C. Howardwilliams et al., PATTERNS OF AQUATIC WEED REGROWTH FOLLOWING MECHANICAL HARVESTING IN NEW-ZEALAND HYDRO-LAKES, Hydrobiologia, 340(1-3), 1996, pp. 229-234
Mechanical harvesting is used to control submerged aquatic weeds in pa
rts of the hydro-lakes in New Zealand's North Island. Problem species
are Egeria densa and Lagarosiphon major (Hydrocharitaceae), and Cerato
phyllum 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 har
vested and control (unharvested) plots. Regrowth of C. demersum was de
pendent on the prior establishment of the rooted Hydrocharitaceae. Reg
rowth of the Hydrocharitaceae was inhibited where significant water mo
vement occurred. Regrowth declined after 3 six-monthly harvests allowi
ng the establishment of low growing native Nitella spp. beds in the sm
aller clear water lake. In Lake Ohakuri there was a change in species
dominance from Ceratophyllum to Elodea canadensis in shallow (1-2 m) w
ater. No change in species dominance was observed in deeper (>2 m) wat
er and native species were not able to re-establish. The recommended c
utting frequency for management of surface weed growths was only once
per year in Lake Aratiatia, but twice per year in Lake Ohakuri.