Carbon quality regulates the temperature dependence of aquatic ecosystem respiration

Ecosystems are responding to broad‐scale changes in climate and other factors in complex ways. Two key changes occurring in many inland waterbodies are increasing temperatures and increasing terrestrial dissolved organic matter (tDOM) inputs. Due in part to tDOM inputs, inland waters play an important role in the global carbon cycle, with release of CO₂ from these ecosystems a substantial portion of the combined carbon sinks of terrestrial ecosystems and the oceans. Ecosystem respiration (ER) is an important mechanism regulating CO₂ in aquatic systems, and ER rates are temperature‐dependent. However, it is unknown how ER rates will change in the future because ER is sensitive to factors other than temperature, such as the concentration and source of organic matter. Theory posits that ER rates supported by refractory carbon compounds should exhibit a greater temperature dependence than ER rates supported by labile carbon compounds, but empirical tests of this theory are inconclusive. We experimentally manipulated temperature and carbon quality in mesocosms while monitoring dissolved oxygen (DO) with in situ high frequency DO sensors. We used two highly divergent, natural DOM sources and diluted the more refractory source so that dissolved organic carbon concentrations were similar in all mesocosms. We calculated daily ER rates over 27 days in each mesocosm. We found that ER increased more with temperature when tDOM dominated the organic carbon pool. Given widespread increases in temperature and tDOM inputs to aquatic ecosystems, these results highlight important interactions governing how ecosystems may respond to concurrent environmental changes and alter their contributions to the global carbon cycle.