Variation and controls of sediment oxygen demand in backwater lakes of the Upper Mississippi River during winter

Many ecological processes affect the availability of winter dissolved oxygen (DO) concentrations in rivers, a key feature of overwintering fish habitat. Sediment oxygen demand (SOD) contributes to DO depletion, particularly during ice-covered periods, and may cause hypoxic conditions in backwater lakes, affecting the availability of suitable overwintering habitat. Understanding the drivers of SOD on habitat conditions during winter is critical for the management of the Upper Mississippi River System (UMRS). We measured SOD rates in three different habitat types (shallow, non-vegetated; deep, non-vegetated; and vegetated) within 12 backwater lakes in Pools 4, Pool 8, and Pool 13 of the UMRS in January and February 2022. Sediment physicochemical characteristics were measured to identify potential drivers of winter SOD rates. Measured SOD rates ranged from 0.04–0.44 g O₂/(m²d) at in situ temperatures, and 0.14–1.46 g O₂/(m²d) when corrected to 20°C. There were no statistically significant relations between in situ SOD and most sediment characteristics. SOD was positively associated with aquatic vegetation presence and negatively associated with sediment pH, water depth, flow velocity, and ice depth. SOD was typically higher at vegetated sites with lower flow velocity, with four of the five highest SOD rates measured at vegetated sites. Vegetation presence, depth, and flow velocity played greater roles in controlling SOD rates than sediment characteristics. Additionally, DO concentrations near the sediment–water interface at deep sites (> 1.5 m) were much lower than DO concentrations 0.2 m under the water surface, indicating that SOD was influencing DO concentrations and may be affecting overwintering habitat in backwater lakes.