Age and hydrological significance of lichen limits on sandstone river channels near sydney, australia

Abstract

Trimmed lichen communities (lichen limits) are abrupt changes from a lichen community to a scoured bare rock surface and have been used to determine bankfull channel capacity on bedrock channels and their response to the combined disturbances of flow regulation and climate change. They can also be used to set flushing flows in bedrock channels. In sandstone gorges of the Nepean River, Australia, the crustose lichen, Lecidea terrenaNyl, was common at both gorge and cemetery (sandstone headstones) sites, enabling construction of growth curves for above and below dam areas. Growth curves were used to date lichen colonisation of sandstone surfaces in rivers. The oldest, highest lichen limit at all sites represented the pre‐flow regulation lichen community because its characteristics above and below epean am were similar and were trimmed to a level that produced consistent discharges across a range of catchment areas. They corresponded to return periods of less than 2 years on the annual maximum series and was developed during the flood‐dominated regime of 1857–1900. Lichen limits form by the phycobiont dominating the mycobiont and hence degrading lichen thalli due to water inundation causing weak or dead thalli to be scrubbed from the rock surface. Trimming to the unregulated lichen limit represents a small flood of frequent occurrence appropriate for flushing bedrock channels. A lower lichen limit was only found below a diversion weir and was formed by frequent dam spills between 1950 and 1952 during an extraordinary wet period at the start of the between 1949 and 1990. Lichens colonised exposed sandstone between the level of frequent flows from 1949 to 1952, and the high lichen limit. On the von iver, an additional lower limit reflected a massive downward shift in flow duration following the start of interbasin diversions to ollongong in 1962.

Key words:

• lichen growth curves
• Lecidea terrena-Nyl
• alternating flood‐ and drought‐dominated regimes
• flushing flows
• flow regulation
• phycobiont
• mycobiont

Acknowledgements

We are grateful to Professors K. Gregory, G. Petts, R.F. Warner and M.D. Melville for helpful comments on this work. The journal editor and referees improved the manuscript with their constructive comments. We thank the following for assistance with fieldwork: C. Peacock, N. Terrazzolo, K. Tilse‐Sammut, M. Saynor, F. Dorman, A. Erskine and P. Scott. We are grateful to Sydney Catchment Authority for access to restricted areas and for hydrological data. Dr John Elix kindly identified lichens of several genera for this study. The Australian Research Council, Nepean Dredging and the University of New South Wales funded our research.