A monitoring program designed to identify the role of water and sediments in the transport of phosphorus (P) was conducted at two contrasting tributaries of the River Kennet, U.K. The River Lambourn flows through chalk with low seasonal variation in discharge (2 year range 0.7 – 3.0 cumecs) and extensive macrophyte beds. The River Enborne cuts through low permeability sands, silts and clays and had peaky, variable flow (2 year range 0.1 to 25 cumecs). Water, suspended sediment and bedload samples were collected under daily baseflow and hourly stormflow conditions for two years.

Soluble P fractions were diluted by high flows (SRP means 0.079 and 0.186 mg P/l, in the Rivers Lambourn and Enborne respectively). Conversely, suspended sediment (SS) (means 9 and 34 mg/l), particulate P (PP) (means 0.028 and 0.101 mg P/l) and bedload (means 65 and 378 g/m/week) increased in high flows. Sediment supply pathways, macrophyte trapping and bed armouring are suggested as important controls of P transport. Calculation of critical entrainment thresholds indicated that different flows transported different size fractions of the bedload. In the Enborne, bedload was dominated by fine sand-sized particles (0.250 – 0.063 mm). Preferential transport of finer material (<0.063 mm) occurred during storm events. In the Lambourn however, a more even distribution of particle sizes were observed with granule gravel and coarse sand (>0.250 mm) and clays (<0.038 mm) transported in baseflow conditions. These contrasts were thought to be a function of different sediment supply and critical shear velocities for the two rivers. Storm events in the River Enborne were more significant in the downstream transport of P than in the River Lambourn, especially during August to October when dry antecedent conditions were observed in the catchment. SS and PP behaved similarly during these events, displaying multiple peaks in concentration, indicating the importance of terrestrial flow pathways to the channel, bank erosion and bed sediment re-suspension.

Phosphorus concentrations in bedload (means of 1.1 and 1.5 mg P/g dry sediment, in the Rivers Lambourn and Enborne respectively) were generally lower than in SS (means of 1.6 and 2.0 mg P/g dry sediment). In addition, specific types of sediment particles tended to store P in the river channel (small particle size, large surface area, high Fe, Al, Mn, & Ca concentrations, high organic matter content and the presence of clays, feldspars & calcite). A novel SEM digital mapping overlay method found evidence for the retention of P by calcite co-precipitation and metal oxyhydroxide adsorption. Release of this stored P back into the water column appeared to occur concomitantly with increasing pH, temperature, soluble Ca concentration, re-suspension of sediment and decreasing oxygen concentration.

Overall, net retention of P in the River Lambourn and Enborne study reaches was observed (1.5 and 10.7 tonnes P/year, respectively) and the implications for this upon catchment management were reviewed. The complex set of storage, release and transformation mechanisms that were observed while P was in the river channels would have great biological significance for the aquatic system in which they operated.

E-mail: d.j.evans@qub.ac.uk