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Multi-decadal trajectory of riverine carbon, nitrogen and phosphorus dynamics in rural catchments

By Rémi Dupas, Gérard Gruau, Chantal Gascuel-Odoux and Camille Minaudo

Abstract

International audienceHuman activities have substantially altered riverine exports of carbon, nitrogen and phosphorus in the Anthropocene,causing major alterations of freshwater and marine ecosystems worldwide. Documenting anthropogenicalterations of carbon and nutrient cycles in aquatic ecosystems is a challenge as industrialization in westerncountries began in the 18th century and the green revolution in the mid-20th century, while river monitoringbecame widespread only since the 1990’s. Brittany - France, is one of the few noticeable exceptions of a regionwith multi-decadal datasets of nitrate (NO3-), soluble reactive phosphorus (SRP), total phosphorus (TP) anddissolved organic carbon (DOC) in meso-scale (< 5000 km2) rural catchments.Here we analyze these multi-decadal and multi-parameter time series with the objectives to investigate: i)long term trajectory of C, N, P concentrations in relation to the history of diffuse and point source pressuresin seven catchments since the 1970’s, ii) medium term variations in relation to inter-annual climate variability(during 5-year wet-dry cycles) and iii) seasonal variations (and their long term evolution) in relation to catchmentsproperties and history. Statistical analyses include a seasonal-trend decomposition procedure based on Loess(STL) and long term input-output hysteresis plots.Results show that long term NO3- concentration is controlled by the agricultural N surplus with a hysteresiseffect, indicating a time lag of approximately 10 years between decrease in N inputs and decrease in NO3-loads. Long term SRP and TP concentrations are mainly controlled by point source emissions that have decreasedcontinuously since the 1980’s. Long term DOC concentration shows little variation contrary to many catchmentsof the northern hemisphere. Medium term inter-annual NO3- concentration is controlled by discharge, with anincrease in concentrations during wet years and a hysteresis effect indicating the flushing of a pool of nitrate storedin the catchments during antecedent drier years. Medium term SRP concentration is also controlled by discharge,with a dilution during wet years supporting the hypothesis of point-source dominated emissions. Medium termvariations in TP and DOC concentrations are less clear, possibly because of opposite flushing and dilution effectsduring wet years. Seasonal variations exhibited winter maxima for NO3- and summer maxima for SRP, TP andDOC, confirming the dominant diffuse nature of NO3- emissions, the point-source dominated P emissions andpossibly autochthonous DOC production during the summer period.Despite the study catchments being of relatively small size (median 800 km2), they were large enough tocomprise a variety of point and diffuse sources which were not monitored individually, and the effect of hillslopeprocesses could often not be disentangled from in stream processes. Therefore, future work will focus oncombining monitoring in smaller headwater catchments, where few sources are present and in stream processesminimal, and in downstream stations representing the integrated signal discharging into the se

Topics: [SDU.STU]Sciences of the Universe [physics]/Earth Sciences
Publisher: HAL CCSD
Year: 2018
OAI identifier: oai:HAL:insu-01767764v1
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