Ocean Acidification

It has been proposed that increasing levels of pCO₂ in the surface ocean will lead to  more partitioning of the organic carbon fixed by marine primary production into the dissolved  rather than the particulate fraction. This process may result in enhanced accumulation of  dissolved organic carbon (DOC) in the surface ocean and/or concurrent accumulation of transparent  exopolymer particles (TEP), with important implications for the functioning of the marine carbon  cycle. We investigated this in shipboard bioassay experiments that considered the effect of four  different pCO₂ scenarios (ambient, 550, 750 and 1000 μatm) on unamended  natural phytoplankton communities from a range of locations in the northwest European shelf  seas. The environmental settings, in terms of nutrient availability, phytoplankton community  structure and growth conditions, varied considerably between locations. We did not observe any  strong or consistent effect of pCO₂ on DOC production. There was a significant but  highly variable effect of pCO₂ on the production of TEP. In three of the five  experiments, variation of TEP production between pCO₂ treatments was caused by the  effect of pCO₂ on phytoplankton growth rather than a direct effect on TEP production. In  one of the five experiments, there was evidence of enhanced TEP production at high pCO₂  (twice as much production over the 96 h incubation period in the 750 μatm  treatment compared with the ambient treatment) independent of indirect effects, as hypothesised by  previous studies. Our results suggest that the environmental setting of experiments (community  structure, nutrient availability and occurrence of phytoplankton growth) is a key factor  determining the TEP response to pCO₂ perturbations.

MacGilchrist G. A., Shi T., Tyrrell T., Richier S., Moore C. M., Dumousseaud C. & Achterberg E. P., 2014. Effect of enhanced pCO₂ levels on the production of DOC and TEP in short-term bioassay experiments. Biogeosciences Discussions 11:3701-3730. Article.