Hurricane pulses: Small watershed exports of dissolved nutrients and organic matter during large storms in the Southeastern USA

Extreme weather events, such as hunicanes, can cause ecological disturbances that alter energy and nutrients across terrestrial-aquatic boundaries. Yet, relatively few studies have considered the impacts of extreme weather events on biogeochemical dynamics in watersheds at larger spatial scales. Here, we assessed the effects of Hurricanes Harvey and Irma on the export of dissolved organic matter (DOM) and nutrients in Len watersheds from five southeastern states of Lhe United Slates. We quantified the magnitude of dissolved organic carbon (DOC) and nutrients exported during the storms and assessed Lhe changes in DOM sources and bioreactiviLy after storms. Our results show that Lhe storm-mobilized DOC and nutrients fluxes were primarily driven by water discharge. The proportions of terrestrial, humic-like DOM compounds increased, and percent auLochthonous, protein-like DOM decreased during high flows. Percent bioreacLive DOC decreased with increasing discharge. BioreactiviLy increased with increasing nitrate concentration, but decreased as percent terrestrial humic-like DOM, aromaLicity, and molecular weight increased. These observations suggest that storms may have shifted flow paths to shallower depths that promoted the addition of biorefractory organic matter from topsoils into the water column. Notably, the total flux of bioreactive DOC was at least nearly twice as high at peak discharge, indicating materials transported by large storm flows could strongly enhance microbial activity in streams, although the position of storm -mediated microbial hotspots would depend on the flow rate and other instream parameters. Additionally, compared to forest -dominated watersheds, urban watersheds exported high loads of nutrients and bioreactive DOC, and a wetland -dominated watershed had a prolonged, but relatively subdued export of DOC and nutrients. Together, our findings highlight the ecological significance of extreme weather and climate events in leading to rapid, large -magnitude changes in energy and nutrient availability within drainage networks, and the potential interactions between land use and climate change on watershed biogeochemistry. (C)D 2019 Elsevier B.V. All rights reserved.