[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-12] Changes in phytoplankton photophysiology during pre-bloom and bloom phases in the coastal Oyashio waters

*Tomonori Isada¹, Toru Hirawake², Koji Suzuki³, Jun Nishioka⁴, Hiromi Kasai⁵ (1.Field Science Center for Northern Biosphere, Hokkaido University, 2.Faculty of Fisheries Sciences, Hokkaido University, 3.Faculty of Environmental Earth Science, Hokkaido University, 4.Institute of Low Temperature Science, Hokkaido University, 5.Japan fisheries research and education agency)

Keywords:phytoplankton, photosynthetic parameters, spring bloom, Coastal Oyashio

Measurement of phytoplankton biomass and productivity in the ocean serve fundamental information about marine biogeochemical cycles and the trophic status of marine ecosystems. Photosynthesis–irradiance (PE) curve experiment can provide a means of comparing the photosynthetic characteristics of phytoplankton across different natural populations. Here, we investigated changes over time in phytoplankton community structure and their photosynthetic properties in the coastal Oyashio region, NW Pacific, where massive spring phytoplankton blooms occur every year and the blooms contribute to the highest seasonal biological drawdown effect on seawater pCO₂ in the world’s oceans, from March 2015 to January 2016 through HPLC-CHEMTAX pigment analyses and the PE curve experiments with ¹³C tracer technique. Our CHEMTAX outputs showed that diatoms were consistently predominant from the pre-bloom to bloom periods, although chlorophyll (Chl) a concentrations during bloom were significantly higher than those in pre-bloom. We found the E_k-independent variability associated with the parallel changes in the Chl a specific maximum photosynthetic rate (P^B_max) and the initial slope of the PE curve (α^B) in each phase, suggesting the different photoacclimation strategies of phytoplankton assemblages between pre-bloom and bloom phases. These results could be of help for the parameterization of primary production models using satellite ocean color data and of ecosystem models in the coastal Oyashio waters.