Environmental conditions for phytoplankton influenced carbon dynamics in boreal lakes

The partial pressure of CO2 (pCO2) in lake water, and thus CO2 emissions from lakes are controlled by hydrologic inorganic carbon inputs into lakes, and in-lake carbon transformation (mainly organic carbon mineralization and CO2 uptake by primary producers). In boreal lakes, CO2 uptake by phytoplankton is often considered to be of minor importance. At present, however, it is not known in which and how many boreal lakes phytoplankton CO2 uptake has a sizeable influence on the lake water pCO2. Using water physico-chemical and phytoplankton data from 126 widely spread Swedish lakes from 1992 to 2012, we found that pCO2 was negatively related to phytoplankton carbon in lakes in which the phytoplankton share in TOC (Cphyto:TOC ratio) exceeded 5%. Total phosphorus concentration (TP) was the strongest predictor of spatial variation in the Cphyto:TOC ratio, where Cphyto:TOC ratios > 5% occurred in lakes with TP > 30 µg l−1. These lakes were located in the hemi-boreal zone of central and southern Sweden. We conclude that during summer, phytoplankton CO2 uptake can reduce the pCO2 not only in warm eutrophic lakes, but also in relatively nutrient poor hemi-boreal lakes.