Stream morphology controls ammonium retention in tropical headwaters

Ammonium retention was studied in 12 tropical headwater streams differing in channel morphology and hydraulic characteristics. Results from NaCl and NH4Cl injections combining a one-dimensional transport model with the nutrient spiraling concept were evaluated. Transient solute storage was large in swamp reaches, with ratios of storage zone and main channel cross-sectional area (As/A) of up to 18, and fractions of residence time due to transient storage (Fmed 200) of up to 94%. Moreover, transient storage was at an intermediate level in step-pool and meandering reaches and low in run reaches. Maximum ammonium uptake potentials (UMAX) varied by a factor of 15 due to differing stream morphologies, and ammonium retention was high in swamp and step-pool reaches, with UMAXvalues as high as 1.8 mg N· m-2· min-1. Between 52% and 85% of ecosystem retention occurred in storage zones. Additionally, differences in ammonium uptake efficiency in the storage zones of contrasting stream morphotypes were negligible; hence, ecosystem ammonium uptake potential was determined by storage zone size. Interestingly, surface transient storage, which is usually neglected in stream tracer studies, was both an important transient storage mechanism and a strong determinant of ammonium uptake potentials. Here, we demonstrated that channel morphology controlled transient storage and ammonium retention potentials in these tropical ecosystems.