Hidden Processes During Seasonal Isolation of a High-Altitude Watershed

Abstract

Biogeochemical processes capable of altering global carbon systems occur frequently in Earth’s Critical Zone–the area spanning from vegetation canopy to saturated bedrock–yet many of these phenomena are difficult to detect. Observation of these processes is limited by the seasonal inaccessibility of remote ecosystems, such as those in mountainous, snow- and ice-dominated areas. This isolation leads to a distinct gap in biogeochemical knowledge that ultimately affects the accuracy and confidence with which these ecosystems can be computationally modeled for the purpose of projecting change under different climate scenarios. To examine a high-altitude, headwater ecosystem’s role in methanogenesis, sulfate reduction, and groundwater-surface water exchange, water samples were continuously collected from the river and hyporheic zones (HZ) during winter isolation in the East River (ER), CO watershed. Measurements of continuously collected ER surface water revealed up to 50 μM levels of dissolved methane in July through September, while samples from 12 cm deep in the hyporheic zone at the same location showed a spring to early summer peak in methane with a strong biogenic signature (<65 μM, δ13C-CH4, −60.76‰) before declining. Continuously collected δ18O-H2O and δ2H-H2O isotopes from the water column exhibited similar patterns to discrete measurements, while samples 12 cm deep in the hyporheic zone experienced distinct fluctuations in δ18O-H2O, alluding to significant groundwater interactions. Continuously collected microbial communities in the river in the late fall and early winter revealed diverse populations that reflect the taxonomic composition of ecologically similar river systems, including taxa indicative of methane cycling in this system. These measurements captured several biogeochemical components of the high-altitude watershed in response to seasonality, strengthening our understanding of these systems during the winter months.

Determining the biogeochemical transformations of organic matter composition in rivers using molecular signatures

“…Regardless of seasonal hyporheic flow and groundwater discharge, permanently saturated sediments are continuously influenced by solute flushing and hyporheic mixing. During upwelling events-as seen during low flow events-hyporheic zones oxygen availability is limited, driving the chemical and microbial community toward a more reduced environment which likely has a major influence on the carbon processing rate and products (Danczak et al, 2016;Saup et al, 2019;Buser-Young et al, 2021). Sediment PBT were not strongly correlated to surface water DO concentrations or the sediment respiration rates, suggesting groundwater is dominating leading to a reduced environment (Nelson et al, 2019).…”

Biogeochemical Dynamics of a Glaciated High‐Latitude Wetland

“…This relationship between methane concentrations and temperature is evidenced in the CRD by a buildup event in TS pond during a brief warming event in January, yet it is unlikely temperature had a significant influence on methane production as methane accumulation did not occur during the fall months that exceeded freezing temperatures. An increase in temperature may similarly affect hydrologic discharge from upstream glaciers and seasonal snowmelt, causing changes in the surface-subsurface exchange of available solutes (Buser-Young et al, 2021;Winnick et al, 2017). These differential concentration-discharge relationships provide the ponds with available organic matter for microbial mineralization, ultimately stimulating biotic metabolism in the sediments perhaps as observed by DelSontro et al (2016) in a system similar to the CRD.…”