Patterns of composition and abundance in macroinvertebrate egg masses from temperate Australian streams

Abstract

Quantitative samples of lotic invertebrate egg masses were collected on two occasions from three riffles along two rivers within the Acheron River catchment, south-eastern Australia. Sampling was stratified to compare the abundance and composition of egg masses on submerged rocks with rocks that protruded above the water surface within each of three substrate size classes. The egg mass fauna of both rivers was found to be both species rich and abundant. Egg masses belonging to Diptera and Trichoptera represented the majority of material collected, with approximately 70% of all taxa common to both rivers. In particular, egg masses of Chironomidae, Hydrobiosidae, Hydroptilidae and Hydropsychidae dominated most samples numerically. Descriptions of the egg masses belonging to 17 taxa are provided, as well as preliminary estimates of hatching times and observations of egg-laying and post-hatching behaviour for some species. Large rocks (> 30 cm maximum diameter) that protruded above the water surface consistently yielded the highest number of egg masses for all common taxa. For common taxa, between 74% and 100% of all egg masses were found on large emergent rocks. Of the common taxa, little variation in egg mass abundance was detected between sampling times or between sites within each river. Exceptions were hydroptilids and chironomids, which were completely absent from some time/site combinations, suggesting a degree of synchrony in egg-laying behaviour. For all common taxa, the greatest source of variation in egg mass numbers appeared at the level of individual rocks, where a high degree of spatial aggregation was apparent (as described by Lloyd's index of patchiness).