Interannual surface water CO ₂ and O ₂ dynamics during fall in a small headwater lake

Lake metabolism is often quantified using continuous measures of dissolved oxygen (O ₂ ), where a 1: -1 stoichiometry with carbon dioxide (CO ₂ ) is assumed because of their roles in photosynthesis and respiration, respectively. However, many other physical, chemical, and biological processes decouple dissolved O ₂ and CO ₂ concentrations in lakes. Tracking departures from 1:-1 stoichiometry may provide insights into larger scale ecosystem functioning, particularly during fall when temperatures change and destratification occurs. Using continuous measures of both dissolved O ₂ and CO ₂ in a small temperate headwater lake, we looked at the interannual gas departure signals during fall over seven years. The beginning of fall, defined here as the start of leaf colour change, differed among years but coincided well with the onset of lake destratification and a shift in surface gas concentrations. Fall surface CO ₂ accumulation rates varied considerably, whereas O ₂ depletion rates were rather similar among years. Departure signals were broadly related to interannual differences in climate: more CO ₂ accumulated in the surface during the hottest-wettest fall compared to the coldest-driest one (0.81 and 0.37 µmol L ^-1 d ^-1 , respectively), presumably from more catchment than hypolimnetic inputs. Lower CO ₂ accumulation occurred during years with prolonged hypolimnetic hypoxia potentially through enhanced CO ₂ consumption by methanogenesis. Other internal biological phenomena influenced fall departure signals, including a large metalimnetic oxygen peak, and higher fall surface primary production. We suggest gas departures during fall provide an integrative metabolic fingerprint for temperate stratified lakes, as well as insights into winter-priming conditions.