Permanently stratified lakes provide an opportunity to study microbially-driven biogeochemical processes at oxic/anoxic interfaces. Such lakes are useful for understanding past or future ocean scenarios (e.g. stratified Precambrian oceans or expanding oxygen minimum zones), and the influence that anthropogenic perturbations (e.g. road salt, alum treatments) have on the biogeochemistry of lakes. Here I present seasonal geochemical, microbiological, & mineralogical data to document permanent stratification of two ferruginous (i.e. anoxic and Fe(II)-rich) lakes from the Upper Midwest, USA: Brownie Lake in Minneapolis, MN and Canyon Lake in the Upper Peninsula, MI. These two lakes vary in their range of salinity, nutrients (N, P), light penetration depth, anthropogenic impact, and time since onset of stratification. These parameters influence the extent of microbial elemental cycles and resulting chemistry and mineralogy within the water column, as well the sedimentary record. Brownie Lake supports a community of anoxygenic photosynthetic bacteria capable of oxidizing Fe(II). Yet, the emergence of a mid-summer sulfur cycle has implications for how the stratified microbial community affects and responds to changing redox regimes. The methane flux from Brownie Lake rivals reports from most other lakes in the literature, but shows a strong seasonality, hinting at seasonal control over (an)aerobic methane oxidation. The transition between oxic and anoxic waters at Canyon Lake is meters long, providing a natural laboratory to study microaerophilic oxidation pathways for Fe, S, and methane, as well as the mineral precipitates that form from such “oxygen minimum zones”. I will combine examples from the literature, and the geological characteristics of the upper Midwest to suggest that such permanently stratified or prone-to-stratifying lakes may not be so rare. In addition to the accessibility of stratified Midwestern lakes to study, the microbial-driven biogeochemistry in these lakes may become increasingly important in regulating global elemental cycles in the future.<br/>

Relevance of stratified Midwestern lakes to past, present, and future microbial biogeochemistry

255th National Meeting (2018)

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