Nitrogen and Phosphorus Storage in Contrasting Reaches of a Sub-tropical River System

Abstract

This study investigated the storage of nitrogen (N) and phosphorus (P) in the biomass, bed sediments and water column of representative reaches of a sub-tropical river, the upper Brisbane River (UBR), Queensland, Australia, and contrasted instream storage with total wet season exports. In reaches which contained accumulated fine sediments, more than 87% of total P and between 50% and 92% of total N were stored in the surface sediments. The lower proportion of N in sediment at some sites was attributed to substantial differences in the N/P ratios of sediments and macrophytes. At one site, the riverbed was dominated by cobbles and boulders and total nutrient stocks were comparatively low and dominated by the biomass. In reaches with a narrow channel and intact riparian cover, biomass N and P were stored predominately in leaf litter, while in wider unshaded reaches, macrophytes dominated. Total instream storage in the mid to lower reaches of the UBR was ∼50.9 T for N and ∼18.1 T for P. This was considerably higher than total wet season N (∼15.6 T) and P (∼2.7 T) exports from the UBR. The first flow event in the river after a prolonged period of no flow resulted in the export of free-floating, emergent species Azolla. The estimated biomass of Azolla in the mid to lower reaches of the river was equivalent to approximately 24% and 9% of the total N and P flux, indicating that this may be a significant, previously unaccounted for, source at peak flow.