Fractional-derivative model simulations of reach-scale uptake and transport dynamics of natural fluorescent dissolved organic matter in a temperate forested stream in southeastern US

Dissolved organic matter (DOM) entering the river networks affects aquatic ecosystems, yet it remains difficult to quantify the strength of various environmental factors in controlling reach-scale uptake and transport of DOM. This study combined reach-scale field experiments and stochastic modeling to investigate the uptake and transport dynamics of natural fluorescent DOM (FDOM) in a forested coastal plain stream located in the southeastern U.S. FDOM from litter leachate and sodium chloride (NaCl) were injected into the stream under base flow conditions (with different flow rates) on three separate occasions over a 3-month period. Field measurements showed that FDOM and NaCl exhibited different anomalous dynamics characterized by the peak and tailing behaviors of concentration breakthrough curves (BTC), indicating that the transport of FDOM was regulated more by physical retention and/or biogeochemical recycling than that of NaCl. The preferential removal of humic fluorescence than protein fluorescence before the BTC peak suggests that physical sorption was more important than biological degradation in early FDOM removal at the field site. A tempered fractional advection-dispersion-reaction model (TFADRM) was proposed to describe the FDOM migration process and estimate the FDOM spiraling metrics, and the resultant metrics were compared to those from conventionally used BTC-integrated approach. Results revealed that solute retention can lower the peak concentration and enhance the late-time tail of the FDOM BTC, whereas irreversible biogeochemical reaction processes can consume FDOM and decrease the BTC peak. Relative to the commonly used OTIS model, TFADRM can better capture the observed BTC late-time tail by considering multiple rates for the mass exchange/uptake of FDOM in streams, and therefore, this study provided a more reasonable estimate of FDOM spiraling metrics and transport dynamics at the reach scale.

Automated summary

This study investigated the uptake and transport dynamics of FDOM in a forested coastal plain stream through reach-scale field experiments and stochastic modeling, revealing that the transport of FDOM is regulated more by physical retention and/or biogeochemical recycling. The tempered fractional advection-dispersion-reaction model (TFADRM) proposed in the study provided a more reasonable estimate of FDOM spiraling metrics and transport dynamics at the reach scale compared to conventionally used approaches.