Hyporheic Oxygen Dynamics in the East River, Colorado: Insights From an In‐Situ, High Frequency Time Series During Two Distinct Flow Seasons.

Dissolved oxygen (DO) is critical for aquatic ecosystems, however, few studies have focused on the long‐term DO dynamics in hyporheic zones, which are a function of both transport (hydrologic exchange between river and hyporheic zone) and uptake by biogeochemical reactions or respiration. We explore the dynamics of temperature and DO at 10, 20, and 35 cm depth beneath the East River, Colorado, from July–October 2017 (relatively normal water year) and April to October 2018 (comparatively low flow year), enabled by distinctive, in‐situ, high frequency (Δt = 5 min) sensors that provided continuous time‐series from the undisturbed study site over 14 months. We expect that hyporheic DO, which has a regular daily fluctuation pattern, is supplied by the surface water (at all times we estimate downwelling) and that diurnal hyporheic DO temporal patterns should be aligned with diurnal hyporheic temperature patterns. However, this was not found to be the case. Hyporheic DO becomes depleted briefly at 20 and 35 cm depths in 2017, and at all three hyporheic depths for extended periods in 2018. Whereas diurnal temperature fluctuations have consistent timings of maxima and minima, hyporheic DO rarely has as regular a pattern, and daily ranges are inconsistent. Rainfall events caused some of these changes to diurnal hyporheic DO patterns without repeatable effects. Antecedent snowpack conditions influence streamflow dynamics and therefore hyporheic DO dynamics in this alpine river. These results also point to the strong and variable influence of hyporheic microbial communities regulating hyporheic DO. Plain Language Summary: Much like humans, river ecosystems rely on oxygen (in this case dissolved into river water) to remain healthy and function properly. River water is known to exchange through riverbed sediments (this is called the hyporheic zone) carrying dissolved materials and heat into the bed. In the East River, a subalpine river outside of Crested Butte, Colorado, we deployed sensors in the river and at 10, 20, and 35 cm into the riverbed to measure temperature and dissolved oxygen concentrations every 5 min from July 2017 to October 2018. These data indicate that the riverbed goes anoxic (no oxygen) for short periods (a few days) at 20 and 35 cm depths in 2017, a typical flow year. But in the low flow summer of 2018, anoxia persists through the entire riverbed from mid‐June to October. Temperature in the riverbed follows a repeated daily pattern of high and low values at all locations, but the daily fluctuation of dissolved oxygen in the riverbed does not. During rainstorms, riverbed dissolved oxygen is affected. These results indicate that hyporheic bacteria vary their consumption of oxygen day‐to‐day. Key Points: Subdaily to annual hyporheic dissolved oxygen dynamics revealed by continuous in‐situ measurements in an undisturbed riverbed for >14 monthsIn two contrasting flow seasons, hyporheic dissolved oxygen is strongly controlled by both river discharge and antecedent conditionsCompared to repeated daily and seasonal patterns of hyporheic temperature, dissolved oxygen patterns are heterogeneous, aperiodic [ABSTRACT FROM AUTHOR]