→low), coupling with phases of climate indices. However, interestingly, following the recent rapid rise in greenhouse gas emission (since 2007), phytoplankton community, even under continued coupling of climate indices and physical factors, showed only dominance of pico-phytoplankton, decoupling with the phases of climate indices. These findings suggest that the contribution of pico-sized plankton to NPSG phytoplankton community will increase gradually in response to the acceleration of the global warming.