High flow events as hot moments of reactive Fe and P export: impacts of land cover and seasonality

High flow events often comprise the majority of annual discharge and riverine geochemical flux of phosphorus (P) and metals such as iron (Fe) and manganese (Mn) due to glacial melt, snowmelt, and storm-driven sustained high flow. Aquatic ecosystem productivity in receiving water bodies such as Lake Champlain and the Gulf of Alaska (GoA) are impacted by the riverine import of nutrients. The magnitude of these high flow events can be a strong predictor of receiving water body conditions, and in some cases can contribute to eutrophication. We explore the intersection of high flow events and land cover in contrasting catchments in Vermont and Alaska, covering a range of land covers including glacial, boreal-forested, mixed hardwood-conifer forested, and agricultural. In Vermont, we explore the hypothesis that riverine dissolved and suspended sediment P loads during spring runoff have a particularly high proportion of reactive species due to unique hydrologic pathways and the association of P with Fe. We compared spring runoff and summer storm concentrations and distribution of dissolved P (DP), dissolved and colloidal metals, and redox sensitive suspended sediment P (RSP). Agricultural catchments in Vermont were characterized by enrichment in RSP and DP during both spring runoff and summer storms, particularly at the onset of snowmelt. In 2014, 82% of the annual DP and 74% of annual RSP loads were delivered to Missisquoi Bay during spring runoff, with the majority of suspended sediment significantly more redox sensitive, and carrying potentially bioavailable P, than typical inputs to limnological models, suggesting that the reactivity of this load is systematically underestimated. In Alaska, we investigate Fe size partitioning and flux throughout the hydrologic year, with additional high-resolution sampling during discrete storm events in adjacent forested and glacierized catchments typical of coastal Alaska. There are clear differences between these catchments during individual storm events, and across seasons, reflecting widely varying source environments for Fe. The geochemical character of river water exported from the forested catchment, dominated by dissolved Fe and DOC, reflects the influence of peatlands and organic-rich soil as the dominant source of Fe and P, while the glacial catchment exports significantly more material derived from glacial weathering of bedrock, reflected in higher sediment and colloidal concentrations. Phosphorus concentrations in both watersheds are very low throughout the year, but significantly higher in the forested catchment, driven by organic matter decomposition. Both Vermont and Alaska are likely to be significantly impacted by climate change, with an increase in the frequency of heavy precipitation events, and continued glacial recession in Alaska driven by rising temperatures. Changes in the timing, provenance, and severity of high flow associated with climate and land cover change will have dramatic impacts on total riverine P and Fe loads, and their potential reactivity and bioavailability in receiving water bodies. Development of conceptual models that incorporate the intersection between high flow events (hot moments) and land cover source environments (hot spots) is critical to understanding how these systems are likely to change in the future.