Potential Lake Rotorua scenarios

Abstract

Lakes need weeds, good ones (native species) not bad (invasive exotic species), although even exotic species are better than no weeds. Without aquatic macrophytes, a lake can become phytoplankton dominated with low clarity and high chlorophyll concentrations, a condition from which it is difficult to recover. The challenge is to manage the macrophytes to the advantage of the lake to obtain the highest practical water quality that is acceptable to lake users, within the financial constraints of a rates-based budget. Lake Rotorua has sufficient surface area and depth that macrophytes, by themselves, are not likely to control water clarity and phytoplankton growth. However, their management may be a critical factor in the restoration of Lake Rotorua. In this talk I look at potential scenarios associated with managing aquatic macrophytes in Lake Rotorua. With the availability of a weed harvester dedicated to the Te Arawa/Rotorua lakes, there are a number of options that can be applied to Lake Rotorua that would be beneficial to the lake and the people who use the lake. For example, the weed harvester can be used to mow the weed beds to remove part of the nutrient load from the lake and thus improve the lake water quality. The Lake Rotorua Action Plan is looking to reduce the N load on the lake by an additional 50 t y-1. Weed harvesting has the potential to achieve this. Removal of the tops of surface-reaching macrophyte beds around some stream mouths may improve trout fishing for anglers while maintaining the habitat that the trout need in shallow water. Clearance of boat ramps and maintenance of lake access are obvious applications. So is the mowing of the large areas of macrophyte beds adjacent to Rotorua city lake front to reduce plant break-off in wave action and thereby reduce the incidence of macrophytes washing ashore. These are just some of the potential Lake Rotorua scenarios.