Submerged vegetation and spread ofEgeria densaPlanchon in Lake Rotorua, central North Island, New Zealand

Abstract

Abstract SCUBA records on the distribution and abundance of macrophytes in Lake Rotorua show marked changes between 1980 and 1988. Extensive weed beds of Lagorosiphon major (Ridley) Wager, present since the mid‐1950s, disappeared from the lake by 1982. The loss of the L. major weed beds coincided with marked reductions in water clarity in the early 1980s and with a storm in April 1982. Egeria densa Planchon spread rapidly from a small infestation in July 1983 to become the most abundant macrophyte in the lake by December 1988 (occupying about 475 ha), equivalent to the historic L. major weed beds. We predict that E. densa will be dispersed to become a major problem weed in other Rotorua lakes in the future.