Competitive performance of Hydrilla verticillata in New Zealand

In outdoor tanks, Hydrilla verticillata was grown in equal start, and estab lished, cultures with each of four other submerged adventive aquatic macrop hytes Egeria densa, Lagarosiphon major, Ceratophyllum demersum and Elodea c anadensis. Plant growth and performance were monitored for a year, then all the plants and H, verticillata tubers (subterranean turions) were harveste d. There was a significant difference in H. verticillata biomass obtained w hen grown with each of the competitor species and the highest and lowest bi omass values were obtained with C. demersum and E, densa, respectively. Pla nting time also affected H. verticillata biomass. In equal start experiment s with any competitor species biomass was significantly greater than when t he competitor species had between 2-8 weeks to establish. Similarly, H. ver ticillata tuber numbers were greater when H. verticillata was grown with C. demersum compared with other hydrocharitaceans. Although its growth varied significantly, depending on the species with which it was planted, and it encroached on the space occupied by its competitors, H. verticillata appear ed to have little impact on the biomass of its potential competitors over t he duration of this study. Nevertheless, the capacity of H. verticillata to grow and compete effectively with other adventive aquatic macrophyte speci es that already cause problems in lakes and waterways was evident and highl ights the highly invasive nature and threat that H. verticillata poses to t he New Zealand aquatic environment. (C) 1999 Elsevier Science B.V. All righ ts reserved.