Nutrient Uptake in the Supraglacial Stream Network of an Antarctic Glacier

Abstract

In polar regions, where many glaciers are cold based (frozen to their beds), biological communities on the glacier surface can modulate and transform nutrients, controlling downstream delivery. However, it remains unclear whether supraglacial streams are nutrient sinks or sources and the rates of nutrient processing. In order to test this, we conducted tracer injections in three supraglacial streams (62 to 123 m long) on Canada Glacier in the Taylor Valley, of the McMurdo Dry Valleys, Antarctica. We conducted a series of additions including nitrate (N), N + phosphate (P), N + P + glucose (C), and N + C. In two reaches, N-only additions resulted in N uptake. The third reach showed net N release during the N-only addition, but high N uptake in the N + P addition, indicating P-limitation or N + P colimitation. Coinjecting C did not increase N-uptake. Additionally, in these systems at low N concentrations the streams can be a net source of nitrogen. We confirmed these findings using laboratory-based nutrient incubation experiments on sediment collected from stream channels on Canada Glacier and two other glaciers in the Taylor Valley. Together, these results suggest there is active biological processing of nutrients occurring in these supraglacial streams despite low sediment cover, high flow velocities, and cold temperatures, modifying the input signals to proglacial streams. As glaciers worldwide undergo rapid change, these findings further our understanding of how melt generated on glacier surfaces set the initial nutrient signature for subglacial and downstream environments. Plain Language Summary In polar regions most glaciers are frozen to their beds, meaning that meltwater from these glaciers comes from the surface, or "supraglacial" environment. Active biological communities exist in this supraglacial environment where they generate and recycle nutrients. However, it remains unclear whether nutrients are removed by the supraglacial streams, and what limits this removal process. We did experimental nutrient additions in three streams on Canada Glacier in the Taylor Valley of the McMurdo Dry Valleys, Antarctica. We found that nitrogen in the form of nitrate was quickly removed. In one of three locations, the addition of phosphorus stimulated more nitrogen removal than just adding nitrogen alone. We also found that at low nitrogen concentrations, the streams can be a net source of nitrogen. We confirmed these field experiment findings using laboratory nutrient incubation experiments on sediment collected from stream channels on Canada Glacier and two other glaciers in the Taylor Valley. Supraglacial streams act as a filter and can remove nitrogen from meltwater before it leaves the glacier and affect the quantity and type of nutrients transported to downstream ecosystems. This is important worldwide because glaciers are often the headwaters of the stream network.

Seasonal and Morphological Controls on Nitrate Retention in Arctic Deltas

“…TE is determined as: $$$\mathrm{T}\mathrm{E}={e}^{-{k}_{i}\times \tfrac{{w}_{i}{l}_{i}{d}_{i}}{{Q}_{i}}\times {\theta }^{T-20}}$$$ where w i , l i and d i are channel width, length, and depth in meters of link i , k i is the first order removal rate constant of nitrate estimated from literature based on spatially referenced regressions of contaminant transport on watershed attributes (SPARROW) regression (Alexander et al., 2000; Howarth et al., 1996; Smith et al., 1997), θ T− 20 is a simplified temperature‐dependent expression of the Arrhenius equation (Ambus, 1993; Boyer et al., 2006; Chapra, 1997) for observed values of temperature ( T ) at the delta apex (Holmes et al., 2012) in units of centigrade and expresses the temperature dependence of biologically mediated reactions across seasons and climate (Table 1). We express the aggregate effect of removal processes (e.g., denitrification, and plant uptake) and processes that add nitrate (e.g., nitrification) as a loss rate since removal processes often outweigh regeneration in estuarine environments (Delaune et al., 2005; Howarth et al., 1996; Seitzinger et al., 2006) including high latitude streams (Bergstrom et al., 2020; Blaen et al., 2014; Gooseff et al., 2004). Because the major mechanisms controlling in‐stream removal of nitrogen operate at the channel bed or hyporheic zone, deeper streams are modeled using lower magnitudes of processing rates, that is, k in Equation (Table 1; Howarth et al., 1996; Smith et al., 1997; Alexander et al., 2000; Howarth et al., 1996; Smith et al., 1997).…”

Glacial Water: A Dynamic Microbial Medium

“…It is unclear whether the englacial realm also has a role in nutrient and carbon transformation and is characterized by a microbial community specific to englacial pathways conditions [ 150 ]. Microbial nutrient cycling observed in fast-flowing supraglacial streams [ 49 ] suggests that microbial processes may be significant in fast-flowing englacial conduits. In addition, ice cores collected from englacial systems indicate that microorganisms are not quiescent but maintain an active metabolism [ 151 ].…”

“…Whereas an active englacial system (i.e., with flowing water) has been observed in a wide variety of glacial thermal regimes [36,47,48], temperate glaciers have the most developed water discharging system compared to the other glacier types due to the higher permeability and malleability of temperate ice [1]. Differences in permeability, therefore, lead to the development of different water discharge networks: e.g., because of the less developed englacial channel network in cold glaciers, water is discharged mainly through the supraglacial environment in most systems during the summer [49], where supraglacial features are more developed compared to those in the other ice systems [50].…”

Spatial Patterns of Major Ions and Their Relationship to Sediment Concentration in Near Surface Glacier Ice, Taylor Valley Antarctica

“…However, under the warmest conditions that may connect and drain cryoconite holes to the downstream system, they may only contribute 13% of total runoff (Fountain et al., 2004). Supraglacial ponds are more integrated into the drainage network where they do exist (Bagshaw et al., 2010), and transformations of meltwater chemistry can occur in the drainage network (Bergstrom, Gooseff, Singley, et al., 2020), but the regular glacier ice is the zone of the vast majority of meltwater generation, and thus should set the initial water chemistry, making it the focus of this study.…”