Abstract
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.
Highlights
Oceanic concentration of CO2 has increased by ∼ 42 % over pre-industrial levels, with a continuing annual increase of ∼ 0.4 %
chlorophyll a (Chl a) concentration in the western English Channel (WEC) at station L4 from 30 September to 6 October 2015 varied between 0.02 and 5 mg m−3, with a mean concentration of ∼ 1.6 mg m−3 (Fig. 1a)
These treatments were slowly acclimated to increasing levels of pCO2 over 7 days, while the control and hightemperature treatments were acclimated at the same ambient carbonate system values as those measured at station L4 on the day of sampling
Summary
Oceanic concentration of CO2 has increased by ∼ 42 % over pre-industrial levels, with a continuing annual increase of ∼ 0.4 %. The projected increase in atmospheric CO2 and corresponding increase in ocean uptake is predicted to result in a decrease in global mean surface seawater pH of 0.3 units below the present value of 8.1 to 7.8 (Wolf-Gladrow et al, 1999). Under this scenario, the shift in dissolved inorganic carbon (DIC) equilibria has wide ranging implications for phytoplankton. Keys et al.: Effects of elevated CO2 and temperature on phytoplankton community biomass photosynthetic carbon fixation rates and growth (Riebesell, 2004)
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