Seasonality and Oxygen Control Dissolved Organic Carbon Quantity, Quality, and Greenhouse Gases in a Eutrophic, Freshwater Reservoir

Abstract:

Simultaneously measuring dissolved organic carbon (DOC) quantity and quality plus dissolved greenhouse gases (GHGs; carbon dioxide, methane) is necessary for identifying the dominant environmental controls on different components of the carbon cycle, yet these carbon pools are rarely measured in concert. In particular, understanding how DOC-GHG coupling varies under different redox conditions is important as this coupling may be changing in response to increased prevalence of hypolimnetic anoxia due to land use and climate change. To disentangle the effect of oxygen from seasonal changes in temperature, we conducted a whole-ecosystem oxygenation experiment in which we switched a reservoir’s hypolimnion from oxic to anoxic conditions every two weeks over the course of a summer stratified period. We measured DOC concentration, fluorescent dissolved organic matter (i.e., excitation emission matrices, EEMs), and dissolved GHG concentrations (carbon dioxide, methane) throughout the summer from surface to bottom waters. Preliminary results suggest the DOC pool is largely derived from autochthonous sources with increasingly lower humification indices (HIX) and higher biological indices (BIX) at all depths through the summer stratified period, as determined via EEMs. There appears to be little variation in DOC quality as a result of oxygenation yet a substantial increase in methane in the hypolimnion under anoxic versus oxic conditions. While DOC quality is mainly driven by seasonal changes and watershed inputs, methane is largely influenced by oxygen concentrations. These results suggest that the dominant controls on DOC quality versus GHG concentrations in freshwater reservoirs may be different, highlighting the importance of studying multiple pools of the carbon budget of inland waters.