Riverine macrophytes control seasonal nutrient uptake via both physical and biological pathways |
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Authors: | Tenna Riis Jennifer L Tank Alexander J Reisinger Antoine Aubenau Kevin R Roche Peter S Levi Annette Baattrup-Pedersen Anette B Alnoee Diogo Bolster |
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Institution: | 1. Department of Bioscience, Aarhus University, Aarhus, Denmark;2. Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, USA;3. Soil and Water Sciences Department, University of Florida, Gainesville, FL, USA;4. Lyles School of Civil Engineering, Purdue University, West Lafayette, IN, USA;5. Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN, USA;6. Environmental Science and Policy, Drake University, Des Moines, IA, USA;7. WATEC, Aarhus University Center for Water Technology, Aarhus C, Denmark;8. Department of Bioscience, Aarhus University, Aarhus, Denmark
Department of Bioscience, Aarhus University, Silkeborg, Denmark |
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Abstract: | - In low-gradient, macrophyte-rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics. Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision-making and management outcomes.
- We explored the relationships among seasonal differences in reach-scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte-rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach-scale metabolism, and surface transient storage in spring, summer, and autumn.
- We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach-scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte-rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reach-scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake.
- In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short-circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers.
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Keywords: | nitrogen phosphorus river stream transient storage |
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