Conveyance of Inorganic and Organic Carbon in Mountainous Rivers: A Case Study from the Haast River, Southern Alps, New Zealand

Abstract

There have been longitudinal studies on rivers around New Zealand, however, these studies have not assessed the carbon flux, rather they were interested in the impact of land use. There is a clear gap in knowledge in New Zealand’s interplay between the atmospheric processes, tectonic processes and the solutes in the alpine rivers. Specifically, the flux, movement and transformation of organic and inorganic carbon, and the impact this has on the potential drawdown of carbon dioxide. This thesis presents a study on seasonal and longitudinal trends of carbon in the pristine alpine Haast River, Southern Alps, New Zealand. The catchment was sampled in each season between March 2016 –March 2017, and during an event, in spring 2016. The dissolved inorganic carbon flux from the Haast River is 38,534 t a-1 and specific yield ranges between 8.9 – 22.1 t km2 a-1, the dissolved organic carbon flux is 12,891 t a-1 and specific yield ranges between 3.9 –17.6 t km2 a-1, and particulate organic carbon flux is 5,733 t a-1 specific yield ranges between 0 – 30.7 t km2 a-1. Inorganic flux is dominant over organic carbon flux in the Haast River, which is due to catchment characteristics, including high uplift, high runoff, mechanical weathering dominance, and schist lithology. Carbonate weathering dominates the Haast River catchment, which is attributed to the high precipitation limiting chemical erosion and therefore, the most soluble materials are eroded and are found in the river, resulting in high calcium concentration. As carbonates weather at a faster rate than silicates, the carbonates can play an important role regulating atmospheric CO2. Downstream changes were observed in dissolved inorganic carbon, calcium and specific electrical conductance, all of which were impacted by tributary input, specifically the Landsborough River. The lithology of the Landsborough River, consists of semi-schist with pockets of sandstone, and it also has steep slopes and, large active glaciers. Therefore, contributes a large amount of DIC (10,782 t a-1) and calcium (15,020 t a-1) to the Haast River. Particulate organic carbon and dissolved organic carbon had relationships with discharge, during the event with extremely high flow there was a flushing effect of particulate organic carbon. During extremely low flow (57 m3s-1) there was an increase in dissolved organic carbon concentration (4.6 mg C L-1). However, the spike in DOC concentration was during autumn sampling, and can therefore also be attributed to seasonal changes in biological uptake and production. Ratios of DIC:DOC showed which type of carbon increase relative to the other. Autumn and summer had an increase in DIC downstream, compared to winter and spring, which had an increase in DOC downstream. The results from this study is important contribution to global and regional studies, specifically the seasonal trends and specific flux calculations.