Effects of elevated CO2 and temperature on phytoplankton community biomass, species composition and photosynthesis during an experimentally induced autumn bloom in the western English Channel

The combined effects of elevated pCO(2) and temperature were investigated during an experimentally induced autumn phytoplankton bloom in vitro sampled from the western English Channel (WEC). A full factorial 36-day microcosm experiment was conducted under year 2100 predicted temperature (+4.5 degrees C) and pCO(2) levels (800 mu atm). Over the experimental period total phytoplankton biomass was significantly influenced by elevated pCO(2). At the end of the experiment, biomass increased 6.5-fold under elevated pCO(2) and 4.6-fold under elevated temperature relative to the ambient control. By contrast, the combined influence of elevated pCO(2 )and temperature had little effect on biomass relative to the control. Throughout the experiment in all treatments and in the control, the phytoplankton community structure shifted from dinoflagellates to nanophytoplankton. At the end of the experiment, under elevated pCO(2) nanophytoplankton contributed 90 % of community biomass and was dominated by Phaeocystis spp. Under elevated temperature, nanophytoplankton comprised 85 % of the community biomass and was dominated by smaller nanoflagellates. In the control, larger nanoflagellates dominated whilst the smallest nanophytoplankton contribution was observed under combined elevated pCO(2) and temperature (similar to 40 %). Under elevated pCO(2), temperature and in the control there was a significant decrease in dinoflagellate biomass. Under the combined effects of elevated pCO(2) and temperature, dinoflagellate biomass increased and was dominated by the harmful algal bloom (HAB) species, Prorocentrum cordatum. At the end of the experiment, chlorophyll a (Chl a) normalised maximum photosynthetic rates (P-m(B)) increased > 6-fold under elevated pCO(2) and > 3-fold under elevated temperature while no effect on P-m(B) was observed when pCO(2) and temperature were elevated simultaneously. The results suggest that future increases in temperature and pCO(2) simultaneously do not appear to influence coastal phytoplankton productivity but significantly influence community composition during autumn in the WEC.