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

MARTÍ, EUGÈNIA; FEIJOÓ, CLAUDIA; VILCHES, CAROLINA; FERREIRO, NICOLÁS; GANTES, PATRICIA; RANIERI, CONSTANZA; TORREMORELL, ANA; RODRIGUEZ CASTRO, MARÍA CAROLINA; GULTEMIRIAM, MARÍA LOURDES; GIORGI, ADONIS; SABATER, FRANCESC

Freshwater Science

chicago press

Año: 2020 vol. 39 p. 268 - 280

2161-9549

In-stream nutrient retention is an important ecosystem function because it can regulate nutrient fate andexport to downstream ecosystems. Temporal variation in nutrient retention in streams has been studied extensivelyat the annual and seasonal scale but less thoroughly at the diel scale. However, understanding temporal variability innutrient uptake at the diel scale can increase understanding of the role of photoautotrophic primary production onnutrient uptake in streams, especially open-canopy streams. We hypothesized that nutrient retention mostly de-pends on autotrophic demand in open-canopy streams and that it varies following the diel pattern of gross primaryproduction (GPP). We therefore evaluated the temporal variation in phosphate (PO432) and ammonium (NH41) up-take at a daily scale in a highly-productive Pampean stream that is dominated by a dense assemblage of macrophytesand filamentous algae. We conducted 6 slug additions of PO432and NH41over a 24-h period and quantified reach-scale nutrient uptake concurrently with measurements of whole-stream metabolism and chemical variables duringadditions (including nitrates and nitrites). The study stream had extremely high uptake of PO432and NH41(>90 and>75% retention of the P and N mass added, respectively). Uptake of PO432did not vary throughout the day. EstimatedPO432uptake from GPP accounted for only a small fraction of observed PO432uptake. Thus, another mechanism,such as heterotrophic demand by microbial assemblages or adsorption onto sediments, could also have contributedto PO432uptake in the study stream. In contrast, NH41uptake clearly varied throughout the day. Up to 48% of theobserved NH41uptake rate could be explained by NH41estimated from GPP, and NH41demand was positively as-sociated with GPP, indicating a high dependence on photoautotrophic demand. An increase of nitrite (NO22) con-centration during additions (representing up to 70% of the added mass of NH41) suggests that nitrification contrib-uted to the diel pattern of NH41uptake. Our results indicate that nutrient uptake does not always rely on autotrophicdemand in open-canopy streams and that other abiotic and dissimilatory mechanisms may explain the diel patternsof nutrient retention. In addition, our study highlights the need to measure uptake metrics throughout the day toobtain an accurate estimate of nutrient retention on a daily scale.