Interannual surface water CO2 and O2 dynamics during fall in a small headwater lake

Lake metabolism is often quantified using continuous measures of dissolved oxygen (O2), where a 1: -1 stoichiometry with carbon dioxide (CO2) is assumed because of their roles in photosynthesis and respiration, respectively. However, many other physical, chemical, and biological processes decouple dissolved O2 and CO2 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 O2 and CO2 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 CO2 accumulation rates varied considerably, whereas O2 depletion rates were rather similar among years. Departure signals were broadly related to interannual differences in climate: more CO2 accumulated in the surface during the hottest-wettest fall compared to the coldest-driest one (0.81 and 0.37 {micro}mol L-1 d-1, respectively), presumably from more catchment than hypolimnetic inputs. Lower CO2 accumulation occurred during years with prolonged hypolimnetic hypoxia potentially through enhanced CO2 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. HighlightsO_LIAnnual fall surface water CO2 accumulation rates vary more than O2 depletion rates. C_LIO_LIExternal fall inputs and internal processes influence gas departures differently. C_LIO_LIFall gas departures may act as an integrative metabolic signal in temperate lakes. C_LI