| Abstract: | Changes in vegetation of Bushells Lagoon, a freshwater body fed intermittently by the Hawkesbury River in the Sydney basin, were studied over three scales of time: broad fluctuations in areas occupied by emergent plants over 28 years; fluctuations in their percentage cover over 21 months; and, over 6 months, seasonal variation in biomass of emergent plants of two native species Ludwigia peploides (Kunth) Raven ssp. montevidensis (Spreng.) Raven and Marsilea mutica Mett., and the introduced Myriophyllum aquaticum (Vellozo) Verde. Differences in distribution and abundance of plants observed between the three species were related to environmental variation in time and space. Wind and wave action varied along the shoreline, and emergent shoots of Myriophyllum were most abundant in sheltered conditions, floating leaves of Marsilea most abundant in relatively exposed conditions and emergent shoots of Ludwigia most abundant in somewhat intermediate conditions in the shallows. Chemical characteristics of the water varied in time but, unlike water temperature, had no obvious seasonal pattern. Seasonal change occurred in the biomass of Marsilea, and to a lesser extent in that of Myriophyllum, with smaller proportions of leaf material present in winter than in summer. Though water levels were apparently stable over a period of some years (1965–74), at other times they varied greatly through flooding and drought with related major change in cover of the three species. Co-existence of the three species in the lagoon may be related to differences between their plants in establishment and growth following disturbance (regeneration niche), tolerance of wind and wave action (habitat niche), or seasonal growth (phenological niche). |
