Invertebrate Activities in Wetland Sediments Influence Oxygen and Nutrient Dynamics at the Sediment-water Interface

In flooded soils and sediments, bioturbating invertebrates rework sediment and convey oxygenated surface water through burrowing, creating a mosaic of adjacent anoxic and oxic patches while simultaneously translocating and transforming nutrients as they feed and excrete. We investigated the impacts of two functionally contrasting bioturbators (gallery-network burrower Lumbriculus variegatus and U-shaped burrower Ephemera simulans) on oxygen availability and nutrient fluxes in wetland sediments. To assess excretion contributions, we also incubated bioturbators in sand-water microcosms. Fine-scale oxygen measurements combined with flux rates of redox-sensitive and conservative ions reveal that both bioturbators introduced oxygen to sediments. U-shaped burrowers facilitated measurable oxygen introduction into sediments while gallery-network burrowers did not. However, gallery-network burrowers showed evidence of oxidizing reduced solutes in sediments, which suggests that oxygen is being introduced. At high densities, both bioturbators promoted sufficient iron oxidation to sequester phosphorus from surface water into sediments, effectively counteracting phosphorus release from excretion. Conversely, bioturbation caused nitrate release into surface water, likely driven by excretion of ammonia followed by nitrification. Gallery-network burrowers facilitated P retention in sediments but contributed N to surface water, while U-shaped burrowers showed similar, but less pronounced trends. Bioturbators have profound, but variable, effects on sediment-surface water nutrient exchange in wetlands. Sediment characteristics, bioturbator density, and bioturbation mode regulate both the amount of oxygen introduced to normally anoxic sediments and its reactions with sediment substrates, shaping the magnitude and direction of bioturbator-induced nutrient fluxes.