Agricultural activities can affect the delivery of nutrients to streams, riparian canopy cover, and the capacity of aquatic systems to process nutrients and sediments. There are few measures of nutrient uptake and metabolism from tropical or subtropical streams in general, and even fewer from tropical regions of South America. We examined ammonium (NH4+) and soluble reactive phosphorus (SRP) retention in streams in Brazil and Argentina. We selected 12 streams with relatively little or extensive agricultural activity and conducted whole‐stream nutrient additions and measurements of gross primary production and ecosystem respiration. We used multiple linear regression to determine potential drivers of nutrient uptake metrics across the streams. Nutrient concentrations and retention differed significantly between land use categories. Both NH4+and SRP concentrations were higher in the agricultural sites (means of 161 and 495 μg l–1, respectively), whereas metabolic rates were slower and transient storage was smaller. Our analysis indicated that agriculture increased ambient uptake lengths and decreased uptake velocities. The regression models revealed that ambient SRP had a positive effect on NH4+uptake and vice versa, suggesting strong stoichiometric controls. Drivers for nutrient uptake in streams with low‐intensity agriculture also included canopy cover, temperature, and ecosystem respiration rates. Nutrient assimilation in agricultural sites was influenced by a higher number of variables (gross primary production for SRP, discharge, and transient storage for both nutrients). Our results indicate agricultural activity changes both the magnitude of in‐stream nutrient uptake and the mechanisms that control its variation, with important implications for South American streams under agricultural intensification.
Stream restoration efforts have aimed at increasing hydraulic residence time (HRT) and transient storage (TS) to enhance nutrient uptake, but there have been few controlled studies quantifying HRT and TS influences on nutrient uptake dynamics. We assessed the effects of HRT and TS on ammonium (NH4+) and phosphate (PO43−) uptake through controlled experiments in an artificial channel draining a pristine tropical stream. We experimentally dammed the channel with artificial weirs, to progressively increase HRT, and performed NH4+and PO43−additions to estimate uptake each time a weir was added. We also ran consecutive additions of NH4+and PO43−with no weirs, to evaluate short‐term changes in uptake metrics. Also, NH4+was injected alone to assess potential nitrification. We observed that NH4+and PO43−uptake rates were much greater in the very first addition, probably due to luxury uptake. The weirs increased mean HRT (from 8.5 to 12 min) and depth (from 6.5 to 8.9 cm) and decreased mean water velocity (0.40–0.28 m s−1). Surprisingly, damming decreased the relative size of transient storage zone (storage zone area/channel area,
- NSF-PAR ID:
- 10067342
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Ecohydrology
- Volume:
- 11
- Issue:
- 7
- ISSN:
- 1936-0584
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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