ALBERT

Description: Critical transitions in ecosystem states are often sudden and unpredictable. Consequently, there is a concerted effort to identify measurable early warning signals (EWS) for these important events. Aquatic ecosystems provide an opportunity to observe critical transitions due to their high sensitivity and rapid response times. Using palaeoecological techniques we can measure properties of time series data to determine if critical transitions are preceded by any measurable ecosystem metrics, i.e. identify EWS. Using a suite of palaeoenvironmental data spanning the last 2,400 years (diatoms, pollen, geochemistry and charcoal influx) we assess whether a critical transition in diatom community structure was preceded by measurable EWS. Lake Vera, in the temperate rainforest of western Tasmania, Australia, has a diatom community dominated by Discostella stelligera and undergoes an abrupt compositional shift at ca. 820 cal yr BP that is concomitant with increased fire disturbance of the local vegetation. This shift is manifest as a transition from less oligotrophic acidic diatom flora ( Achnanthidium minutissimum , Brachysira styriaca and Fragilaria capucina ) to more oligotrophic acidic taxa ( Frustulia elongatissima , Eunotia diodon and Gomphonema multiforme ). We observe a marked increase in compositional variance and rate-of-change prior to this critical transition, revealing these metrics are useful EWS in this system. Interestingly, vegetation remains complacent to fire disturbance until after the shift in the diatom community. Disturbance taxa invade and the vegetation system experiences an increase in both compositional variance and rate-of-change. These trends imply an approaching critical transition in the vegetation and the probable collapse of the local rainforest system.