Seasonal changes in phytoplankton nutrient limitation: Lake Rotorua

Abstract

The management of macronutrient availability is central for reducing harmful algal blooms and the restoration of aquatic ecosystems. While there is some dispute as to the need for phosphorus (P) only control versus P and nitrogen (N) control, scientific opinion generally supports dual nutrient control in New Zealand lakes. Previous nutrient limitation studies of Lake Rotorua conducted during the 1970s and 1980s reported N limitation to varying degrees. This period coincided with the disposal of sub-optimally treated municipal wastewater to the lake, resulting in significant P loading and resultant algal blooms. Land disposal of wastewater was initiated in 1991 reducing nutrient loads to the lake, however, no further nutrient limitation studies were conducted until the summer of 2004 when N and P co-limitation was reported. From the mid-2000s water quality improved from a Trophic Lake index (TLI) of approximately 4.8 to 4.2, a change likely driven, in part, by sediment P depletion following the change to land disposal of wastewater and the initiation of alum dosing in 2006.