Nutrient limitation of periphyton growth in tributaries and the mainstem of a central North Island river, New Zealand

Abstract

The management of nutrient inputs into New Zealand waterways is a key focus for environmental agencies, but few North Island studies have examined whether nitrates, phosphates or a combination of nutrients limit periphyton proliferations. The underlying volcanic geology that the Rangitikei River, a large central North Island river, drains would suggest nitrogen (N) as a likely limiting nutrient for periphyton growth. However, examination of nutrient ratios in water samples indicated N‐limitation consistently at only 7 of 11 sites sampled on the mainstem and tributaries of the Rangitikei River. The actual effect of nutrient limitation on periphyton growth was investigated at those same sites using in situ nutrient diffusing substrates. Nutrient limitation was also examined upstream and downstream of sewage discharges from the townships of Taihape, Hunterville, and Bulls. The nutrient diffusing substrates indicated N was the limiting nutrient for periphyton growth at most sites in the mainstem and several tributaries of the Rangitikei River. Sewage discharges at both Taihape and Bulls seemed to alter the nutrient balance in the receiving waterways to the extent that the nutrient diffusing substrates did not indicate any nutrient limitation.