Exports of dissolved ammonium (NH4 +) during storm events across multiple catchments in a glaciated forested watershed

Abstract

Storm event exports of dissolved NH(4)(+) were explored for multiple events in the Point Peter Brook watershed (PPBW), a glaciated, forested watershed located in Western New York, USA. Investigations were performed across four catchments (1.6-696 ha) with varying topography and the extent of surface-saturated areas. While wetland and riparian waters were important sources of NH(4)(+) during non-storm periods, throughfall and litter leachate were the dominant contributors of NH(4)(+) during storm events. Ammonium concentrations in catchment discharge displayed a sinusoidal seasonal pattern with a maximum during early spring (March) and a minimum in late summer (August-September). Storm event concentrations of NH(4)(+) in streamflow were much greater than baseflow values and showed a consistent temporal pattern with an increase in concentrations on the hydrograph rising limb, a peak at or before the discharge peak, followed by a decline in concentrations. Storm event patterns of DON were similar to NH(4)(+) while the patterns of [Formula: see text]differed from NH(4)(+) for the summer and fall events. The storm event expression of NH(4)(+) was attributed to throughfall and throughfall-mediated leaching of the litter layer. The reactive behavior of NH(4)(+) precluded its use in an end member mixing model (EMMA) for predicting streamflow concentrations. While concentrations of NH(4)(+) in precipitation and streamflow were high for the spring events, exports of NH(4)(+) in streamflow were highest for the large and intense storm events. Baseflow NH(4)(+) concentrations increased with the percent wetland/saturated area in the catchment but the same trend did not hold for storm-event concentrations.