Speciation and distribution of organic phosphorus in river sediments: a national survey

Abstract

Organic phosphorus (P) can play a role in in-stream productivity (e.g., periphyton or macrophyte growth), but little is known of the largest likely source of organic P—bed sediment. A survey was conducted of 76 bed sediment samples of rivers within the New Zealand National River Water Quality Network in an effort to determine the concentration and form of organic P species, and variation according to catchment and sediment characteristics or classifications used to characterise anthropogenic P inputs (e.g., as baseline [viz. reference] or impact sites) and therefore likely in-stream productivity. Sediments were analysed for a range of physiochemical properties. NaOH-EDTA extracts were analysed by 31P nuclear magnetic resonance spectroscopy (31P NMR), which isolated orthophosphate from other P species including orthophosphate monoesters, orthophosphate diesters and pyrophosphate. More P was extracted from sediments at impact compared to baseline sites, while the proportion of organic P increased where microbial processing (as reflected by site elevation) decreased. Analysis by 31P NMR of sediments showed they contained fewer and less-enriched forms of organic P than profiles of typical lake sediments or soils (e.g., no phosphonates or polyphosphates). This was hypothesised to reflect either low P inputs via catchment runoff, or efficient utilisation due to strong P limitation. Some evidence was found of changes in the distribution and form of organic P species in relation to anthropogenic activity and sediment processes. However, organic P concentrations were small and only represented a single sampling. Hence, additional work needs to examine if these changes relate to changes in in-stream productivity over time.