Abstract. The combined effects of elevated pCO2 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 ∘C)
and pCO2 levels (800 µatm). Over the
experimental period total phytoplankton biomass was significantly influenced
by elevated pCO2. At the end of the experiment, biomass increased
6.5-fold under elevated pCO2 and 4.6-fold under elevated temperature
relative to the ambient control. By contrast, the combined influence of
elevated pCO2 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 pCO2
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
pCO2 and temperature (∼ 40 %). Under elevated
pCO2, temperature and in the control there was a significant decrease
in dinoflagellate biomass. Under the combined effects of elevated pCO2
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 (PmB) increased
> 6-fold under elevated pCO2 and > 3-fold under
elevated temperature while no effect on PmB was observed when
pCO2 and temperature were elevated simultaneously. The results suggest
that future increases in temperature and pCO2 simultaneously do not
appear to influence coastal phytoplankton productivity but significantly
influence community composition during autumn in the WEC.