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Fluid transit time is understood to be an important control on the shape of concentration-discharge (C-q) relationships, yet empirical evidence supporting this linkage is limited. We investigated C-q relationships for weathering-derived solutes across seven Antarctic glacial meltwater streams. We hypothesized that (H1) solute fluxes in McMurdo Dry Valley streams are reaction limited so that C-q relationships are characterized by dilution and that (H2) transit time explains between-stream variability in the degree of C-q dilution. Results show that C-q relationships are chemostatic because solute equilibrium times are shorter than stream corridor fluid transit times. Between-stream variability in the efficiency of solute production is positively correlated with transit time, suggesting that transit time is an important control on the solute export regime. These results provide empirical evidence for the controls on weathering-derived C-q relationships and have important implications for Antarctic ecosystems and solute export regimes of watersheds worldwide.

Plain Language Summary Relationships between solute concentration and stream discharge (i.e., streamflow) rates in rivers contain important information regarding how water moves through a watershed. In this research, we use a combination of observations and mathematical modeling to show that the shape of concentration-discharge relationships is related to hydrologic transit time, the duration of time a parcel of water spends moving through the watershed. Because the time scales of chemical reactions are shorter than hydrologic transit times, solute concentration is invariant with discharge. Results of this work empirically corroborate previous theoretical work regarding the physical controls on concentration-discharge relationships, and have important implications for understanding chemical concentration patterns in rivers worldwide.