Channelizing Streams for Agricultural Drainage Impairs their Nutrient Removal Capacity

In agricultural basins, fluvial ecosystems can work as filters when retaining the nutrient excess from agricultural activities, mitigating the impacts downstream. In frequently flooded areas, like the Pampas Region of Argentina, natural streams are being channelized to reduce flood frequency and intensity, thus increasing land suitability for crop production, but the impact of these interventions on nutrient removal capacity by streams is unknown. To evaluate the effects of channelizing streams on the assimilation rate of nitrate, ammonia, and phosphorus, nutrient addition experiments were performed in streams of the southern Pampas under three different conditions: (i) channelized reaches without Schoenoplectus californicus (C.A. Mey.) Palla (reeds), (ii) unchannelized reaches without reeds, and (iii) unchannelized reaches with reeds. Assimilation rates were estimated by applying the one‐dimensional transport with inflow and storage (OTIS) model, which considers the solute transport with lateral flow and storage. Nitrate and ammonia uptake rates were higher in unchannelized than in channelized stream reaches, and a higher nitrate assimilation rate was found in the presence of reeds, indicating an important role of this macrophyte in the nitrate uptake. In the case of phosphorous, uptake rates were higher in unchannelized reaches with reeds than in the channelized reaches. These results suggest that channelizing first‐order streams in agricultural landscapes of the Argentine Pampas may significantly reduce the ability of streams to mitigate nutrients loss to continental and marine water sinks. Core Ideas Channelizing streams significantly reduces their capacity of nutrient removal. Assimilation velocity and areal uptake rate (U) were the main affected parameters. Median reductions of nitrogen U due to channelization varied between 34 and 86%. Median reductions of phosphorus U due to channelization varied between 63 and 75%. Nutrients removal in nonchannelized streams is enhanced by reed cover.