Abstract
We studied the effect of ocean acidification (OA) on a coastal North Sea plankton community in a long-term mesocosm CO2-enrichment experiment (BIOACID II long-term mesocosm study). From March to July 2013, 10 mesocosms of 19 m length with a volume of 47.5 to 55.9 m3 were deployed in the Gullmar Fjord, Sweden. CO2 concentrations were enriched in five mesocosms to reach average CO2 partial pressures (pCO2) of 760 μatm. The remaining five mesocosms were used as control at ambient pCO2 of 380 μatm. Our paper is part of a PLOS collection on this long-term mesocosm experiment. Here, we here tested the effect of OA on total primary production (PPT) by performing 14C-based bottle incubations for 24 h. Furthermore, photoacclimation was assessed by conducting 14C-based photosynthesis-irradiance response (P/I) curves. Changes in chlorophyll a concentrations over time were reflected in the development of PPT, and showed higher phytoplankton biomass build-up under OA. We observed two subsequent phytoplankton blooms in all mesocosms, with peaks in PPT around day 33 and day 56. OA had no significant effect on PPT, except for a marginal increase during the second phytoplankton bloom when inorganic nutrients were already depleted. Maximum light use efficiencies and light saturation indices calculated from the P/I curves changed simultaneously in all mesocosms, and suggest that OA did not alter phytoplankton photoacclimation. Despite large variability in time-integrated productivity estimates among replicates, our overall results indicate that coastal phytoplankton communities can be affected by OA at certain times of the seasonal succession with potential consequences for ecosystem functioning.
Highlights
Atmospheric CO2 partial pressure is currently rising at an unprecedented rate due to anthropogenic activities
primary production (PPT) was higher in the fjord during phase I, while PPT was higher in the mesocosms during phase II
PPT amounted to highest values of 11.3 ± 5.0 μmol C L-1 d-1 at t53 for high pCO2, and 6.0 ± 1.9 μmol C L-1 d-1 at day t57 for low pCO2 At the peak of the second bloom, PPT appeared to be higher at high pCO2, though this difference was marginally significant and dependent on time (Table 2; repeated measures Analysis of Variance (rmANOVA), Time x CO2 treatment, P = 0.098)
Summary
Atmospheric CO2 partial pressure (pCO2) is currently rising at an unprecedented rate due to anthropogenic activities This leads to enhanced CO2 uptake by the oceans and a decrease in PLOS ONE | DOI:10.1371/journal.pone.0172594. [17,18,19]), which further complicates predictions on OA-driven changes in primary production To test these effects directly, numerous studies have exposed natural phytoplankton communities to high pCO2, either in bottle incubations or mesocosms, often finding higher rates of CO2 fixation under OA [20]. In these experiments, which lasted only a couple of days up to a month, the effects were yet relatively small. We assessed the light dependency of CO2 fixation by performing photosynthesis-irradiance response curves in short incubations (80 min.)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
