Planktonic respiration and organic matter cycling using short-term in situ measurements

Planktonic community respiration (CR) is a fundamental driver of biogeochemical processes in aquatic environments and plays a critical role in the cycling of organic matter (OM). Despite the importance of CR in OM metabolism and mineralization, studies have shown that respiration from the rapid turnover of highly labile OM may be underestimated using standard day- to multi day long dark incubation protocols. Here we first compiled patterns of plankton CR from short-term, in situ incubations from dissolved oxygen (DO) loss rates from incubations conducted in diverse freshwater and marine systems. These incubations showed widespread rapid, non-linear rates of DO loss over short timescales (minutes) and more linear, but still elevated rates over intermediate timescales (hours). Next, we used a controlled experimental approach at one site (Lake Washington, USA) to define whether extreme non-linear DO loss in incubations seen in the initial minutes of incubations actually reflects preferential removal of highly bio-labile OM, or a different aspect of OM cycling that may be consuming DO (e.g., OM oxidation and molecular modification). The controlled experiment revealed that removal of labile, low molecular weight OM formulae was not the dominant process across short (0 to 10 minute) timescales. Rather, DO uptake may have been related to the transformation of OM. These preliminary observations suggest that while rapid cycling of DO may be common in many surface waters, further investigation of these approaches is needed to improve our understanding of the implications for OM cycling and overall ecosystem functioning.