Impacts of streamflow variability and antecedent conditions on the magnitude, timing, and form of watershed carbon and nutrient export from a coastal Alaskan watershed

Abstract

The coastal watersheds of Southeast Alaska have diverse hydrologic regimes driven by differences in the contribution of glacial, snow, and rainwater inputs to streamflow. Climate change is set to shift watershed streamflow regimes, which has the potential to impact the source, processing, and export of materials (e.g., organic matter, nutrients, drifting macroinvertebrates) from watersheds to nearshore ecosystems. However, the impact of streamflow variability, such as droughts and floods, on material transport is poorly understood in Southeast Alaska. My study evaluated how streamflow variability impacts the magnitude and composition of materials exported from a predominantly rain-fed watershed in Juneau, Alaska. I collected stream drift and water chemistry samples at least twice per week from late April through October 2021, capturing peaks and troughs in stream flow during the main runoff season. I found that dissolved organic carbon (DOC) dominated streamwater carbon export, except during low flow periods in July and August when alkalinity was the dominant form of carbon export. Relative to DOC and alkalinity, streamwater export of particulate organic matter and invertebrates was small, but these carbon sources could be disproportionately important to downstream food webs because of their higher quality, assessed from molar ratios of C to N. Furthermore, hourly measurements of DOC and alkalinity collected with in situ sensors revealed differing seasonal relationships between concentration and discharge (CQ), as well as potential effects of antecedent stream flow conditions. Shifts in streamflow regimes and/or enhanced variability in flow owing to climate change will alter the form, magnitude, and timing of material exports from rain-fed watersheds in Southeast Alaska. Shifting patterns of material export could in turn alter land-to-ocean linkages that may be important for productivity and resilience of freshwater and marine food webs.