Neal, C.; Rowland, A.P.; Neal, M.; Morris, P.N.; Lead, J.R.; Lawlor, A.J.; Woods, C.; Vincent, C.; Guyatt, H.; Hockenhull, K.. 2012 Role of riverine colloids in macronutrient and metal partitioning and transport, along an upland-lowland land-use continuum, under low-flow conditions. Science of the Total Environment, 434. 171-185. 10.1016Environment, 434. 171-185. 10. /j.scitotenv.2011 Contact CEH NORA team at noraceh@ceh.ac.ukThe NERC and CEH trademarks and logos ('the Trademarks') are registered trademarks of NERC in the UK and other countries, and may not be used without the prior written consent of the Trademark owner. An integrated assessment is made of the role of riverine colloids in macronutrient (nitrogen, 21 phosphorus and carbon), metal and trace element partitioning and transport, for five rivers in the 22Ribble and Wyre catchments in north-western England, under baseflow/near-baseflow 23 conditions. Cross-flow ultrafiltration was used to separate colloidal (<0.45 >1kDa) and truly 24 dissolved (<1kDa) fractions from river water. Clear patterns were observed, along the upland-25 lowland land use continuum, in the partitioning and transport of macronutrients and metals 26 between the colloidal, truly dissolved and acid-available particulate (>0.45µm, suspended) 27 fractions. Of these operationally-defined fractions measured, colloids were generally more 28 important for both macronutrient and metal transport in the upland than in the lowland rivers. 29The results suggest that organic moieties in truly dissolved form from sewage effluent may have 30 a greater capacity to chelate metals. Organic-rich colloids in the upland moorlands and metal 31 oxide colloidal precipitates in the industrial rivers had a higher capacity for binding metals than 32 2 the colloidal fractions in the urban and agricultural lowland rivers. Aggregation of these colloids 1 may provide an important mechanism for formation of larger suspended particulates, accounting 2 for a much higher degree of metal enrichment in the acid-available particulate fractions of the 3 upland moorland and lowland industrial rivers than in the lowland agricultural and urban rivers. 4This mechanism of transfer of contaminants to larger aggregates via colloidal intermediates, 5 known as 'colloidal pumping' may also provide a mechanism for particulate P formation and the 6 high proportion of P being transported in the particulate fraction in the uplands. The cross-flow 7 ultrafiltration data also allowed refinement of partition coefficients, by accounting for colloids 8 within the solids phase and replacing the filtered (<0.45µm) fraction with the truly dissolved 9 (<1kDa) concentrations. These provided a clearer description of the controls on metal and P 10 partitioning along the upland-lowland continuum. 11 12