Abstract

The 2019/2020 southeast Australian fires ravaged the environment and threatened endemic vegetation groups, including the Tasmanian montane rainforest. This endemic biome, dominated by Athrotaxis species and Nothofagus gunnii, is declining due to increased aridity and fire frequency (years between fire events). Little is known about the impacts of fire and the montane rainforest decline on aquatic ecosystems in the region, yet aquatic ecosystems are strongly reliant on the terrestrial environment for nutrients and humic acids to support their ecosystem health. Here we evaluate the impacts of repeat fires and decline in montane rainforest species on the aquatic ecosystem of Lake Osborne, Tasmania, Australia, during the past 6500 years using a palaeoecological approach. Newly obtained data including organic carbon (δ13C) and nitrogen (δ15N) isotope composition, visible reflectance spectroscopy (R650–700 as a measure of chlorophyll a and derivatives), and diatom remains are compared with previously published charcoal, pollen, micro-X-Ray fluorescence, magnetic susceptibility, and organic carbon and nitrogen elemental data. Results suggest repeat fire occurrence from 6300 to 4200 years ago caused a decline in montane rainforest, increased erosion, and high aquatic productivity, pH, and conductivity (as indicated by diatoms Epithemia species, Fragilaria type species, Karayevia clevei, and Tabellaria flocculosa). Recovery of montane rainforest due to low fire activity from 4200 to 3000 years ago caused an anomalous assemblage of diatoms dominated by Aulacoseira species along with a less productive aquatic environment (inferred from low δ13C and δ15N, R650–700, and percent macrophytes and algal remains), higher lake level and clearer waters at Lake Osborne. A fire event 2500 years ago caused the removal of montane rainforest and a shift to Eucalyptus dominance within the catchment, leading to an increase in aquatic productivity, and a shift toward benthic diatom taxa dominant in clearer waters-characteristic of eastern Tasmanian sites. The aquatic environment at Lake Osborne for the past 6500 years has responded to increased fire frequency, declines in the montane rainforest and climate change. Fire disturbance removes montane rainforest, burns the underlying soils resulting in erosion of terrigenous material and increases aquatic productivity with communities that favour higher conductivity and low light conditions. With projected increases in fire frequency and loss of rainforest, freshwater ecosystems are vulnerable to changes in physical characteristics, productivity, species assemblages, and ecological resilience.