A comparison between pulse and constant rate additions as methods for the estimation of nutrient uptake efficiency in-streams

SummaryNutrient retention efficiency, an indicator of stream function, has become a significant focus of research in fluvial biogeochemistry. The most popular method used to measure nutrient retention efficiency is constant rate addition (CRA), although studies using pulse addition (PA) as an alternative have been published. The similarities of the results produced by the two methods have not yet been explored. With the aim of clarifying this uncertainty, the present study assessed the similarities between the results of the CRA and PA experiments. Thirteen phosphate CRA–PA paired additions were performed in two different Mediterranean streams. We used ANOVA to compare the results of the uptake rate coefficients from the CRA (KtCRA) and the PA (KtPA) experiments. Results from CRA and PA were significantly different, with KtCRA being higher than KtPA in most cases, however, the difference between KtCRA and KtPA decreased as specific water residence time increased. When specific water residence time exceeded a specific threshold value, they became equal (KtCRA ≈ KtPA). Results suggested that the differences in nutrient retention estimates between the two methods may be a consequence of the difference in solute spreading along the stream reaches that shape the duration and magnitude of solute interaction across the surface water and streambed interface.