Abstract
Abstract. The impact of ocean acidification and increased water temperature on marine ecosystems, in particular those involving calcifying organisms, has been gradually recognised. We examined the individual and combined effects of increased pCO2 (180 ppmV CO2, 380 ppmV CO2 and 750 ppmV CO2 corresponding to past, present and future CO2 conditions, respectively) and temperature (13 °C and 18 °C) during the exponential growth phase of the coccolithophore E. huxleyi using batch culture experiments. We showed that cellular production rate of Particulate Organic Carbon (POC) increased from the present to the future CO2 treatments at 13 °C. A significant effect of pCO2 and of temperature on calcification was found, manifesting itself in a lower cellular production rate of Particulate Inorganic Carbon (PIC) as well as a lower PIC:POC ratio at future CO2 levels and at 18 °C. Coccosphere-sized particles showed a size reduction with both increasing temperature and CO2 concentration. The influence of the different treatments on coccolith morphology was studied by categorizing SEM coccolith micrographs. The number of well-formed coccoliths decreased with increasing pCO2 while temperature did not have a significant impact on coccolith morphology. No interacting effects of pCO2 and temperature were observed on calcite production, coccolith morphology or on coccosphere size. Finally, our results suggest that ocean acidification might have a larger adverse impact on coccolithophorid calcification than surface water warming.
Highlights
The global atmospheric carbon dioxide (CO2) concentration has increased from a pre-industrial value of about 280 ppmV to 379 ppmV in 2005 (IPCC, 2007)
As a general feature in our culture experiments, Chlorophyll a (Chl-a) concentration and the particulate component increased while nutrients were consumed; a stationary phase was reached where PO4 became depleted and Chl-a levels slowly decreased while Particulate Organic Carbon (POC) and Particulate Inorganic Carbon (PIC) accumulated in the culture
This evolution was monitored in cultures subjected to different treatments of pCO2 and temperature to assess the effect of ocean acidification and global warming on the organic and inorganic carbon production of E. huxleyi
Summary
The global atmospheric carbon dioxide (CO2) concentration has increased from a pre-industrial value of about 280 ppmV to 379 ppmV in 2005 (IPCC, 2007). Global warming results in an enhancement in vertical stratification of the water column, leading to a decreased mixing between the surface ocean and the deeper layers with a consequent decrease in the supply of nutrients (Bopp et al, 2001) and Dissolved Inorganic Carbon (DIC) (Borges et al, 2008). Increasing stratification results in a shoaling of the upper mixed layer leading to an increase in the light availability in this layer (Bopp et al, 2001) Both ocean acidification and warming influence the distribution of DIC for calcifying organisms and have the potential to alter the particulate inorganic and/or organic carbon production, which would affect the efficiency of particle export. By photosynthesis in the photic zone, phytoplankton draws down CO2: 16 CO2 + NO−3 + H2 PO−4 + H+ + 122 H2O (1) ↔ (CH2O)106 (NH3) H3 PO4 + 138 O2
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