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Rewriting the Redox Paradigm: Dynamic Hydrology Shapes Nutrient and Element Transformations in a Great Lakes Coastal Estuary

Active Dates 9/1/2021-8/31/2024
Program Area Environmental Systems Science
Project Description
Fine-scale biological, geochemical, and chemical reactions underlie environmental system function. Many of these processes, including oxygenic photosynthesis and aerobic respiration, are coupled reduction-oxidation, or “redox,” reactions. Redox reactions transfer energy by moving electrons from an electron donor, such as an organic molecule, to an electron acceptor, often oxygen. In flooded organic soils, oxygen is depleted by microbial respiration more rapidly than it is regenerated. When oxygen is depleted, organisms can use other molecules to accept electrons in a specific order, in part because the energy yields of redox reactions differ based on thermodynamic principles. When competing for labile organic substrates, organisms that can capture more energy should outcompete organisms using metabolisms that yield less energy, in what is sometimes called the “thermodynamic exclusion principle.” However, recent research challenges predictions based on this “redox ladder” paradigm. At small scales, it is now apparent that rather than exclude one another, redox processes of variable energy yield co-occur in space and time, such that the theoretically discrete “rungs” of this metaphorical ladder to seem out of place or even intermingled.

Microbes that mediate environmental processes experience conditions at much smaller scales than scales at which we typically detect them. Thus, we hypothesize that fine scale heterogeneity causes apparent departures from “redox ladder” predictions at larger scales. We predict that we will detect this heterogeneity in electrochemical signatures measured as redox potential (Eh) and zero resistance ammetry (ZRA) at fine spatial scales and high temporal resolution, and that this heterogeneity has whole-ecosystem consequences. Our objectives are to combine custom-built sensors, direct measurements of biogeochemistry, and process-based modelling in a Great Lakes river mouth wetland system (Old Woman Creek, OH) to (1) relate dynamic hydrology to redox regimes in contrasting soils; (2) determine how redox heterogeneity in space and time drives C, N, P, S, Fe, and Mn cycling, and (3) assess the sensitivity of ecosystem scale process-based models to fine-scale variability in redox conditions.

Wetland systems, including river mouths and estuaries at terrestrial-aquatic interfaces, are often called “the kidneys of the landscape” because of their potential to filter, store, and transform contaminants. Many of the processes that drive “purifying” functions are redox reactions, and thus understanding how fine scale redox processes relate to whole-ecosystem function is highly relevant to wetland and watershed management. Current policies and best practices are based largely on structural indices of wetland function, and thus visible variability in habitat is a common goal. Less common, but often implied or assumed, is that structural heterogeneity will promote nutrient removal by creating redox heterogeneity. This project will formalize relationships between hydrology, electrochemical conditions, and biogeochemical processes at nested scales to inform ecosystem- and watershed-scale management for nutrient and elemental cycling.
Award Recipient(s)
  • Kent State University (PI: Kinsman-Costello, Lauren)