Nitrogen and phosphorus concentrations in the Manawatu River, New Zealand, are among the highest nationally. To target policies to address these nutrient levels effectively, this study sought information on relative contributions of soluble nutrients from point and diffuse (non-point) sources at various river flows and in relation to concentration-based regulatory targets using load calculations. In the upper Manawatu and Mangatainoka sub-catchments of the Manawatu River, measured nutrient loads were 55Á154% greater than target nutrient loads. Measured loads were predominately from diffuse sources, which contributed 98% or more of the soluble inorganic nitrogen (SIN) and 84Á88% of the dissolved reactive phosphorus (DRP) at all flows. At low flows, point source inputs contributed up to 64% of the DRP in the upper Manawatu sub-catchment. This study suggests policy to manage nutrient enrichment in these areas should target inputs from diffuse sources at all flows, along with management of point sources at low flows.
Dissolved phosphorus (P) from various sources contributes to periphyton growth in rivers. The Manawatu catchment is in the southern North Island, and since it flows through pastoral farmland with soft rock geology, it carries nitrogen, P and sediment that lead to growth of periphyton. We estimated long-term average sediment P in rivers in the catchment above Palmerston North (PNth) (3910 km 2 ) with the NZeem † model, and also measured the total P on the bed of the river. The annual loss of P in eroded sediment made up 95% of the P flux, (1500 t P yr(1 at PNth); 96% of the erosion occurred under pastures and 4% under forest. Dissolved P was measured at Hopelands and at PNth, and its origins*point sources, forests, the bed of the river and farmland*were assessed. The annual average dissolved P was estimated to be 75 t P at PNth, with 34 t P coming from sheep and beef farms, and this could be reduced with targeted planting of trees; 19 t P came from dairy farms, and could be reduced with changes to management of effluent, limiting soil Olsen P to the optimum agronomic range, and excluding cows from streams, banks and channels. Dissolved P from point sources (15 t P) could be decreased with changes to management of effluent. During low flows, sediment on the bed of the river released approximately 1.3 t dissolved P. These particulate-bound P losses could be reduced by targeted planting of trees on highly erodible land. The Upper Manawatu Water Management Zones (a priority catchment for mitigation) above Hopelands Bridge were also modelled. Council programmes to reduce soil erosion and total P, and to reduce dissolved P during low flow have the potential to reduce P loads in the river. Monitoring of P in the river should be carried out to define a more precise base line, and to monitor improvements to water quality as the programmes progress.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.