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

Abstract

Storm event exports of dissolved ${\text{NH}}^{ + }_{4}$ 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 ${\text{NH}}^{ + }_{4}$ during non-storm periods, throughfall and litter leachate were the dominant contributors of ${\text{NH}}^{ + }_{4}$ 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 ${\text{NH}}^{ + }_{4}$ 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 ${\text{NH}}^{ + }_{4}$ while the patterns of ${\text{NO}}^{ - }_{3}$differed from ${\text{NH}}^{ + }_{4}$ for the summer and fall events. The storm event expression of ${\text{NH}}^{ + }_{4}$ was attributed to throughfall and throughfall-mediated leaching of the litter layer. The reactive behavior of ${\text{NH}}^{ + }_{4}$ precluded its use in an end member mixing model (EMMA) for predicting streamflow concentrations. While concentrations of ${\text{NH}}^{ + }_{4}$ in precipitation and streamflow were high for the spring events, exports of ${\text{NH}}^{ + }_{4}$ in streamflow were highest for the large and intense storm events. Baseflow ${\text{NH}}^{ + }_{4}$ concentrations increased with the percent wetland/saturated area in the catchment but the same trend did not hold for storm-event concentrations.