Diel variation of nutrient retention is associated with metabolism for ammonium but not phosphorus in a lowland stream

In-stream nutrient retention is an important ecosystem function because it can regulate nutrient fate and export to downstream ecosystems. Temporal variation in nutrient retention in streams has been studied extensively at the annual and seasonal scale but less thoroughly at the diel scale. However, understanding temporal variability in nutrient uptake at the diel scale can increase understanding of the role of photoautotrophic primary production on nutrient uptake in streams, especially open-canopy streams. We hypothesized that nutrient retention mostly depends on autotrophic demand in open-canopy streams and that it varies following the diel pattern of gross primary production (GPP). We therefore evaluated the temporal variation in phosphate (PO43-) and ammonium (NH4+) uptake at a daily scale in a highly-productive Pampean stream that is dominated by a dense assemblage of macrophytes and filamentous algae. We conducted 6 slug additions of PO(4)(3-)and NH(4)(+)over a 24-h period and quantified reach-scale nutrient uptake concurrently with measurements of whole-stream metabolism and chemical variables during additions (including nitrates and nitrites). The study stream had extremely high uptake of PO(4)(3-)and NH4+(>90 and >75% retention of the P and N mass added, respectively). Uptake of PO(4)(3-)did not vary throughout the day. Estimated PO(4)(3-)uptake from GPP accounted for only a small fraction of observed PO(4)(3-)uptake. Thus, another mechanism, such as heterotrophic demand by microbial assemblages or adsorption onto sediments, could also have contributed to PO(4)(3-)uptake in the study stream. In contrast, NH(4)(+)uptake clearly varied throughout the day. Up to 48% of the observed NH(4)(+)uptake rate could be explained by NH(4)(+)estimated from GPP, and NH(4)(+)demand was positively associated with GPP, indicating a high dependence on photoautotrophic demand. An increase of nitrite (NO2-) concentration during additions (representing up to 70% of the added mass of NH4+) suggests that nitrification contributed to the diel pattern of NH(4)(+)uptake. Our results indicate that nutrient uptake does not always rely on autotrophic demand in open-canopy streams and that other abiotic and dissimilatory mechanisms may explain the diel patterns of nutrient retention. In addition, our study highlights the need to measure uptake metrics throughout the day to obtain an accurate estimate of nutrient retention on a daily scale.