18 Oct 2019

18 Oct 2019

Review status: this preprint was under review for the journal BG but the revision was not accepted.

Phosphorus attenuation in streams by water-column geochemistry and benthic sediment reactive iron

Zachary P. Simpson1, Richard W. McDowell1,2, and Leo M. Condron1 Zachary P. Simpson et al.
  • 1Department of Soil and Physical Sciences, Lincoln University, Lincoln, 7647, New Zealand
  • 2AgResearch, Lincoln Science Centre, Lincoln, 7647, New Zealand

Abstract. Streams can attenuate inputs of phosphorus (P) and, therefore the likelihood of ecosystem eutrophication. This attenuation is, however, poorly understood, particularly in reference to the geochemical mechanisms involved. In our study, we measured P attenuation mechanisms in the form of (1) mineral (co-)precipitation from the water-column and (2) P sorption with benthic sediments. We hypothesized that both mechanisms would vary with catchment geology and, further, that P sorption would depend on reactive Fe content in sediments. We sampled 31 streams at baseflow conditions, covering a gradient of P inputs (via land use), hydrological characteristics, and catchment geologies. Geochemical equilibria in the water-column were measured and benthic sediments (< 2 mm) were analyzed for sorption properties and P and iron (Fe) fractions. Neither P-containing minerals (e.g., hydroxylapatite) nor calcite-phosphate co-precipitation had the potential to occur. In contrast, in-stream dissolved reactive P (DRP) correlated with labile sediment P (water-soluble and easily reduced Fe-P), but only for streams where hyporheic exchange between the water-column and the coarse sediment porewaters was likely sufficient. The non-labile P fractions contained most of sediment P (generally > 90 %) and varied with parent geology. Similarly, most sediment Fe was in a recalcitrant form (generally > 90–95 %). However, despite its small contribution to total sediment Fe, the pool of surface-reactive Fe was a strong predictor for sediment P sorption potential. Our results suggest that, in these streams, it is the combination of biogeochemical Fe and P cycles and the exchange with the hyporheic zone that attenuates DRP in baseflow. Such combinations are likely to vary spatiotemporally within a catchment and must be considered alongside inputs of P and sediment if the P concentrations at baseflow – and eutrophication risk – are to be well managed.

Zachary P. Simpson et al.

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Zachary P. Simpson et al.

Data sets

Sediment survey data Z. Simpson

Zachary P. Simpson et al.


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Short summary
Streams buffer phosphorus (P), making it difficult to know how mitigation measures work. We examined this P buffer across 31 diverse streams in Canterbury, New Zealand. We found catchment geology to be a major control for precipitation reactions in the water and the forms of P and iron (Fe) in sediments. Poorly crystalline Fe oxides in sediments predicted chemical sorption of P. These reactions may determine the P available for biota, and further, that Fe cycling is closely related to that of P.