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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 (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.

Indicators of nutrient removal efficiency for riverine wetlands in agricultural landscapes of Argentine Pampas.

Main objectives of this study were (a) to assess wetlands contribution to regulation of surface water quality of riverine wetlands in agricultural landscapes through their nutrient removal efficiency (RE), (b) to understand how RE of wetlands is related to hydrological, morphological, chemical and biological attributes, and (c) to identify RE indicators suitable for remote RE assessment. Macrophytes composition, hydrological, chemical, and morphological properties were estimated for 14 riverine wetlands of the Argentinean Pampas, and related to empirically estimated removal-exportation levels of phosphorus (dissolved and total) and nitrogen (inorganic and total). Nutrient inputs and outputs were assessed in four opportunities, two under baseline and two after storm events. A discriminant function based on remotely assessed wetland attributes was able to discriminate three wetland groups according to their contrasting mean RE for total phosphorus and total nitrogen. Descriptors of wetland size (area, length, perimeter) and vegetation (cover of the tall emergent macrophytes) showed the main weights and hence the main value as indicators for conservation and/or management of wetlands according to their nutrient removal capacities.

Areal changes of lentic water bodies within an agricultural basin of the Argentinean pampas. Disentangling land management from climatic causes.

Wetland loss is a frequent concern for the environmental management of rural landscapes, but poor disentanglement between climatic and land management causes frequently constrains both proper diagnoses and planning. The aim of this study is to address areal changes induced by non-climatic factors on lentic water bodies (LWB) within an agricultural basin of the Argentinean Pampas, and the human activities that might be involved. The LWB of the Mar Chiquita basin (Buenos Aires province, Argentina) were mapped using Landsat images from 1998-2008 and then corrected for precipitation variability by considering the regional hydrological status on each date. LWB areal changes were statistically and spatially analyzed in relation to land use changes, channelization of streams, and drainage of small SWB in the catchment areas. We found that 12 % of the total LWB in the basin had changed (P < 0.05) due to non-climatic causes. During the evaluated decade, 30 % of the LWB that changed size had decreased while 70 % showed steady increases in area. The number of altered LWB within watersheds lineally increased or decreased according to the proportion of grasslands replaced by sown pastures, or the proportion of sown pastures replaced by crop fields, respectively. Drainage and channelization do not appear to be related to the alteration of LWB; however some of these hydrologic modifications may predate 1998, and thus earlier effects cannot be discarded. This study shows that large-scale changes in land cover (e.g., grasslands reduction) can cause a noticeable loss of hydrologic regulation at the catchment scale within a decade.