Abstract

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, that is, 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 rain forest 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
rain forest system.