RIPARIAN EUCALYPTS AND WILLOWS AND THEIR SIGNIFICANCE FOR AQUATIC INVERTEBRATES IN THE RIVER MURRAY, SOUTH AUSTRALIA

The exotic weeping willow Salix babylonica is compared to the native r iver redgum Eucalyptus camaldulensis as a modifier of invertebrate hab itats in the littoral zone of the River Murray. Net samples indicated minor differences in the composition and diversity of invertebrate ass emblages at three willow and three redgum sites. Packs of redgum and w illow leaves in mesh bags were used to assess rates of leaf breakdown. Willow leaves (half-life 14-26 days) decomposed more quickly than red gum leaves (27-50 days), although they were affected more by physical abrasion than biological decomposition. Willow leaves in 2.5-cm mesh b ags lost more weight than those in 300 mu m bags, indicating that feed ing by invertebrates was also a factor. Microbial activity was the mai n agent of weight loss in redgum leaves. After 8 weeks' incubation the re were distinctive invertebrate assemblages associated with leaf pack s at redgum and willow sites. At redgum sites, but not willow sites, t here were different assemblages in willow and redgum packs. Feeding tr ials showed that the common atyid shrimp Paratya australiensis preferr ed microorganism-colonized redgum leaves over colonized willow leaves and fresh leaves, although this may reflect preferences for the associ ated biofilms. Leaf biofilms at redgum sites had relatively high densi ty and diversity of diatoms; redgum leaves generally attracted greater densities of bacteria, and fungi were not prominent on either leaf ty pe. (C) 1997 John Wiley & Sons, Ltd.