Abstract
Abstract. Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. In order to underpin the definition of current limits, this study was set up to identify the pre-industrial background PO4 concentration in surface water of the Scheldt River, a tidal river in Flanders. We used the sedimentary records preserved in tidal marsh sediment cores as an archive for reconstructing historical changes in surface water PO4. For sediment samples at sequential depths below the sediment surface, we dated the time of sediment deposition and analysed the extractable sediment P. The resulting time series of sediment P was linked to the time series of measured surface water-PO4 concentrations (data 1967–present). By combining those datasets, the sorption characteristics of the sediment could be described using a Langmuir-type sorption model. The calibrated sorption model allowed us to estimate a pre-industrial background surface water PO4 levels, based on deeper sediment P that stabilized at concentrations smaller than the modern. In three out of the four cores, the sediment P peaked around 1980, coinciding with the surface water PO4. The estimated pre-industrial (∼1800) background PO4 concentration in the Scheldt River water was 62 [57; 66 (95 % CI)] µg PO4-P L−1. That concentration exceeds the previously estimated natural background values in Flanders (15–35 µg TP L−1) and is about half of the prevailing limit in the Scheldt River (120 µg PO4-P L−1). In the 1930s, river water concentrations were estimated at 140 [128; 148] µg PO4-P L−1, already exceeding the current limit. The method developed here proved useful for reconstructing historical background PO4 concentrations of a lowland tidal river. A similar approach can apply to other lowland tidal rivers to provide a scientific basis for local catchment-specific PO4 backgrounds.
Highlights
Excess phosphorus (P) concentrations in surface waters is a global problem (Azevedo et al, 2015; Dodds and Smith, 2016; Elser et al, 2007)
In the majority of surface waters in lowland regions such as Flanders (Belgium), the local PO4 levels exceed the limits defined by environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions
In all soil cores starting at the surface, the degree of P saturation (DPS) and Pox increased with depth and peaked at about 0.5 m below the surface
Summary
Excess phosphorus (P) concentrations in surface waters is a global problem (Azevedo et al, 2015; Dodds and Smith, 2016; Elser et al, 2007). Eutrophication by excess nutrients, including phosphate (PO4) and nitrogen (N), can lead to hypoxia, acidification, and harmful algal blooms (Azevedo et al, 2015; Correll, 1998; Watson et al, 2018). Limiting P concentrations in the surface water is crucial to ensure a good ecological status. Lowland river systems are at higher risk for eutrophication than upland streams (Watson et al, 2018). Eutrophication of lowland rivers is on the international agenda (Jarvie et al, 2006; Mainstone and Parr, 2002; Reynolds, 2000).
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