Description: We conducted an experiment with large volume in situ mesocosms (~55–60 m3 and 21 m depth) in Raunefjord (Bergen), Norway in 2015 to investigate how ocean acidification (OA) extreme events affect food web structure and carbon export in a natural plankton community, particularly focusing on the keystone species Oikopleura dioica, a globally abundant appendicularian.

Description: In the autumn of 2014, nine large mesocosms were deployed in the oligotrophic subtropical North-Atlantic coastal waters off Gran Canaria (Spain). Their deployment was designed to address the acidification effects of CO2 levels from 400 to 1,400 μatm, on a plankton community experiencing upwelling of nutrient-rich deep water. Among other parameters, chlorophyll a (chl-a), potential respiration (PHi), and biomass in terms of particulate protein (B) were measured in the microplankton community (0.7–50.0 μm) during an oligotrophic phase (Phase I), a phytoplankton-bloom phase (Phase II), and a post-bloom phase (Phase III). Here, we explore the use of the PHi/chl-a ratio in monitoring shifts in the microplankton community composition and its metabolism. PHi/chl-a values below 2.5 μL O2/h/ (μg chl-a) indicated a community dominated by photoautotrophs. When PHi/chl-a ranged higher, between 2.5 and 7.0 μL O2/h/ (μg chl-a), it indicated a mixed community of phytoplankton, microzooplankton and heterotrophic prokaryotes. When PHi/chl-a rose above 7.0 μL O2/h/ (μg chl-a), it indicated a community where microzooplankton proliferated (〉10.0 μL O2/h/ (μg chl-a)), because heterotrophic dinoflagellates bloomed. The first derivative of B, as a function of time (dB/dt), indicates the rate of protein build-up when positive and the rate of protein loss, when negative. It revealed that the maximum increase in particulate protein (biomass) occurred between 1 and 2 days before the chl-a peak. A day after this peak, the trough revealed the maximum net biomass loss. This analysis did not detect significant changes in particulate protein, neither in Phase I nor in Phase III. Integral analysis of PHi, chl-a and B, over the duration of each phase, for each mesocosm, reflected a positive relationship between PHi and pCO2 during Phase II [alpha = 230*10−5 μL O2/h/L/(μatm CO2)/(phase-day), R2 = 0.30] and between chl-a and pCO2 during Phase III [alpha= 100*10−5 μg chl-a/L/ (μ atmCO2)/ (phase-day), R2 = 0.84]. At the end of Phase II, a harmful algal species (HAS), Vicicitus globosus, bloomed in the high pCO2 mesocosms. In these mesocosms, microzooplankton did not proliferate, and chl-a retention time in the water column increased. In these V. globosus-disrupted communities, the PHi/chl-a ratio [4.1 +- 1.5 μL O2/h/(μg chl-a)] was more similar to the PHi/chl-a ratio in a mixed plankton community than to a photoautotroph-dominated one.