Wat er Assessment Aquatic Ecology Report Series Response of the Lake Crescent and Lake Sorell ecosystems to water level variability during 2009: a period of drought and post- drought recovery October 2010 ISSN: 1835-9523 Report No. WA 10/01 Water Assessment Branch W at er and Marine Resources Division D e p a r t m ent of Primary Industries, Parks, Water and Environment Copyright Notice: Material contained in the report provided is subject to Australian copyright law. Other than in accordance with the Copyright Act 1968 of the Commonwealth Parliament, no part of this report may, in any form or by any means, be reproduced, transmitted or used. This report cannot be redistributed for any commercial purpose whatsoever, or distributed to a third party for such purpose, without prior written permission being sought from the Department of Primary Industries, Parks, Water and Environment, on behalf of the Crown in Right of the State of Tasmania. Disclaimer: Whilst DPIPWE has made every attempt to ensure the accuracy and reliability of the information and data provided, it is the responsibility of the data user to make their own decisions about the accuracy, currency, reliability and correctness of information provided. The Department of Primary Industries, Parks, Water and Environment, its employees and agents, and the Crown in the Right of the State of Tasmania do not accept any liability for any damage caused by, or economic loss arising from, reliance on this information. Preferred Citation: DPIPWE (2010). Response of the Lake Crescent and Lake Sorell ecosystems to water level variability during 2009: a period of drought and post-drought recovery. Water Assessment Aquatic Ecology Report Series, Report No. WA 10/01. Water and Marine Resources Division. Department of Primary Industries, Parks, Water and Environment, Hobart, Tasmania. Contact Details: Department of Primary Industries, Parks, Water and Environment Water Assessment 13 St Johns Avenue, New Town Phone: 03 6233 6833 Web: www.dpiw.tas.gov.au Email: [email protected] Cover Page Images: Top left: Lake Crescent from Tea Tree Point boat ramp, April 2009; top right: Boathouse Shore, Lake Crescent, May 2009; bottom left: Clyde Marsh, November 2009; bottom right: Sexually mature female golden galaxias (Galaxias auratus) from Lake Sorell, July 2009. The Department of Primary Industries, Parks, Water and Environment The Department of Primary Industries, Parks, Water and Environment provides leadership in the sustainable management and development of Tasmania’s resources. The Mission of the Department is to advance Tasmania’s prosperity through the sustainable development of our natural resources and the conservation of our natural and cultural heritage for the future. The Water and Marine Resources Division provides a focus for water management and water development in Tasmania through a diverse range of functions including the design of policy and regulatory frameworks to ensure sustainable use of the surface water and groundwater resources; monitoring, assessment and reporting on the condition of the State’s freshwater resources; facilitation of infrastructure development projects to ensure the efficient and sustainable supply of water; and implementation of the Water Management Act 1999, related legislation and the State Water Development Plan. Response of Lake Crescent and Lake Sorell to water level variability during 2009 Summary The interconnected Lake Crescent and Lake Sorell are impounded natural lakes which are located 1 km apart in the upper reaches of the Clyde River catchment (~800 m a.s.l.) in the south-east of the Tasmanian Central Plateau. This lake system contains several distinctive and significant environmental values, including extensive littoral wetlands (one of which is Ramsar-listed), and the freshwater fish golden galaxias (Galaxias auratus Johnston) which is endemic to lakes Crescent and Sorell and listed under Tasmanian and Commonwealth threatened species legislation. Because of these and other environmental values, and their shallow and highly productive states, lakes Crescent and Sorell are considered to be a regionally unique lake system. Between 2000 and 2008, a substantial volume of scientific work was undertaken to examine the influences of low water levels and patterns of water level fluctuations on the Crescent-Sorell ecosystem. This work showed that these components of the hydrological regime of this system directly and indirectly affect three broad aspects of the Crescent-Sorell ecosystem: (1) important aquatic habitats (i.e. wetlands and rocky shores), (2) water quality, and (3) G. auratus populations. The findings of studies during 2000-2003 were used to guide the formulation of water level operating rules in the water