Distribution and density of vegetative hydrilla propagules in the sediments of two New Zealand lakes

The distribution and density of hydrilla (Hydrilla verticillata (L.f.) Royl e) turions and tubers in two New Zealand lakes were assessed by sampling co res of sediment from Lakes Tutira and Waikapiro each year from 1994 to 1997 . Turion and tuber density differed with water depth, with maximum numbers of tubers and turions found in the 1-2 m and 1.5-4 m water depth ranges res pectively. A high turion to tuber ratio was observed, with turions accounti ng for over 80% of the propagules. The relatively low numbers of turions an d tubers compared with other reports, and the distribution of most tubers w ithin the shallow water is likely to be associated with black swan grazing (Cygnus atratus Latham), which maintains a canopy of hydrilla consistently 1 m below the water surface.