Biogeochemical Dynamics During Snow Cover in a Montane Grassland Stream

The recent development of durable aquatic semi-continuous sensors has led to "The Renaissance of Hydrology" by enabling researchers to analyze sub-hourly observations of state variables. While sensor networks are currently being established across a wide range of fluvial systems, there has been limited focus on seasonally ice-covered streams. This gap of knowledge is of importance since many fluvial systems within high alpine and artic regions experience prolonged periods of ice and snow cover. During the winter of 2018 we were able to utilize an EXO 2 and a C-Sense semi-continuous sensor to capture data of key variables at 15-minute intervals at the East Fork Jemez River (EFJR) located in the Valles Caldera National Preserve in North Central New Mexico. During the 2018 the EFJR was ice-covered for approximately 4.5 months, and coupled with meteorological data we were able to observe the coupling between snow accumulation and a dissolved oxygen sag that depleted the oxygen to below 1 ppm for 47 days. We were also able to observe diurnal decoupling between dissolved oxygen, pH, and water temperature with inverse hysteresis loops occurring during initial ice cover. We believe this reflects the importance of snow and ice on the metabolic regime of seasonally ice-covered streams. Snowmelt was a major driver of biogeochemical dynamics with a 500% increase in fluorescence dissolved organic matter, a 60% decrease in specific conductivity concentrations, and a 1.2 increase in pH from pre-freeze concentrations. The dramatic winter changes to the biogeochemistry are an important part of the natural hydrologic cycle within seasonally ice-covered systems; however, climate models are predicting warmer winter in the Northern Hemisphere which will lessen the frequency and duration of ice and snow cover. This will have an overall impact on the entire ecohydrological system and can degrade critical environmental functions and services.