Stream solutes respond differently within and across flow conditions: a comparison of baseflow and higher flow events

Abstract

<p>Streams are dynamic ecosystems susceptible to frequent and long-term physical and chemical changes. Characterizing how solute concentrations change with hydrology is key to understanding solute sources, fate, and transport. Here we tested how solute concentrations respond to changes in flow in a stream draining a mixed land use catchment in Blacksburg, Virginia, USA. To do this, we measured how various solutes (i.e., DOC, DIN, Cl<sup>-</sup>, Na<sup>+</sup>, Mg<sup>+2</sup>, Ca<sup>+2</sup>, SO<sub>4</sub><sup>-2</sup>, K<sup>+</sup>) changed within and across one baseflow period of 24 hours and three high flow events during summer 2021. Solutes concentration relationship with flow dynamics can result in different responses: (1) enrichment (increase in concentration), (2) dilution (decrease in concentration), or (3) chemostasis (no change in concentration). We found that, overall, solutes responded to changes in flow and the patterns observed for each flow event were variable, resulting in both dilution and enrichment. Discharge (Q) ranged from 0.04 - 3.37 m<sup>3</sup>/s during our 8-week sampling period. Dissolved organic carbon (DOC) and dissolved inorganic nitrogen (DIN) concentrations ranged from 2 - 5.7 and 0.23 - 0.94 mg/L, respectively. While DOC exhibited enrichment with increasing Q, DIN, Cl<sup>-</sup>, Na<sup>+</sup>, Mg<sup>+2</sup>, Ca<sup>+2</sup>, SO<sub>4</sub><sup>-2</sup>, and K<sup>+</sup> were mainly diluted during higher flows. However, during baseflow conditions the relationship between Q and solute concentrations was more pronounced (R<sup>2 </sup>>0.30), particularly for DIN and SO<sub>4</sub><sup>-2 </sup>(dilution), and Cl<sup>-</sup> and Na<sup>+</sup> (enrichment). During higher flows, we did not see a general dilution or enrichment pattern for all solutes but there were solute-specific behaviors which were similar among sampling periods. The differences in Q-solute dynamics among the 4 sampling events supports the enhancement of hydrological connectivity and landscape influence during changes in flow and how it can contribute to changes in solute concentration. Additionally, Q-solute patterns observed highlight the importance of time and sampling frequency to develop a well characterization of solute dynamics during changes in flow. Ongoing work is focused on understanding the directionality and timing of responses to further inform changes in solute concentrations during different flow events. Analyses of solute-specific behavior, timing of peak concentrations, and directionality will broaden our understanding of solute chemical dynamics and the different factors that contribute to the variable responses that have been found.</p>