Significant change to the Arctic and sub-arctic water cycle is underway, impacting hydrologic and biogeochemical fluxes. In southcentral Alaska, glacier mass loss, changes to precipitation (including the rain/snow fraction), thawing ground ice, and vegetation encroachment will change both magnitude and timing of water and solute fluxes downstream. Although altered fluxes of limiting nutrients are likely to impact productivity in both freshwater and marine environments, our lack of understanding on these processes limits our ability to predict their changes and the likely effects on downstream ecosystems.
Our Research:
Our research addresses an identified need to better understand hydro-chemical processes within a well-studied, partially-glacierized basin in southcentral Alaska. We build upon projects funded in the past three years that worked to partition water and chemical fluxes among landcover types (glacier, tundra, and forest) for discrete points in time. Our experimental design is unique in that it targets multiple source types at the headwaters, instead of more mixed waters in downstream locations. Our results will provide a minimal context of interannual variability (~5 years of data total) allowing seasonal patterns and variability within seasonality to be evaluated in the context of longer-term climate-driven change.
Why this Research is Important:
Glaciers worldwide play a significant role in the water and biogeochemical cycles of high altitude and high latitude landscapes. Glacier change impacts the discharge and chemistry of rivers, and subsequently the riverine and nearshore marine ecosystems and fisheries of the Gulf of Alaska and Arctic coasts. Improving our understanding of biophysical coupling within these economically-important ecosystems requires integration of physical, chemical, and ecological process studies. Our work contributes to an improved fundamental understanding of these processes, then provides a platform to link glaciological, hydrological and biogeochemical research efforts. Our research will help determine the sources, residence times, flowpaths, and fluxes of water and solutes moving through the Wolverine Glacier Watershed. This watershed includes a large glacier, a region of alpine tundra, an immature forest (Alder, Willow cottonwood) and a more mature lowland spruce-hemlock forest. Developing these relationships will result in more accurate calculations of annual fluxes and prediction of future fluxes and downstream effects of continued glacier mass loss.
From a broad array of physical and biological perspectives, coastal Alaska represents a focal point of climate-forced change. Failure to understand patterns of change will have drastic financial implications, especially related to fisheries and tourism, which together represent a multi-billion dollar a year industry.
Objective(s):
The three questions below summarize our direction and objectives for this project.
What is the role of land ice in water and nutrient budgets as exported into the nearshore ocean? How does southcentral Alaska differ from other studied regions? Informing this question requires quantified solute and DOM fluxes from a glacierized basin. The geometry of Wolverine Glacier catchment and the greater Nellie Juan catchment provide an opportunity to assess the relative contributions of glaciers, tundra and forest landscapes to broader scale water and nutrient budgets.
How will water delivery and associated biogeochemical fluxes (e.g, DOC, N, Fe) change as the ice-covered landscape fraction continues to decline? Will changes be predominantly in magnitude or will timing of delivery undergo more significant change? Can modern day discharge-flux relationships be established, then used for hindcasting, and eventually future projection? Predictive skill for changes in the timing and magnitude of water and solute delivery requires understanding present-day forcing for these fluxes. This work will elucidate similarities and differences in forcing among the various source regions (e.g., forest, glaciers) which can then form the basis for space-for-time substitutions and comparisons with other regions (e.g., southeast AK, Greenland, Antarctica). Source characterizations allow for mixing models to be developed, which sets the stage for scenario-based projection exercise.
Can all first order water budget terms be captured at the basin scale? Can geochemical tracers help to inform ground-water exchange? Geochemical tracers can potentially reveal ground-water exchange at the basin scale which further helps to constrain remotely sensed or model platforms for regional water budgets (e.g., Beamer et al., 2016; Scanlon et al., 2018) and sea level rise. The timing and concentration of solutes like Ca, Mg, As that characterize groundwater can be used to better understand groundwater- surface water exchange and fill a major knowledge gap in the water cycle.
Methods:
- Use automated sensors (Exo 2) to measure solute and total suspended sediment (TSS; turbidity) concentrations in select glacier-fed, tundra, and forest streams.
- Use field campaigns and synoptic samplings to develop robust relationships between sensor output and actual concentrations in order to compute fluxes. Explore the use of auto-samplers for more frequent grab samples.
- Measure fractions of C, N and P on TSS from discrete samples. If range is tight enough, we can go directly from turbidity to POC, TN, and TP concentrations / fluxes.
- Quantify the magnitude and uncertainty of solute and TSS fluxes estimated with automated sensors.
- Use an endmember mixing analysis (EMMA) model to determine the major catchment water and solute sources using preliminary endmembers determined by GRIP student Bergstrom and data from an additional five years of data.
- Determine water isotope (d18O and d2H) signatures of source waters to evaluate mixing proportions and constrain EMMA model.
- Assess organic carbon quality on a subset of samples to infer temporal and spatial shifts in sources relative to land cover.
