The macroinvertebrate colonisation of the tropical, seasonally-inundated Magela Creek floodplain


Internal Report 250
Gunn B
Supervising Scientist Division
Department of Environment, Sport and Territories

About the report

This study examined the colonisation of the Magela Creek floodplain by aquatic macroinvertebrates. The Magela Creek floodplain is located in Northern Australia which has a tropical, monsoonal climate. During the Wet-season, floodplain waters cover an area of over 150 km2.  As the Dry-season progresses, water recedes forming several large permanent billabongs. The penn anent billabongs, floodplain soils and the aerial stages of aquatic insects were identified as the likely sources of colonising fauna. The taxa occurring in each one of these sources, at the end of the Dry-season, was determined. The floodplain soil fauna was examined by taking soil cores from five locations along a transect adjacent to Djabiluka and Island billabongs. The cores were artificially wetted and the emerging fauna identified. Aerial insects were sampled using Malaise traps.

After floodplain inundation, the colonising fauna was collected along the same transects used for the soil fauna determination. Samples were collected from the Island site on days 1, 4, 7, 14 and 21 after inundation. Samples from the Djabiluka site were collected on days 3, 9, 17 and 24 after inundation. Due to time constraints only one location from the Island transect and four locations along the Djabiluka transect could be examined. Changes in taxon richness and relative abundance of the colonising fauna with time and distance from the billabongs was examined using an ANOVA on the results of a principal coordinates analysis.

The Oribatida, Oligochaeta, Chironomidae and Nematoda were the most important taxa in colonisation. Within the Chironomidae, all locations at the Djabiluka site became dominated by the genera Chironomus and Parachironomus. The success of Chironomus was attributed to a combination of high fecundity, adaptations to low oxygen conditions, and competitive superiority. The source of Parachironomus was attributed to dispersal of larvae from the billabongs, rather than egg-laying by flying adults, as a result of age determinations on colonising individuals. Sources of colonisation for Chironomus were probably from the billabongs, as well as, rain-pools present on the floodplain prior to inundation. During early inundation, one location at both Island and Djabiluka transects closely resembled the billabong chironomid fauna. This was probably due to remnant pools at each of these two locations. The oribatid fauna was dominated by the species Trhypochthoniellus sp. This species was found in both the floodplain soils and the penn anent billabongs, and has an asexual mode of reproduction. While asexual reproduction may be an adaptation to an aquatic life-style, it is likely that this species is pre-adapted to such an environment by its asexual ancestry.