Patterns of aquatic weed regrowth following mechanical harvesting in New Zealand hydro-lakes

Mechanical harvesting is used to control submerged aquatic weeds in parts of the hydro-lakes in New Zealand's North Island. Problem species are Egeria densa and Lagarosiphon major (Hydrocharitaceae), and Ceratophyllum demersum. Experiments were conducted in two contrasting hydro-lakes. Lake Aratiatia; clear water (K-o 0.2 m(-1)) and a low residence time (<8 h), and Lake Ohakuri; turbid water (K-o = 0.6) and a longer residence time (>5 days). Growth rates were measured underwater in harvested and control (unharvested) plots. Regrowth of C. demersum was dependent on the prior establishment of the rooted Hydrocharitaceae. Regrowth of the Hydrocharitaceae was inhibited where significant water movement occurred. Regrowth declined after 3 six-monthly harvests allowing the establishment of low growing native Nitella spp. beds in the smaller clear water lake. In Lake Ohakuri there was a change in species dominance from Ceratophyllum to Elodea canadensis in shallow (1-2 m) water. No change in species dominance was observed in deeper (>2 m) water and native species were not able to re-establish. The recommended cutting frequency for management of surface weed growths was only once per year in Lake Aratiatia, but twice per year in Lake Ohakuri.