Testing the direct effect of CO₂ concentration on a bloom of the coccolithophorid Emiliania huxleyi in mesocosm experiments

Abstract

494Engel et al. AbstractWe studied the direct effects of CO 2 and related changes in seawater carbonate chemistry on marine planktonic organisms in a mesocosm experiment. In nine outdoor enclosures (ϳ11 m 3 each), the partial pressure of CO 2 (pCO 2 ) in the seawater was modified by an aeration system. The triplicate mesocosm treatments represented low (ϳ190 parts per million by volume (ppmV) CO 2 ), present (ϳ410 ppmV CO 2 ), and high (ϳ710 ppmV CO 2 ) pCO 2 conditions. After initial fertilization with nitrate and phosphate a bloom dominated by the coccolithophorid Emiliania huxleyi occurred simultaneously in all of the nine mesocosms; it was monitored over a 19-day period. The three CO 2 treatments assimilated nitrate and phosphate similarly. The concentration of particulate constituents was highly variable among the replicate mesocosms, disguising direct CO 2 -related effects. Normalization of production rates within each treatment, however, indicated that the net specific growth rate of E. huxleyi, the rate of calcification per cell, and the elemental stoichiometry of uptake and production processes were sensitive to changes in pCO 2 . This broad influence of CO 2 on the E. huxleyi bloom suggests that changes in CO 2 concentration directly affect cell physiology with likely effects on the marine biogeochemistry.

A data–model synthesis to explain variability in calcification observed during a CO<sub>2</sub> perturbation mesocosm experiment

“…The initial abundance of picoplankton (mainly Micromonas spp.) and their decline was observed during the early pre-bloom period of the PeECE-I experiment (Engel et al, 2005). This explains why our simulated Chl a and PON concentrations are lower compared to observations between day 1 and day 4.…”

“…Detritus comprises a variety of components with particles of different sizes and sinking rates (Fasham et al, 1990). The detritus resolved by our model simply combines dead plankton biomass and fecal pellets.…”

“…Thus, for each of these three (LC, MC, and HC) situations we find three mesocosms that were subject to three different CO 2 levels (initial 700, 370, and 180 ppmV). By adapting the same nomenclature as in Engel et al (2005) assign the mesocosms M1, M6, and M8 to those with low calcification rates (highest TA), M2, M5, and M7 to the ones with medium calcification, and finally M3, M4, and M9 to mesocosms with high calcification rates (lowest TA).…”

“…We will also address the problem of resolving the variability observed in the accumulation of PIC and how this variability is related to the expression of the CO 2 effect introduced to the model. (Engel et al, 2005;Delille et al, 2005). The objective of this study was to investigate OA effects on marine calcifying algae (coccolithophores) captured in polyethylene bags of enclosed water volumes (mesocosms) and perturbed by different levels of CO 2 concentrations.…”

“…For this we take a modelling approach to simulate environmental conditions and the predominant dynamics of nine individual mesocosms as described in Engel et al (2005) and in Delille et al (2005). Joassin et al (2011) presented a dynamical model to simulate the mass flux of carbon (C), nitrogen (N), and phosphorus (P) for the same PeECE-I experiment.…”