Estimation of the natural background of phosphate in a lowland river using tidal marsh sediment cores

Abstract. Elevated phosphate (PO4) concentrations can harm the ecological status in water by eutrophication. In majority of surface waters lowland regions such as Flanders (Belgium), local PO4 levels exceed limits defined environmental policy and fail to decrease, despite decreasing total phosphorus (P) emissions. order underpin definition current limits, this study was set up identify pre-industrial background concentration Scheldt River, a tidal river Flanders. We used sedimentary records preserved marsh sediment cores an archive for reconstructing historical changes PO4. For samples at sequential depths below surface, we dated time deposition analysed extractable P. The resulting series P linked measured water-PO4 (data 1967–present). By combining those datasets, sorption characteristics could be described using Langmuir-type model. calibrated model allowed us estimate levels, based on deeper that stabilized smaller than modern. three out four cores, peaked around 1980, coinciding with estimated (?1800) River 62 [57; 66 (95 % CI)] µg PO4-P L?1. That exceeds previously natural values (15–35 TP L?1) is about half prevailing limit (120 L?1). 1930s, were 140 [128; 148] L?1, already exceeding limit. method developed here proved useful river. A similar approach apply other rivers provide scientific basis catchment-specific backgrounds.