08 Apr 2022
08 Apr 2022
Status: this preprint is currently under review for the journal HESS.

Seasonal variation and release of soluble reactive phosphorus in an agricultural upland headwater in central Germany

Michael Rode1,2, Jörg Tittel3, Frido Reinstorf4, Michael Schubert5, Kay Knöller6, Benjamin Gilfedder7, Florian Merensky-Pöhlein4, and Andreas Musolff8 Michael Rode et al.
  • 1Department Aquatic Ecosystem Analysis, UFZ - Helmholtz-Centre for Environmental Research, Brückstr. 3a, 39114 Magdeburg, Germany
  • 2Institute of Environmental Science and Geography, University of Potsdam, Potsdam-Golm, Germany
  • 3Department of Lake Research, UFZ - Helmholtz-Centre for Environmental Research Brückstr. 3a, 39114 Magdeburg, Germany
  • 4Department for Water, Environment, Construction and Safety, Magdeburg-Stendal University of Applied Sciences, Breitscheidstr. 2, 39114 Magdeburg, Germany
  • 5Department Catchment Hydrology,UFZ - Helmholtz Centre for Environmental Research, Permoserstr. 15, 04318 Leipzig, Germany
  • 6Department Catchment Hydrology UFZ - Helmholtz Centre for Environmental Research, Theodor-Lieser-Str. 4, 06120 Halle, Germany
  • 7University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth, Germany
  • 8Department of Hydrogeology, UFZ - Helmholtz-Centre for Environmental Research GmbH, Leipzig, Germany

Abstract. Soluble reactive phosphorus concentrations (SRP) in agricultural headwaters can display pronounced seasonal variability at low flow, with the highest concentrations occurring in summer. These SRP concentrations often exceed eutrophication levels but their main sources, spatial distribution, and temporal dynamics are often unknown. The purpose of this study is therefore to differentiate between potential SRP losses and releases from soil drainage, anoxic riparian wetlands and stream sediments in an agricultural headwater. To identify the dominant SRP sources we carried out three longitudinal stream sampling campaigns on SRP fluxes. We used salt dilution tests and 222Rn to determine water fluxes in different sections of the stream, and carried out specific sampling for SRP, iron and 14C-DOC to examine possible redox-mediated mobilization from riparian wetlands and stream sediments. The results indicate that a single short section in the upper headwater reach was responsible for most SRP losses to the stream. Analysis of samples taken under summer low flow conditions revealed that the stream-water SRP concentrations, SRP-fraction for dissolved P (DP) and DOC radiocarbon ages matched those in the groundwater entering the gaining section. We argue that the seasonal variation of SRP concentrations was mainly caused by variations in the proportion of groundwater present in the streamflow, and was thus highest during summer low flow periods. Stream-sediment pore water showed evidence of reductive mobilization of SRP but the exchange fluxes were probably too small to contribute substantially to SRP stream concentrations. Examination of the combined results of this campaign and previous monitoring confirms that groundwater is also the main long-term contributor of SRP at low flow and that seepage from agricultural phosphorous is largely buffered in the soil zone. In this headwater, stream SRP loading during low flow is therefore mainly geogenic, while agricultural sources play only a minor role in SRP loading, with the dominant SRP sources being the local Paleozoic greywacke and Devonian shale. Because it is also possible for similar seasonal SRP dilution patterns to be generated by enhanced mobilization in riparian zones or wastewater inputs, precise knowledge of the different input pathways is important to the choice of effective management measures.

Michael Rode et al.

Status: open (until 15 Jun 2022)

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  • RC1: 'Comment on hess-2022-126', Anonymous Referee #1, 10 May 2022 reply

Michael Rode et al.

Michael Rode et al.


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Short summary
Agricultural catchments often show elevated phosphorus concentrations during summer low flow. In a typical agricultural headwater we found out that geogenic phosphorus in groundwater was the major source of stream water phosphorus during these low-flow conditions and that stream sediments derived from farmland are unlikely to have increased stream phosphorus concentrations during low water. Agricultural land use was not the main cause of P loading in the stream during low-flow conditions.