Evidence for catastrophic shifts in the trophic structure of flood-plain lakes associated with soil erosion

The introduction of European agricultural practices to southeastern Australia during the 1800s was associated with a period of extensive erosion in upland areas. The effects of this erosion on aquatic systems were substantial. Deep gullies replaced natural "chain of ponds" systems in headwaters, and the resultant "sand slugs" reduced substrate complexity and stability in middle reaches. The impact of this period of intensive erosion in lowland reaches is less obvious. This study presents the results of palaeoecological reconstructions from several flood-plain lakes on the Murray River that cover the period prior to the introduction of agriculture to the present. These records show a consistent pattern whereby benthic algae are replaced by planktonic algae concomitant with peaks in indicators of high sediment input. This pattern supports studies which have utilised different indicators at other sites in the region and is interpreted as resulting from light attenuation due to high suspended-sediment loads during a phase of intensive sediment erosion and transport during the mid to late 1800s. The maintenance of this macrophyte-free state in the absence of continued high sediment loads up to the present day is thought to reflect the existence of "alternative stable states" in these systems.