Phosphorus bioavailability in rivers flowing through contrasting land uses

Abstract

Bioavailable phosphorus (BAP) is a fraction of total phosphorus (TP) that accelerates algae growth in aquatic ecosystems. BAP is comprised of dissolved phosphorus (DP) and a portion of particulate phosphorus (PP). Algal assays and chemical extractions can be applied to determine BAP. However, several constraints such as long extraction time and high costs, hinder their use. We used a new rapid procedure which involves ultrasonic treatment to extract BAP from suspended sediment (SS) samples. This study aimed to assess particulate bioavailable phosphorus (PBAP) and BAP in runoff from rivers flowing through different land uses, and the relationship with SS and TP respectively. PP was the dominant fraction of TP at high flow and between 5 to 54% of PP was estimated to be bioavailable. A strong statistical relationship was observed between PBAP and SS in the river classed as agricultural (r2 = 0.79), in contrast to the urban one where no correlation was observed, implying that the particle size and composition of SS in both rivers was markedly different. A strong correlation was observed between BAP and TP in the urban river (r2 = 0.78), indicating that merely using TP as a substitute for BAP is questionable as this would overestimate the eutrophication potential of some phosphorus sources. The BAP and TP relationship in the agricultural river revealed that elevated PP concentration at high flow may not translate into higher BAP in runoff, implying that quantification of BAP in SS is critical for the effective implementation of watershed management programs.