Aquatic macrophytes of the Macquarie and South Esk Rivers, Tasmania

Aquatic macrophytes are the flowering plants and larger algae growing submerged in or emerging from water. Macrophytes are an essential component of riverine ecosystems: they produce oxygen, filter out sediments and pollutants and provide habitat and food sources for invertebrates, fish and mammals. The Macquarie and South Esk Rivers in Tasmania are the largest rivers of the northern central plain, and are unique in Tasmania in having long stretches of relatively stable and abundant macrophytic vegetation along their mid- to lower reaches. The macrophyte communities of the mid- to lower reaches of the two rivers are described by classification into groups with similar species composition. Significant environmental variation between groups is determined. Depth, substrate type and distance upstream are the environmental factors most strongly associated with variation between the distribution of individual species/species assemblages. Distance upstream, percentage shading, river form, stream width, substrate type and bank height are the factors most strongly associated with variation between groups of sites. Bank vegetation type, distance upstream, percentage shading, level of stock damage and stream width are found to be the environmental factors most strongly associated with differences in richness and diversity. Percentage shading and bank vegetation type are the factors most strongly associated with differences in cover. The two rivers are found to differ significantly in percentage cover and total species richness. The associated environmental factors that vary significantly between the rivers are percentage shading, bank height, bank vegetation type, level of stock damage and stream width. The species rich and abundant macrophyte communities in the mid-reaches of the Macquarie River and in some parts of the South Esk are found to have high conservation value. A vulnerable marginal species, Persicaria decipiens, is also of high conservation value. Willow infestation and changes to flow regimes or water quality are seen as being the greatest threats to these communities. The importance of management of stock access to river edges and the potential value of buffer zones are discussed.