Understanding microbial respiration and energy flow in seasonally hypoxic streams via in situ and in silico experimentation

Seasonal hypoxia occurs in low gradient streams worldwide. Low dioxygen sags are a frequent focus of river management and waste water effluent regulations but are rarely studied by stream ecologists. Investigators often exclude low gradient streams with periodically lentic reaches as many methods in stream biogeochemistry rely on the assumptions of advective transport. In our study system in the NC Piedmont, USA, low summer and fall baseflow and shallow channel slopes lead to a slow moving, ponded system where dioxygen is frequently depleted via microbial respiration. When the water becomes hypoxic, anaerobic metabolic pathways dominate resulting in dioxygen depletion and greenhouse gas production.

To understand the drivers of these metabolic shifts, we manipulated water column chemistry in the stream channel. Subsequently, using the stoichiometric- and thermodynamic-constraned GANGSTA simulation framework, we generated a series of models that simulate microbial respiration and energy flow in silico . Field and model manipulations across a range of dioxygen conditions and alternative electron-acceptor concentrations indicate that hypoxia at the scale observed in our study river has a substantial impact on net energy flow and the rate and diversity of metabolic pathways utilized by microbes. Incorporating the periodic hypoxia of lentic reaches into the conceptual models of river-network biogeochemistry changes our understanding of how water chemistry and hydrology drive microbial metabolism in rivers.