Surface water CO concentration influences phytoplankton production but not community composition across boreal lakes.

Recent experimental evidence suggests that changes in the partial pressure of CO (pCO ), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO , and where phytoplankton structure and activity are under complex control of diverse local and regional factors. Here, we use a large-scale comparative study of 69 boreal lakes to explore the influence of existing CO gradients (c. 50-2300 μatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO , demonstrating that environmental context is key in determining pCO -phytoplankton interactions. We further argue that increased atmospheric CO is unlikely to influence phytoplanktonic composition and production in northern lakes.