In situ plankton community respiration measurements show low respiratory quotients in a eutrophic lake

Planktonic community respiration is an important carbon cycling process, typically quantified by converting measured values of dissolved O-2 consumption rates into CO2 production rates assuming a respiratory quotient of 1 (RQ = CO2 per O-2 by moles). However, the true variability in planktonic RQs between different aquatic ecosystems is poorly understood. We conducted in situ RQ measurements in a eutrophic lake dominated by algal-derived substances and found that RQs were significantly below 1. In fact, many RQ values were extremely low (0.2-0.6), below theoretical RQs for oxidation of algal organic matter substrates (0.7-0.8), suggesting that other factors than substrate control need to be considered to understand the RQ. This view was further supported by lack of correlations between RQ and microbial variables known to be strongly substrate dependent, including bacterial growth efficiency and the functional capacity of the bacterioplankton community to degrade different compounds. Based on the measured dynamics in methane and nutrient pools, we discuss that methane oxidation and nitrification likely occurred in the lake, contributing to the unusually low RQs. Our findings demonstrate that planktonic RQs in productive lakes can systematically be below 1, suggesting that CO2 emissions from these lakes may currently be overestimated.