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

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

Freshwater Science

The Society for Freshwater Science

Lugar: Chicago; Año: 2020 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 dependson autotrophic demand in open-canopy streams and that it varies following the diel pattern of gross primaryproduction (GPP).Wetherefore evaluated the temporal variation in phosphate (PO432) and ammonium (NH41) uptakeat 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 PO432 and NH41 over a 24-h period and quantified reachscalenutrient uptake concurrently with measurements of whole-stream metabolism and chemical variables duringadditions (including nitrates and nitrites). The study stream had extremely high uptake of PO432 andNH41 (>90 and>75% retention of the P andNmass added, respectively). Uptake ofPO432 did not vary throughout the day. EstimatedPO432 uptake from GPP accounted for only a small fraction of observed PO432 uptake. Thus, another mechanism,such as heterotrophic demand by microbial assemblages or adsorption onto sediments, could also have contributedto PO432 uptake in the study stream. In contrast, NH41 uptake clearly varied throughout the day. Up to 48% of theobserved NH41 uptake rate could be explained by NH41 estimated from GPP, and NH41 demand was positively associatedwith GPP, indicating a high dependence on photoautotrophic demand. An increase of nitrite (NO22) concentrationduring additions (representing up to 70% of the added mass of NH41) suggests that nitrification contributedto the diel pattern ofNH41 uptake. 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.