Long term trends in pCO2 in lakes following rebrowning trends in south-central Ontario from 1980 to 2017

Abstract:

Over the last 30 years, lake browning has increased in many northern lakes. In Central and Eastern Ontario (Canada) these browning trends have been linked to the decline in bulk deposition of SO₄ and the recovery from the impacts of the acid deposition. As a result, many acid-sensitive lakes that were heavily impacted have seen increases in pH and changes in their chemical composition. As DOC level are now approaching past baseline in most of these lakes, the extent to which these long-term changes have affected the C balance of lake, and CO₂ dynamics in particular, is unclear because there are few long term records of lake CO₂ concentrations. We have studied the temporal trends in partial pressure of CO₂ in 15 lakes over 40 years, calculated on the basis of chemical (Alkalinity, DIC, pH) measurements, following the browning trend and the abatement of SO₄ deposition in two sets of lakes in Ontario. We have further explored links to trends in lake DOC, pH, water temperature and catchment properties. Most lakes were classified as moderately to highly acid-sensitive, and positive trends in pH were observed in 60% of the lakes. While from 1980 to 2000, all of the study lakes did not show significant changes in pCO₂ over time, a significant and net increase was observed from 2000 to present in 10 study lakes near the Dorset Environmental Science Centre (DESC) with a mean annual increase of 56 ppm. Over the same time period, all five lakes in the Turkey Lake Watershed (TLW) have shown a significant increase, averaging 95 ppm per year starting in 2009. Significant trends in lake pCO₂ in DESC were linked to positive temporal trends in DOC. However the lack of significant pCO₂ and DOC trends in TLW, located 300 km from DESC, indicates that the trajectory of lake response is not uniform across watersheds, even among lakes within the same region, which has implications for predicting future CO₂ emissions. Moreover, all these lakes have undergone a significant decline in pH over the past decade, following 2 decades of slow pH recovery. This recent downward shift may be linked to the observed increases in lake pCO₂ that may be related to rebrowning and other regional processes at the landscape level.