Evidence for a Drought-driven (pre-industrial) Regime Shift in an Australian Shallow Lake

Abstract:

We present a 750-year record of ecosystem response to long-term drought history from Lake Colac, Victoria. Using multiple lines of evidence, we test the sensitivity and resilience of Lake Colac to independently reconstructed drought history. The sedimentary archive shows that Lake Colac appears to be sensitive to periods of drought. Following drought conditions c. CE 1390, the lake ecosystem indicates signs of recovery. A succession of droughts in the early 1500s initiates a change in the diatom flora, with freshwater species declining and replaced by saline tolerant species, though there is little interpretable change in aquatic palynomorphs. An inferred drought, around CE 1720 appears to precede a major switch in the lake’s ecosystem. The lake became increasingly turbid and saline and there is a distinct switch from a macrophyte-dominated system to an algal-dominated system. The arrival of Europeans in Victoria (CE1840) appears to have little effect on the lake’s ecosystem, but the terrestrial vegetation indicates regionally established changes including declines in native trees, especially Casuarina, and arrival and expansion of exotic shade or plantation trees Pinus and Cupressus as well as native and introduced weeds. As European impact in the catchment increases, nutrients appear to play a role in the modification of the lake’s ecosystem. A long-term drying trend from c. CE 1975 is evident, culminating in the Millennium Drought, which suggests unprecedented conditions in the ecological history of the Lake.