The Invasion of Lake Taupo by the Submerged Water Weed Lagarosiphon major and Its Impact on the Native Flora

The waterweed Lagarosiphon major (Hydrocharitaceae) was first recorded in Lake Taupo in 1966, and by 1979 it had occupied most, if not all, the potentially colonisable sites in the lake. The aim of this study was to examine the distribution, abundance and growth forms of L. major in Lake Taupo and to assess the effects of this plant on the native aquatic vegetation. Studies involved underwater mapping of the 161 km length of littoral zone of the lake and detailed analyses at 42 sites around the lake at which L. major biomass, height and density were measured together with the species composition of each site. Five growth forms of L. major were identified, ranging from absent to beds of continuous cover greater than one metre in height. The relationship between these growth forms and height and biomass was established. Height and biomass were negatively related to fetch, and positively related to littoral slope angle and proportion of fine sediment. The number of native species decreased markedly as L. major height and biomass increased. The most noticeable decrease was at 4 m depth. Large weed beds of L. major attract herbivorous birds and detritivores which also adversely affect the native flora. Until recently the implications of the spread of L. major for the conservation of the indigenous flora has been overlooked.