Interactive effects of CO2 and temperature on the dynamics of phytoplankton communities in the subarctic Pacific

Chair: Alistair Hobday

Koji Sugie (1)*, Sohiko Kameyama (2), Takeshi Yoshimura (3), Hiroshi Uchida (1), Jun Nishioka (4), Naomi Harada (1), Akihiko Murata (1)

1 Research & Development Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokosuka, Kanagawa, 237-0061, Japan
2 Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido, 060-0810, Japan
3 Environmental Sciences, Central Research Institute of Electric Power Industry, Abiko, Chiba, 270-1194, Japan
4 Institute of Low Temperature Science, Hokkaido University, Sapporo, Hokkaido, 060-0819, Japan

Background
Increasing atmospheric CO2 from human activities induce the increase in ocean acidity and temperature. These environmental pressures may impact marine organisms in a variety of ways, but the knowledge of these interactive effects on marine phytoplankton are still very limited.

Methods
We conducted CO2 and temperature manipulation experiments in three stations in the subarctic Pacific to investigate the interactive effects of these factors on the dynamics of marine phytoplankton. Stations are located in 41°53’N 145°50’E, 47°01’N 160°01’E, and 47°00’N 148°02’W, and experiments were conducted in April, 2015, August 2014, and August 2014, respectively. Our study could provide many insights into the effects of CO2 and temperature on plankton dynamics in the spatiotemporally different environment.

Findings
Net phytoplankton growth rate based on chlorophyll-a was enhanced by the increase in temperature in four of the six cases, whereas it slightly but often insignificantly decreased by the increase in CO2 levels in the control temperature treatments. Such slight decrease in net growth rate in the high CO2 condition increases temperature sensitivity, which estimated from Q10 in each CO2 conditions. Cyanobacteria growth was enhanced by increasing temperature in two experiments and synergistically enhanced by increasing both temperature and CO2 in another experiment. Net growth rate of pico-sized eukaryotic phytoplankton was enhanced mainly by the increasing CO2 whereas temperature rarely affected their dynamics. Nano-sized eukaryotic phytoplankton growth was not significantly affected by changing both CO2 and temperature.

Conclusions
We found that the increasing Q10 in the high CO2 condition is partly derived from the positive response of pico-sized phytoplankton species. These results suggest that future high-CO2 and temperature conditions could shift phytoplankton community to fast growth rate with small cell size, which negatively affect trophic transfer efficiency and biological carbon pump in the ocean.