management plan (WMP) for lakes Crescent and Sorell, which took effect in November 2005. Since the late 1990s, the Crescent-Sorell ecosystem has been severely stressed by prolonged periods of low water levels, which have been primarily caused by below average and unseasonal rainfall (including significant El Niño-induced drought events). While there was a slight increase in water levels in both lakes during 2003-2005, between 2007 – early 2009, water levels fell substantially in both lakes. These low water levels were thought to have the potential to further degrade water quality and habitats in the lakes, and cause additional declines in G. auratus populations. This study took advantage of a unique opportunity to examine several components of the Crescent- Sorell ecosystem during a period (i.e. 2009 – early 2010) of extreme water level variation, which included very low water levels in summer-autumn 2009, followed by rapid and substantial seasonal rises in levels during winter-spring 2009. These conditions also provided quite different circumstances to undertake the second environmental water level manipulation in this lake system; with the first being undertaken in 2007 when water level minima were higher and only moderate natural seasonal rises in levels occurred in Lake Crescent and Lake Sorell. Similar to during 2007, during 2009 the response of G. auratus populations (at several life stages) in Lake Crescent and Lake Sorell to the water level manipulation was monitored. Using long-term data sets, which included data collected during the present study, several previously established correlations and relationships between water levels and parameters of the Crescent-Sorell ecosystem were re- i Response of Lake Crescent and Lake Sorell to water level variability during 2009 examined. Furthermore, the resistance/resilience of some environmental values in lakes Crescent and Sorell in relation to impacts of low water levels and recovery during a period of substantial rises in levels was investigated. This study confirmed many findings from previous studies in lakes Crescent and Sorell. It also provided new information in some areas which is highly relevant to the management of water levels in these lakes, and hence water resources in the Clyde River catchment. Principally, this study furthered understanding of: (1) water level patterns in Lake Crescent and Lake Sorell, (2) the effects of water levels on water quality and the ecosystems of these lakes, (3) relationships between water levels and the inundation and condition of littoral habitats in these lakes, (4) the spawning strategy of G. auratus, (5) relationships between water levels and the abundance of juvenile G. auratus, (6) the status of G. auratus populations, and (7) the environmental benefits of water level manipulations in these lakes. Based on the findings of the present study and previous studies, components of the water level regimes of Lake Crescent and Lake Sorell which are important to the management of their ecosystems were synthesized. In this synthesis, water level thresholds which related to: (1) the inundation of important littoral habitats (rocky shores and wetlands) in Lake Crescent and Lake Sorell, (2) maintaining water quality (particularly low turbidity levels) in Lake Crescent and Lake Sorell, and (3) allowing mass spawning of the G. auratus population in Lake Crescent, were identified. Furthermore, the importance of maintaining water level fluctuations in lakes Crescent and Sorell that follow historical seasonal patterns (which relate to climatic conditions in the area) is also specified. Based on this synthesis, to sustainably manage the Crescent-Sorell ecosystem it is recommended that the: Critical minimal operating water level in Lake Crescent is 802.200 m AHD Critical minimal operating water level in Lake Sorell is 803.200 m AHD Preferred minimum operating water level in Lake Crescent is 802.700 m AHD Preferred minimum operating water level in Lake Sorell is 803.500 m AHD Preferred operating range for water levels in Lake Crescent is 802.200-803.800 m AHD Preferred operating range for water levels in Lake Sorell is 803.500-804.360 m AHD. The findings of the present study, and those of DPIW (2008), have demonstrated that appropriately timed releases of water from Lake Sorell to Lake Crescent can be used to manage the Crescent- Sorell ecosystem. This innovative approach has been trialled twice, under quite different water level conditions in these lakes, and positive responses in environmental parameters in the system on both occasions show that it provides a powerful tool for managing these lakes. Therefore, additional to the environmentally important components of the water level regimes of lakes Crescent and ii Response of Lake