Assessing the potential and kinetics of coupled nutrients uptake in mesotrophic streams in Chaohu Lake Basin, China

Interactions among multiple nutrients uptake certainly have a great effect on their retention in headwater streams, yet little research has been made to explore the quantitative characteristics of their interactions, especially in mesotrophic streams. In response, we conducted an identical series of instantaneous nutrient addition experiments, using ammonium nitrogen (NH 4 -N) and phosphate phosphorus (PO 4 -P) alone or together, in two mesotrophic agricultural headwater streams in Chaohu Lake Basin, China, to quantify the relationships between nutrient concentrations and uptake rates, and examine how NH 4 -N and PO 4 -P interact to affect their individual uptake. Both the Michaelis-Menten (M-M) equation and response surface model were utilized to analyze coupled NH 4 -N and PO 4 -P uptake patterns across a range of nutrient concentrations, by fitting the kinetic processes of NH 4 -N and PO 4 -P uptake in single- and dual-nutrient additions. The capacity of both NH 4 -N and PO 4 -P uptake was increased in different degrees in dual-nutrient additions. Response surface models could quantitatively characterize the three-dimensional dynamic evolution trend of NH 4 -N or PO 4 -P uptake rates at different concentrations. The influence of PO 4 -P additions on NH 4 -N uptake was generally greater than that of NH 4 -N on PO 4 -P uptake in the five tracer tests. In addition, results of correlation analysis indicated that water temperature might be the main factor affecting the coupling of N and P uptake in mesotrophic streams and followed by hydrological factors (e.g., discharge) and channel geomorphology.