Suspended particles and their role in biological uptake of nitrogen during storm events

Abstract

Nitrogen (N) cycling in streams is consequential to ecosystem functioning and water quality at a watershed scale. Research concerning the microbial uptake of N in streams has primarily focused on the streambed, since at base flow conditions the streambed provides a large surface area relative to water volume allowing microbes there to cycle significant amounts of N from the water column. During storm events, however, both stream water volume and concentrations of suspended particulate matter (SS) increase, generating a larger surface area in the water column for microbial nutrient uptake. Both water column N uptake and the potential mechanisms controlling uptake on SS have been understudied, so it is unclear what conditions may moderate these processes during storm events. We collected stream water samples during storms and conducted analyses of SS size, SS concentration, and nutrient availability across different events. We related these characteristics to microbial respiration on SS which we measured using 5-day biological oxygen demand (BOD) experiments and microsensor measurements of dissolved oxygen around SS. We also performed a controlled laboratory experiment manipulating sediment source, concentration, and particle size in mesocosms with stream water, and then measured the rates of assimilatory N uptake and dissimilatory N uptake (i.e., denitrification) in each mesocosm over 24 hours. In storm samples, microbial activity on SS was positively correlated with SS concentration and dissolved carbon availability, which varied depending on season and storm size. In mesocosm experiments, both assimilatory N uptake and denitrification were driven by SS concentration. Assimilatory N uptake was also positively correlated with sediment carbon content and particle size, which varied with sediment source. This research demonstrates that water column N uptake during storm events will likely be moderated by the quantity and quality of SS, which change with storm size and season. These types of data should therefore be considered in future work concerning N retention and export in streams.