Volume 15, issue 10 | Copyright
Biogeosciences, 15, 3203-3222, 2018
https://doi.org/10.5194/bg-15-3203-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 30 May 2018

Research article | 30 May 2018

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

Matthew Keys1,2, Gavin Tilstone1, Helen S. Findlay1, Claire E. Widdicombe1, and Tracy Lawson2 Matthew Keys et al.
  • 1Plymouth Marine Laboratory, Prospect Place, the Hoe, Plymouth, PL1 3DH, UK
  • 2University of Essex, Wivenhoe Park, Colchester, CO4 3SQ, UK

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 (800µatm). 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 (PBm) increased  > 6-fold under elevated pCO2 and  > 3-fold under elevated temperature while no effect on PBm 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.

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We conducted a microcosm experiment on a natural phytoplankton community under year 2100 predicted CO2 concentrations and temperature. Biomass and photosynthetic rates were significantly increased by elevated CO2 and elevated temperature. In contrast, the combined influence of these two factors had little effect. This suggests coastal phytoplankton productivity may not be influenced by future conditions. However, the combined influence promoted the greatest diversity and increased HAB species.
We conducted a microcosm experiment on a natural phytoplankton community under year 2100...
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