On the Emergence of Diel Signals in Flowing Waters

Abstract:

Recent advances in in-situ sensors have revolutionized the frequency and precision at which we are able to observe in-stream solute signals, across a broad range of solutes. Despite the coupling implied by autotroph stoichiometry, assimilatory driven diel signals are more common for some solutes, such as DO, than for others such as NO₃^- or PO₄^-. Here we present reactive transport models, calibrated from field observations, which demonstrates that this primarily arises because gaseous solutes such as DO are continuously equilibrating with the atmosphere. This limits the distance over which upstream signal variation driven by watershed processes or discontinuities such as tributary confluences persists, facilitating the emergence of the diel signal due to in-stream biological processing. For non-gaseous solutes the persistence of upstream influence obscures the emergence of the in-stream signal, which must be extracted using a two-station approach utilizing the change in concentration over a finite reach. However, proper application of the two-station method must still account for hydraulic transformation (primarily mixing through dispersion and transient storage zone retention) of the signal rather than assuming plug flow. As the array of solute signals we are capable of detecting continues to expand with advances in sensor technology, rigorous consideration of the processes (both biologic and hydraulic) responsible for signal emergence and downstream evolution is needed. Failure to account for differences in signal emergence across solute type, as well as hydraulic transformations of the signal, may result in incorrect inferences regarding the magnitude and temporal coupling of assimilatory processing of solutes.