Summer Redox Dynamics in a Eutrophic Reservoir and Sensitivity to a Summer’s End Drawdown Event

Abstract

In eutrophic lakes and reservoirs, reduced mixing during stratified conditions limits oxygen (O2) supply to the hypolimnion (that is, bottom waters). In the absence of an O2 supply, microbial decomposers consume alternative electron acceptors, generally in order of their thermodynamic favorability, releasing soluble, reduced manganese (Mn), iron (Fe) and methane (CH4) to the water column with implications for reservoir water quality and greenhouse gas dynamics. Still, there are very few studies that quantify intra- and inter-annual controls on lake and reservoir redox chemistry, especially in managed systems. To address this knowledge gap, we examined redox-sensitive water column chemistry before and during four summer’s end water-level drawdown events (~ 2 m in magnitude) in a eutrophic reservoir. We observed lower dissolved Fe and CH4 concentrations in years with higher hypolimnion O2 and NO3−, suggesting that water column oxidant availability controls the extent of the redox cascade. During drawdowns, dissolved CH4, Mn and Fe concentrations increased in the hypolimnion (on average by 50%, 40% and 175%, respectively) concomitant with order of magnitude increases in methane bubbling rates (that is, ebullition). To our knowledge, this is the first in situ evidence for enhanced flushing of sediment pore water into a reservoir during water-level drawdowns. Furthermore, the mass of CH4, Mn and Fe released varied as a function of summertime redox conditions. Thus, the timing of reservoir water-level drawdowns may determine the degree to which reduced solutes enter the water column during drawdown, with longer pre-drawdown periods of hypolimnion hypoxia leading to higher solute fluxes.