Natural variability of carbon dioxide and net epilimnetic production in the surface waters of boreal lakes of different sizes

Abstract

The variability of surface water carbon dioxide concentration, or partial pressure (pCO2), was studied in 11 lakes of greatly varying size (2.4 ha up to 8 million ha) in Northwest Ontario, Canada. Six of these lakes were chosen to be as similar as possible in all respects except surface area (the Northwest Ontario Lake Size Series [NOLSS], which range from 88 to 35,000 ha). Spatial and temporal variability of pCO2 within a single lake was no greater in the larger lakes than in the smaller lakes. Interannual variability was significant and synchronous, which indicates that weather patterns were important and affected the different lakes within the region in a similar manner. However, annual pCO2 averages were not related to annual differences in planktonic photosynthetic activity, measured by 14CO2 fixation. In the six NOLSS lakes, there was not a significant relationship of average pCO2 with lake size. For all 11 lakes, however, there was a significant negative correlation of pCO2 with lake size, which was likely due to several characteristics of the very small and very large lakes that covaried with size. The larger lakes were deeper and had longer water residence times and lower DOC, which suggests lower CO2 production from allochthonous organic carbon inputs. Also, the ratio of epilimnetic sediment area/epilimnetic volume (Ae/Ve) was smaller in the larger lakes, which likely resulted in lower rates of recycling of fixed carbon to CO2 during summer stratification.