Successive macrophyte invasions within the submerged flora of Lake Tarawera, Central North Island, New Zealand

Abstract

Lake Tarawera is the lake with the clearest water in New Zealand to be invaded by four particularly successful invasive adventive weeds: Ceratophyllum 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. Comparisons of lake survey data for macrophytes in Lake Tarawera from 1988 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. demersum. This species was first recorded in 1988 as infrequent, within 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 average cover estimated to be between 51 and 75% throughout this depth range. The spread of C. demersum has been at the expense of native vegetation (particularly Characean meadows) and E. canadensis. E. densa was first recorded at the same time as C. demersum but has established slowly at localised sites only (<2% shoreline). In Kotukutuku Bay these two species have displaced virtually all other macrophyte species below 2 m depth, with E. densa dominating to 10 m depth and C. demersum as a monospecific stand from 10 to 14.5 m. Their abundance in Lake Tarawera suggests that clear‐water New Zealand lakes and their native vegetation could be severely impacted if these presently restricted species become more widely distributed.