Effect of future CO2 and temperature regimes on phytoplankton community composition, biomass and photosynthetic rates in the western English Channel

CO2 storage in the oceans is strongly affected by biological processes. Production of organic matter through phytoplankton photosynthesis drives CO2 sequestration, which feeds back to atmospheric CO2 and global climate. The ongoing increase in atmospheric CO2 and temperature is strongly associated with changes in ocean chemistry and increasing seawater temperatures. To investigate these impacts on coastal phytoplankton under conditions predicted for the year 2100 (pCO2 elevated to 800 µatm and +4 °C temperature), three factorial experiments were conducted with natural communities sampled from the western English Channel (WEC). Elevated pCO2 increased phytoplankton biomass by up to 20-fold while elevated temperature resulted in an increase of up to 14-fold. Light-saturated photosynthetic carbon fixation rates increased > 6-fold under elevated pCO2 while an increase of up to 3-fold resulted from elevated temperature. The combined effects of elevated pCO2 and temperature reduced biomass in late summer and had no effects on biomass in the autumn with no significant effects on photosynthetic carbon fixation rates in either season. Individual treatments of elevated pCO2 and temperature resulted in near mono-specific communities: diatoms in late summer and nanophytoplankton in autumn. Combined effects of both factors resulted in the most diverse phytoplankton communities and promoted increased dinoflagellate and Synechococcus biomass at the expense of diatoms and nanophytoplankton. Elevated pCO2 alone promoted dominance of the harmful algal bloom (HAB) species, Phaeocystis in spring and autumn, while the combination of elevated pCO2 and temperature promoted biomass of the HAB species, Prorocentrum minimum in autumn. The results indicate that experimental simulations of year 2100 pCO2 and temperature may significantly modify phytoplankton community structure with a positive feedback on atmospheric CO2 in late summer and no change on feedback in autumn. In either scenario, no increase in phytoplankton productivity during a period of changes in bulk carbonate chemistry resulting from ongoing anthropogenic carbon uptake, may be expected to negatively influence carbon biogeochemistry in the WEC.