Effects of hydrological variation and planktivorous competition on macroinvertebrate community structure in experimental billabongs

1. To study two factors which are predicted as causing changes to community structure in cut-off meanders (colloquially known in Australia as billabon gs, a term of aboriginal origin), 16 experimental billabongs were construct ed. These were designed to test two hypotheses: (a) that the structure of m acrophyte and invertebrate communities within billabongs is altered by chan ging the pattern of flooding; and (b) that the presence of small planktivor ous fish alters invertebrate community structure and diversity within billa bongs. 2. An increase in the duration of flooding seems to favour animals better a dapted to a greater availability of macrophyte habitat. Changing the season ality of flooding resulted in prolonging of the time water was available ov er the summer months. 3. The presence of a planktivorous fish appears to affect macroinvertebrate communities through competition with other planktivores. Variable top-down pressure may create differing successional patterns and ultimately differe nt communities at lower trophic levels.