SUCCESSIVE MACROPHYTE INVASIONS WITHIN THE SUBMERGED FLORA OF LAKE TARAWERA, CENTRAL NORTH-ISLAND, NEW-ZEALAND

Lake Tarawera is the lake with the clearest water in New Zealand to be invaded by four particularly successful invasive adventive weeds: Cer atophyllum demersum, Egeria densa, Elodea canadensis, and Lagarosiphon major. E. canadensis was the first to invade the lake followed by L. major which became the dominant tall-growing species to 6 m depth. Com parisons of lake survey data for macrophytes in Lake Tarawera from 198 8 and 1993/94 show that marked changes occurred in the composition and patterns of submerged vegetation as a result of a recent invasion by C. demsrsum. This species was first recorded in 1988 as infrequent, wi thin a restricted area of the lake. Within 5 years it occurred in 52% of the lake profiles, had a depth range of 0.5-15.5 m, and a median av erage cover estimated to be between 51 and 75% throughout this depth r ange. The spread of C. demersum has been at the expense of native vege tation (particularly Characean meadows) and E. canadensis. E. densa wa s first recorded at the same time as C. demersum but has established s lowly at localised sites only (<2% shoreline). In Kotukutuku Bay these two species have displaced virtually all other macrophyte species bel ow 2 m depth, with E. densa dominating to 10 m depth and C. demersum a s a monospecific stand from IO to 14.5 m. Their abundance in Lake Tara wera suggests that clear-water New Zealand lakes and their native vege tation could be severely impacted if these presently restricted specie s become more widely distributed.