Overcoming Rotorua Lakes ecology challenges

Over many years the increasing incidence of nuisance algal blooms in some of the Rotorua Lakes has been the catalyst for studies to identify reasons for the general decline in water quality. These studies cite the combined nutrient inputs from anthropogenic and natural sources in regard to their specific contribution to the problems of lake water quality. Over the last decade there has been a strong focus on setting targets to control nitrogen and phosphorus loads, the main drivers for changes in lake trophic condition. In recognition of the importance of the Rotorua lakes, Bay of Plenty Regional Council has set statutory environmental bottom lines in the form of specific Trophic Level Indices (TLIs) for all the Rotorua lakes in the Water & Land Plan (W&LP). This has lead to investigation and implementation of selected restoration methods to overcome the immense challenges of managing and treating large bodies of lake water. This paper briefly covers the reasons and effects of changes in water quality and introduces methods currently in operation to control excess nutrients. Although the Rotorua Lakes programme targets substantial decreases of both nitrogen and phosphorus loads, this paper is primarily concerned with the control of phosphorus.