Phosphorus dynamics in lowland streams as a response to climatic, hydrological and agricultural land use gradients

Climate and hydrology are relevant control factors for determining the timing and amount of nutrient losses from agricultural fields to freshwaters. In this study, we evaluated the e ect of agricultural intensification on the concentrations, dynamics and export of phosphorus (P) in streams in two contrasting climate and hydrological 5 regimes (temperate Denmark and subtropical Uruguay). We applied two alternative nutrient sampling programmes (high frequency composite sampling and low frequency instantaneous-grab sampling) and three alternative methods to estimate exported P from the catchments. A source apportionment model was applied to evaluate the contribution derived from point and di use sources in all four catchments studied. Cli10 matic and hydrological characteristics of catchments expressed as flow responsiveness (flashiness), exerted control on catchment and stream TP dynamics, having consequences that were more significant than the outcome of di erent TP monitoring and export estimation strategies. The impact of intensification of agriculture di ered between the two contrasting climate zones. Intensification had a significant impact on subtropical 15 climate with much higher total (as high as 4436 µg P L 1 ), particulate, dissolved and reactive soluble P concentrations and higher P export (as high as 5.20 kg P ha 1 year 1 ). However, we did not find an increased contribution of particulate P to total P as consequence of higher stream flashiness and intensification of agriculture. The high P concentrations at low flow and predominance of dissolved P in subtropical streams ac20 tually exacerbate the environmental and sanitary risks associated with eutrophication. In the other hand, temperate intensively farmed stream had lower TP than extensively farmed stream. Our results suggest that the lack of environmental regulations of agricultural production has more severe consequences on water quality, than climatic and hydrological di erences between the analysed catchments. 25