Hawkesbury-Nepean River Environmental Monitoring Program

Macrophytes
Large aquatic plants play an important role in the freshwater aquatic ecosystems of
southeast Australia, absorbing nutrients, providing shelter for some species of fish,
and creating foraging locations for others (Thiebaud and Williams 2008). Water
ribbon (Vallisneria gigantea), a native to eastern Australia, is considered an indicator
of good water quality and is known to be a nursery habitat for juvenile bass and a
foraging habitat for adult bass (Harris 1988).
Many complaints are heard in the warmer months from users of swimming, fishing
and boating facilities in the Hawkesbury-Nepean River about plants that restrict the
recreational use of the waterway (Thiebaud and Williams 2008). Almost invariably,
the plants in question are exotic species (e.g. see Figures 6.33 and 6.34), the
decomposition of which reduces water quality and aesthetic values (Thiebaud and
Williams 2008). Some of these exotic species are capable of reproducing via
fragmentation (Sainty and Jacobs 1981); their dispersion possibilities are therefore
increased through anthropogenic means (e.g., fragments on boat motors or fishing
gear).
Macrophyte monitoring
Williams and Thiebaud (2007) analysed changes to aquatic habitats and adjacent
land use in downstream areas of the Hawkesbury-Nepean River. In that study, the
presence of submerged macrophytes was mapped in early 2004 from Wisemans
Ferry to Warragamba Dam. Other studies of exotic floating macrophytes are currently
under way (R. Coventry, pers. comm., cited by Thiebaud and Williams 2008).
However, no recent studies of the large-scale distribution of all of the three main
categories of aquatic macrophytes (submerged, floating, emergent) within the river
had been reported (Thiebaud and Williams 2008).
Figure 6.33: Salvinia bloom at the junction of the Warragamba and Nepean Rivers,
December 2006
DECC commissioned DPI Fisheries to undertake additional fieldwork to determine
the current distribution of submerged, emergent and floating macrophytes in the
Hawkesbury River. The fieldwork for this study was conducted in autumn and winter
2007, and the distribution of freshwater macrophytes was mapped from Warragamba
Hawkesbury Nepean River Environmental Monitoring Program: Final Technical Report 63