Seasonal export of carbon, nitrogen, and major solutes from Alaskan catchments with discontinuous permafrost

Abstract

Frequent measurements of stream chemistry during snowmelt and summer storms were used in three watersheds that differ in permafrost coverage (high, 53%; medium, 18%; and low, 4%) to determine the role of water flow paths on the fluxes of carbon, nitrogen, and major solutes from Alaskan catchments. Permafrost was important in the seasonal pattern of stream chemistry as there was a distinct shift in chemistry and flow from winter through snowmelt and into summer in the permafrost‐dominated catchment. Furthermore, the active layer above the permafrost was important for the late summer release of NO 3 − and DOC, suggesting a deeper active layer may increase N and C loss in permafrost‐dominated areas. Overall, permafrost constrained water flow to the active layer, resulting in higher DOC but lower dissolved mineral fluxes (Ca 2+ Mg 2+ K + Na + ) in the high‐permafrost watershed than in the watersheds with less permafrost coverage. However, the decline in dissolved mineral fluxes was not linearly related to permafrost coverage across watersheds. The flux of weathering ions may also be explained by total water runoff, since the medium‐permafrost watershed, which had the greatest runoff on an areal basis, yielded the greatest loss of all major elements (Ca 2+ Mg 2+ K + Na + SO 4 2− NO 3 − NH 4 + Cl − ) except DOC. Despite differences among watersheds in permafrost coverage, hydrologic flow paths, area, and total runoff, all watersheds were net sources of every individual ions or elements (Cl − , PO 4 2− , SO 4 2− , DOC, DON, NO 3 − , Na + , K + Mg 2+ , Ca 2+ ) except NH 4 + , which was a small fraction of the total N concentration in streams.