Why did Nadgee Lake estuary change state from benthic to pelagic dominance?

Extensive historical (anecdotal) information covering the past 3 decades indicated that the remote and pristine Nadgee lake estuary in southern NSW had a benthic dominated ecology. All descriptions indicated that it had... [ view full abstract ]

Extensive historical (anecdotal) information covering the past 3 decades indicated that the remote and pristine Nadgee lake estuary in southern NSW had a benthic dominated ecology. All descriptions indicated that it had oligotrophic waters with dense cover of benthic macropyhtes and associated avifauna. When we arrived at Nadgee in late 2008 for the first scientific aquatic survey (ever) it looked nothing like this. The lake was dominated by an intense microalgal bloom and no macrophytes were present. Why?
Entrance opening and closure are the major disturbances in an intermittent estuary like Nadgee, but there are no records of past entrance behaviour for such a remote site. This paper describes the use of Geoscience Australia’s recent compilation and rectification of Landsat images (the “Datacube”), along with the application of a consistent water detection tool for all pixels in that compilation, to determine opening and closing regimes. The output of the analyses provides an indication of whether a pixel was wet or dry (or not able to be determined) for all images over the entire 27 year’s worth of data. Water level records measured by OEH since 2009 were used to ground-truth the remote sensed data.
We can now determine when, over the past 27 years, the Lake opened and how long the water level remained low. From this record, it appears the lake opened in late 2006, resulting in very low water levels. This opening, however, occurred during summer in the millennium drought and the lake did not re-fill for a long time. This information, along with an understanding of the ecology of the primary macrophytes has been used to provide some possible models that explain when and why the fundamental shift from benthic to pelagic may have occurred.