The impact of European settlement on Bolin Billabong, a Yarra River floodplain lake, Melbourne, Australia

Bolin Billabong is a shallow, eutrophic and turbid oxbow lake located on the lower Yarra River floodplain, in suburban Melbourne (Victoria, Australia). A combination of radiometric dating, historical data, fossil markers and mineral magnetics has been used to develop a sediment chronology for the billabong that extends from about AD 1120 to the present. Fossil pollen and diatoms have been used to provide a high‐resolution record of vegetation and aquatic ecosystem change through this period, with the aim of developing a better understanding of human disturbance in floodplain lakes. Specifically we aim to investigate the development and trajectory of eutrophic and turbid conditions that exist in the lake at present. The pre‐European contact diatom assemblage at Bolin Billabong is dominated by a planktonic taxon, Cyclotella stelligera, and had very low diversity, with little evidence of species turnover. This suggests that the billabong had low nutrient concentrations and contrasts with the generally accepted notion of billabongs as naturally diverse, productive and variable systems. The initial period of European occupation was characterized by catchment disturbance with high levels of erosion and sedimentation. Sedimentation rates in the post‐European contact period appear to be 30 times higher than prior to European settlement. Evidence suggests that the Yarra River was not naturally turbid. Changes to the diatom assemblage, reflective of water quality perturbation following European contact, were dramatic and unprecedented. Following an initially high sedimentation rate in the post‐European contact period, the sedimentation rate gradually slowed towards the present day. The increase in nutrients available to the diatom assemblage appears to have been moderate from European contact (c. AD 1840) to until around AD 1920, then more pronounced from this point onwards. Recent changes in the diatom assemblage at Bolin Billabong appear moderate compared with other regulated river floodplain sites studied in southeastern Australia. Copyright © 2005 John Wiley & Sons, Ltd.