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

Abstract

Purpose: The growth of periphyton in streams is enhanced by phosphorus (P) in baseflow. The likely control of P concentrations in baseflow can be approximated by the equilibrium P concentration (EPC₀) of bed sediments. However, sediment composition changes with spatial scale and flood events. It is unknown if this affects EPC₀. Materials and methods: 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 land use (intensive dairying), which minimised the number of factors likely to affect the influence of the storm event. Results and discussion: 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. Conclusions: This work suggests that despite changes in sediment composition due to flood events, EPC₀ is a useful reflection of daytime baseflow-DRP concentrations at sites along a stream network. However, further work is required to clarify if sediment-P exchange or groundwater control baseflow-DRP concentrations. These data also inform our understanding of the influence of sediment on delaying farm and catchment efforts to decrease in-stream DRP concentrations.