Biogeochemical functional responses to flow rate in a low order stream: implications for water quality monitoring

In running waters, the concentration of components that define water quality can be subjected to ample fluctuations quantitatively linked to flow rate. If not properly considered, such variability may hinder assessment of the evolution of water quality, of the effects of management actions, and ultimately the understanding of processes driving water quality. The functional response to flow rate was characterized for multiple biogeochemical variables in a pristine, low order stream. Variability of responses spanned between a factor of 2 and > 34, and in all cases were associated to flow rate according to one of three patterns: positive asymptotic (for variables: seston, suspended particles, total nutrients, dissolved and particulated organic matter, dissolved inorganic nitrogen), negative asymptotic (conductivity and dissolved reactive silicon), and humped (dissolved inorganic phosphorous). Building on those results, a rationale is presented for an unambiguous, cost-effective approach to water quality evaluation in running systems with predominantly diffuse sources.