Understanding variations in the limiting nitrogen and phosphorus status of rivers in the Manawatu-Wanganui Region, New Zealand

摘要

Nitrogen (N) and phosphorus (P) enrichment of waterways can affect their ecological, aesthetic and recreational values by causing excessive growth of nuisance algae and other organisms, collectively known as periphyton.In the past, P was commonly thought to be the 'limiting nutrient' for periphyton growth in many of the Manawatu-Wanganui region's rivers. Management of the Manawatu River in particular relied on reducing P inputs from point sources at low flows to avoid nuisance periphyton growth. Long-term State of the Environment data on N and P were analysed against river flow for a number of sites in the region's river catchments. This paper presents the results from this analysis for the Manawatu and Rangitikei catchments. Nutrient limitation status was found to vary at and between a number of sites in the Manawatu catchment and this variation was highly influenced by river flow. Specific low flow investigations of water quality in two catchments found large spatial variation in limiting nutrient status at a number of sites on the same day, within the same subcatchment. The One Plan (Manawatu-Wanganui combined Regional Plan and Policy Statement) proposes management of both N and P at all flows lower than substrate-disturbing floods (20th percentile flow). As more is learned about the interactions between nutrient concentration, flow and periphyton growth in enriched river catchments, it becomes clear that the best approach is a catchmentspecific framework, based on the combined management of N and P, at relevant flow regimes.