Sediment and water-column phosphorus chemistry in streams at baseflow across varying catchment geologies

Streams can attenuate inputs of phosphorus (P) and therefore dampen the likelihood of ecosystem eutrophication. This P attenuation derives from many processes and remains poorly understood, particularly in reference to the geochemical mechanisms involved. We studied P attenuation in the form of (1) potential for mineral (co-)precipitation from the water column and (2) P sorption with benthic sediments. We hypothesized that both mechanisms vary with catchment geology and that P sorption depends on sediment reactive iron (Fe). We sampled 31 streams at baseflow, 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 were analyzed for sorption properties and P and Fe fractions. Neither P-containing minerals nor calcite-phosphate co-precipitation had the potential to form. However, in-stream dissolved reactive P (DRP) correlated with labile sediment P (water soluble and easily reduced Fe-P) for streams where hyporheic exchange between the water column and sediment porewaters was likely sufficient. Because this labile P was associated with poorly crystalline Fe oxides, which determined P sorption capacity, we observed that more sorptive sediments were positively related to DRP concentrations. Our results suggest that DRP attenuation in these streams at baseflow depends in part on the combination of biogeochemical Fe and P cycles and the hydrological exchange with the hyporheic zone. Such combinations will likely 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.