Nutrient Concentrations in the Rivers of the Southern Alps: A Proxy Indicator for Reference Water Quality Conditions in New Zealand

Abstract

Large-scale transformation of vegetation cover in New Zealand since human occupation has restricted indigenous forest cover to upland and remote regions. The waterways of these remote and undisturbed catchments are typically excluded from regular assessments of water quality, and thus, there is a paucity of longitudinal data from pristine waterways in New Zealand. This thesis presents a two-year study of the water quality of 71 low disturbance catchments draining the Southern Alps. Catchments were sampled every 3 to 6 months from January 2012 to November 2014. In these pristine catchments, phosphate phosphorus and ammoniacal are usually at, or below, detectable limits. The nitrate nitrogen, dissolved inorganic carbon, dissolved organic carbon and silicon all show concentration variability across a seasonal and spatial scale. The nitrate nitrogen was higher during autumn and winter, with a median of 37.5 μg N L-1, compared to 24.5 μg N L-1 during summer. Dissolved organic carbon concentrations were significantly higher on the west coast (1.9 mg C L-1), than the east (1.6 mg C L-1). Silicon showed higher median concentrations on the east coast than the west, 1.6 mg Si L-1 and 1.3 mg Si L-1 respectively. The dissolved inorganic carbon showed no difference between median concentrations, however, the annual specific yield is higher on the west coast equivalent to 20.2 t km2 a-1 versus 8.5 t km2 a-1 on the east coast. All nutrients varied with geology, vegetation and soil type within the catchments identifying the need determine the most influential controls on nutrient concentrations in low disturbance catchments. Multiple regression analysis concluded a combination of controlling nutrient specific variables depicted the nutrient concentrations of the Southern Alps. Phosphate phosphorous, nitrate nitrogen and ammoniacal nitrogen concentrations were predominately controlled by land use factors, whereas dissolved carbon and silicon were controlled by a combination of land use, soil, morphometric, and hydro-climate variables. Dissolved inorganic carbon and silicon, both principally derived from the mineral sources, are controlled by differing factors highlighting the sensitive nature of each nutrient to its surrounding environment and coupling to the hydrologic network. Overall, pristine New Zealand rivers typically have very low nutrient concentrations in the headwaters by global standards. The concentrations and specific yields of the Southern Alps reported in this study provide a robust proxy dataset for establishing reference water quality conditions for New Zealand.