The influence of a flood event on the potential sediment control of baseflow phosphorus concentrations in an intensive agricultural catchment

The growth of periphyton in streams is enhanced by phosphorus (P) in baseflow. The concentration of P in baseflow can be approximated by the equilibrium P concentration (EPC0) of bed sediments. However, sediment composition changes with spatial scale and flood events. We sampled sediments in a main stem and headwater tributary of an agricultural catchment in Reporoa, New Zealand before and after a scouring flood event (99th percentile of flows recorded since 1962). The tributary was chosen for its low slope and predominantly single landuse (intensive dairying), which minimised the number of factors likely to affect the influence of the storm event. EPC0 values were significantly correlated to dissolved reactive P (DRP) in baseflow before, and after, the flood event, despite a decrease in the proportion of fines and total P in bed sediments. Both EPC0 and DRP concentrations increased towards the catchment outlet. This increase likely reflected new P-enriched sediments from dairy-farm runoff, but hyporheic zone samples suggested that shallow groundwater may also have played a role in supplying P to the water column. Despite diel variations in dissolved oxygen, DRP concentrations showed little variation during the day and matched EPC0 estimates. This work suggests that EPC0 could be a useful measurement to reflect daytime baseflow-DRP concentrations at sites along a stream network despite changes in sediment composition due to flood events, but further work is required to clarify if sediment-P exchange or groundwater control baseflow-DRP concentrations.