Impacts of Watershed Glacier Coverage on Interannual Variability in Watershed Organic Matter and Nutrient Concentrations and Fluxes

Abstract:

Glacial melt from the coastal mountain ranges of southeast Alaska provides a substantial amount of water and nutrients to the coastal marine environment during the summer icemelt season. Organic matter and inorganic nutrients exported from the terrestrial environment play an important role in the productivity of near shore fjord ecosystems. This region is experiencing some of the most rapid glacial loss on Earth threatening to disrupt these tight icefield-to-ocean linkages. This research aims to evaluate how watershed glacier coverage affects interannual variability in streamwater concentrations and fluxes of dissolved organic carbon (DOC), nitrogen (N), and phosphorus (P). We sampled streamwater roughly bi-weekly during the primary glacier runoff season (April-October) from three coastal watersheds that vary in glacial coverage from 0 to 48%. Streamwater was collected from 2012 to 2020, which resulted in a total of 725 samples analyzed for DOC, N and P over the nine-year period. Discharge and physical streamwater characteristics (temperature, conductivity, and turbidity) were also measured continuously in all three watersheds during this same period. We used the R LoadEST software package to calculate daily and seasonal fluxes of DOC, N and P and to estimate daily concentrations for the April to October melt season.

The continuum of watershed glacial influence will allow us to use a space for time substitution to infer future trajectories for stream biogeochemistry following watershed deglacierization due to projected climate change. Preliminary analysis results show more stochastic discharge regimes and greater seasonal and inter-annual variability in DOC and total dissolved N (TDN) concentrations and flux in watersheds with lower glacier coverage. As glacier runoff decreases, river discharge and DOC/TDN export will become more stochastic and sensitive to precipitation events. Furthermore, inorganic P concentrations from rock weathering are predicted to decrease but become more variable as glaciers shrink. Our findings suggest that future changes in watershed glacial coverage and regional weather patterns will have significant effects on organic matter and nutrient export from terrestrial landscapes to the coastal ocean.