Terrestrial detritus supports the food webs in lowland intermittent streams of south-eastern Australia : a stable isotope study

1. Large amounts of terrestrial detritus enter many low-order forested streams, and this organic<br />material is often the major basal resource in the metazoan food webs of such systems. However,<br />despite their apparently low biomass, algae are the dominant food of organisms in a number of<br />aquatic communities which conventionally would have been presumed to be dependent on<br />allochthonous detritus, particularly those in the tropics and also in lowland intermittent streams<br />in arid Australia.<br />2. The dual stable isotope signatures (d13C and d15N) of potential primary food sources were<br />compared with the isotopic signatures of common aquatic animals in lowland intermittent<br />streams in south-eastern Australia, in both spring and summer, to determine whether<br />allochthonous detritus was an important nutritional resource in these systems. The isotopic<br />signatures of the major potential allochthonous plant food sources (Eucalyptus, Phalaris and<br />Juncus) overlapped, but were distinct from algae and the dominant macrophytes growing in the<br />study reaches. The isotopic signatures of biofilm were more spatially and temporally variable<br />than those of the other basal resources.<br />3. Despite allochthonous detritus having relatively high C : N ratios compared to other<br />potential basal resources, results from ISOSOURCE mixing model calculations demonstrated<br />that this detritus, and the associated biofilm, were the major energy sources assimilated by<br />macroinvertebrate primary consumers in both spring and summer. The importance of these<br />energy sources was also reflected in animals higher in the food web, including predatory<br />macroinvertebrates and fish. These resources were supplemented by autochthonous sources of<br />higher nutritional value (i.e. filamentous algae and macrophytes, which had relatively low<br />C : N ratios) when they became more prolific as the streams dried to disconnected pools in<br />summer.<br />4. The results highlight the importance of allochthonous detritus (particularly from Eucalyptus)<br />as a dependable energy source for benthic macroinvertebrates and fish in lowland intermittent<br />streams of south-eastern Australia. This contrasts with previous stable isotope studies<br />conducted in lowland intermittent streams in arid Australia, which have reported that the fauna<br />are primarily dependent on autochthonous algae.<br />