Exogenously produced CO sub(2) doubles the CO sub(2) efflux from three north temperate lakes

It is well established that lakes are typically sources of CO sub(2) to the atmosphere. However, it remains unclear what portion of CO sub(2) efflux is from endogenously processed organic carbon or from exogenously produced CO sub(2) transported into lakes. We estimated high-frequency CO sub(2) and O sub(2) efflux from three north temperate lakes in summer to determine the proportion of the total CO sub(2) efflux that was exogenously produced. Two of the lakes were amended with nutrients to experimentally enhance endogenous CO sub(2) uptake. In the unfertilized lake, 50% of CO sub(2) efflux was from exogenous sources and hydrology had a large influence on efflux. In the fertilized lakes, endogenous CO sub(2) efflux was negative (into the lake) yet exogenous CO sub(2) made the lakes net sources of CO sub(2) to the atmosphere. Shifts in hydrologic regimes and nutrient loading have the potential to change whether small lakes act primarily as reactors or vents in the watershed. Key Points * A substantial portion of the CO sub(2) released from three lakes was produced exogenously * Despite positive net ecosystem production, lakes were carbon sources to the atmosphere