Dynamic changes in phosphorus transport in a river system traversing rural and urban landscapes in Christchurch, New Zealand : A thesis submitted in partial fulfilment of the requirements for the Degree of Master at Lincoln University

Abstract

Phosphorus (P) can degrade water quality and the biodiversity of lotic and lentic environments by accelerating the growth of aquatic plants and phytoplankton. The nutrient enters and moves in natural waters in dissolved and particulate forms. The former is more available for uptake by biota and the latter can serve as a long-term reservoir when settled. Partitioning between these forms can be highly variable over time and depends on river flow conditions. The relationship between flow and P form can be used to infer the spatial origin of the source and the dominant P transport pathways; thus, it is essential to assess the fate and behaviour of P forms in the aquatic system. This study investigated the partitioning of P between dissolved and particulate forms in a river that traverses rural and urban areas and examined how well their transport can be measured under variable environmental conditions. Phosphorus and other water quality variables were measured at fortnightly intervals at seven locations along the Cashmere and Heathcote rivers using active and passive sampling methods. Additional sampling was done during high flow and rain events and an autosampler was used to collect high-frequency (two-hourly) samples at one location. Results show that total P (TP) concentrations increased with increased flow. Total P loads were also significantly higher under high flows. The P fraction and its relationship to the river flow varied across different land uses. Dissolved P was more sensitive to changes in flow especially in the Cashmere Stream that is a rural tributary with agricultural land use. Dissolved P was the dominant P fraction both under low and high flows based on the grab sampling method. However, the contribution of PP to the total P load was more evident in high frequency sampling during rain events. Particulate P increased rapidly during rainfall events with response characteristics depending on land use. Overall, the results reveal the complexity of sources, transport pathways and the factors affecting P loss in Cashmere Stream and the Heathcote River. Given the large amount of P being mobilised into the river, especially during rain events via erosion or surface runoff, efforts to adopt high frequency monitoring are recommended to improve estimates of P loads downstream and to inform strategies to minimize P losses into the river.