Abstract
Riverine particulate organic matter (POM) is a critical vector for nutrient cycling of carbon at both regional and global scales. POM and suspended sediment (SS) are transported concomitantly through rivers, and their concentrations change in association to landscape transformations and natural fluctuations. The proportion of POM to SS mobilised in rivers under base flow is an important component of carbon flux to oceans, and is a useful metric for understanding hillslope-river coupling and hydraulic connectivity. We quantified POM during base flow conditions in southern New Zealand to assess the role of different catchment characteristics that control the spatial variability of organic matter in the fluvial environment. The proportion of POM to total suspended material (TSM) can be <5%, or much higher at 50-80% across southern New Zealand, with the highest concentrations associated with lowland agricultural catchments. Particulate organic carbon yields were estimated to be between 0.04 and 2.7 t km(-2) a(-1) discharged into the Pacific Ocean. The POM% responds to both intrinsic and extrinsic catchment characteristics, affecting the availability of organic material and mechanisms for transport including vegetation cover, topographic controls, and hydrological controls. Across southern New Zealand it is evident that riverine organic matter is spatially organised and connected to broader hillslope processes, albeit difficult to predict at the catchment scale. (C) 2019 Elsevier B.V. All rights reserved.