Surface water CO₂ concentration influences phytoplankton production but not community composition across boreal lakes

Recent experimental evidence suggests that changes in the partial pressure of CO₂ (pCO₂ ), in concert with nutrient fertilisation, may result in increased primary production and shifted phytoplankton community composition that favours species lacking adaptations to low CO₂ environments. It is not clear whether these results apply in ambient freshwaters, which are already often supersaturated in CO₂ , 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 CO₂ gradients (c. 50-2300 μatm) on phytoplankton community composition and biomass production. While community composition did not respond to pCO₂ gradients, gross primary production was enhanced, but only in lakes already supersaturated in CO₂ , demonstrating that environmental context is key in determining pCO₂ -phytoplankton interactions. We further argue that increased atmospheric CO₂ is unlikely to influence phytoplanktonic composition and production in northern lakes.