Changes in Terminal Electron Acceptors During Experiment Mixing of a Dystrophic Lake

Changing climate conditions are associated with decreased duration of ice-on periods in lakes worldwide. This trend, coupled with likely increases in extreme weather events, is predicted to disrupt patterns of stratification and mixing in lakes and may result in unexpected, episodic mixing events. Because the mixing regime is a key component of a lake's physical template, regime changes should affect all aspects of the ecosystem. To understand the magnitude and nature of potential ecosystem changes, we experimentally mixed a small dystrophic lake during summer stratification in the Northern Highlands Lake District of Wisconsin (USA). The lake was mixed in July 2008 using a gradual entrainment lake inverter (GELI), a novel apparatus that uses alternating buoyancy stages to oscillate large volumes of water without introducing gas into the water column. Chemical conditions were monitored from spring to fall mixis (April-Sept). Here, we report on changes in the lake's thermal structure and responses of terminal electron acceptors (TEAs) as indicators of microbial activity and biogeochemical dynamics in the water column. Prior to experimental mixing, the lake was completely stratified. As expected, the surface layer (epilimnion) was well-oxygenated and had high concentrations of SO4. Deeper waters of the hypolimnion were anoxic, with detectable levels of H2S, FeII, and CH4. The GELI successfully homogenized the entire water column; the hypolimnion became oxygenated (3-4 mg DO/L), and reduced TEA concentrations declined to detection limits. After mixing ceased, the lake re- stratified but the hypolimnion was 14 degrees warmer than immediately prior to mixing. Re-establishment of chemical profiles was rapid as CH4 and H2S returned to pre-mixing concentrations within 1-2 weeks. The post-mixing stratification period was best characterized by accumulation of NH4-N in the hypolimnion; concentrations reached 1.5 mg N/L by the end of the summer, 4X greater than the pre-mixing level in July. Rapid accumulation of reduced TEAs and NH4-N enrichment both indicate elevated rates of respiration in the hypolimnion. Thus, episodic mixing is expected to increase whole-ecosystem respiration via both re- introducing oxygen needed for aerobic respiration, and by supporting higher anaerobic rates of respiration due to hypolimnetic warming. In turn, by increasing respiration, episodic mixing has the potential to increase the role of lakes as CO2 sources to the atmosphere.