title: River phosphorus cycling: separating biotic and abiotic uptake during short-term changes in sewage effluent loading.
abstract: Medium to small scale point sources continue to threaten river ecosystems through P loadings. The capacity and timescales of within-river processing and P retention are a major factor in how rivers respond to, and protect downstream ecosystems from, elevated concentrations of soluble reactive P (SRP). In this study, the bio-geochemical response of a small river (approximately 40 km(2) catchment area) was determined before, during and after exposure to a fourteen day pulse of treated sewage effluent using an upstream reach as a control. A wide array of approaches (batch and column simulations to in-situ whole stream metabolism) allowed independent comparison and quantification, of the relative contribution of abiotic and biotic processes in-river P cycling. This enabled, for the first time, separating the relative contributions of algae, bacteria and abiotic sorption without the use of labelled P (radioisotope). An SRP mass balance showed that the ecosystem switched from a P sink (during effluent inputs) to a P source (when effluent flow ceased). However, 65-70% of SRP was retained during the exposure time and remained sequestered two-weeks after-effluent flow ceased. Batch studies treated with biocide gave unrealistic results, but P uptake rates derived by other methods were highly comparable. Downstream of the effluent input, net P uptake by algae, bacteria and sediment (including the biofilm polysaccharide matrix) were 0.2 (+/-0.1), 0.4 (+/-0.3), and 1.0 (+/-0.9) mmol m(-2) day(-1) during effluent exposure. While autotrophic production did not respond to the effluent exposure, heterotrophic production increased by 67% relative to the control and this translated into a 50% increase in biological P uptake rate. Therefore, both biological and abiotic components of stream ecosystems uptake P during exposure to treated sewage effluent P inputs, and maintain a long 'memory' of this input in terms of P storage for considerable timescales after loading.
authors.affiliation: The Macaulay Institute, Craigiebuckler, Aberdeen AB15 8QH, UK. m.stutter@macaulay.ac.uk
authors.forename: M I
authors.initials: MI
authors.lastname: Stutter
authors.forename: B O L
authors.initials: BO
authors.lastname: Demars
authors.forename: S J
authors.initials: SJ
authors.lastname: Langan
mesh_terms: D001419:Bacteria / Q000254:growth & development / Q000378:metabolism; D017753:Ecosystem; D004784:Environmental Monitoring / Q000379:methods; D056890:Eukaryota / Q000254:growth & development / Q000378:metabolism; D019015:Geologic Sediments / Q000032:analysis / Q000737:chemistry; D010758:Phosphorus / Q000032:analysis*; D045483:Rivers / Q000737:chemistry*; D012722:Sewage / Q000032:analysis* / Q000737:chemistry; D013997:Time Factors; D014871:Water Microbiology; D014874:Water Pollutants, Chemical / Q000032:analysis*
chemical_list: D012722:Sewage; D014874:Water Pollutants, Chemical; D010758:Phosphorus