Implications of pumping and ponding water on water quality and the development of diverse aquatic ecosystems: Intervention Monitoring of the Hattah Lakes Icon Site 2006/07

"March 2008".

Project Number: Implications of pumping water on the ecology of Hattah Lakes - M/BUS/210.

MDFRC item.

1 of 2 reports associated with project see (Implications of pumping water on the ecology of Hattah Lakes).

The Hattah-Kulkyne Lakes is one of six Living Murray icon sites, located in Hattah-Kulkyne National park in the north west of Victoria. The lakes complex consists of 18 freshwater lakes, 12 of which are Ramsar listed. These lakes periodically fill during high flow events fed by Chalka Creek, an anabranch of the Murray River. The lakes and surrounds contain significant river red gum communities and support a diverse range of waterfowl, native fish and invertebrates during wet phases, making them a significant ecological asset. River regulation and drought have threatened the values of the Hattah Lakes. In particular, the absence of overbank flooding since 2000 led to serious concern regarding the health of river red gum communities. This prompted the delivery of environmental water into the Hattah Lakes from the Murray River as an emergency measure to maintain existing river red gum communities. Approximately 25GL of water was pumped and ponded in Chalka Creek and nine lakes from a series of four pumping events undertaken between April 2005 and December 2006. The Murray Darling Basin Commission (MDBC) engaged the Murray-Darling Freshwater Research Centre (MDFRC) and EPA Victoria to monitor the ecological response of the lakes to pumping and ponding for the 2006/07 period. The project aimed to determine whether pumping is a viable option to meet The Living Murray objectives. The study specifically focused on (i) assessing the response of river red gum condition to the flooding, (ii) identifying water quality threats that may prevent diverse aquatic ecosystems from developing and (iii) monitoring the response of fish, invertebrates and macrophytes by surveying newly filled lakes and those filled for over one year. Key findings from this study are: River red gum trees at inundated lakes (Lakes Mournpall and Brockie) improved their condition (measured as crown density) to a significantly greater degree than at a control lake (Lake Yelwell) that did not receive pumped water. This increase in tree condition is attributed to the pumped watering event. In addition, there was an increase in tree condition at the control lake (to a lesser extent than for the lakes receiving water) and this is attributed to above average rainfall during the study period. Due to a build up of organic matter loads on dry lakebeds, some lakes experienced a brief period of low dissolved oxygen following inundation. Extensive algal blooms also developed over summer, causing large diel fluctuations in dissolved oxygen and pH. These factors were identified as being potential threats to the development of aquatic communities, particularly for macrophyte establishment. An abundant and relatively diverse native fish community has developed within the Hattah Lakes. A total of 16,291 fish representing six native and two exotic fish species were sampled from six lakes and Chalka Creek. Because the lakes were completely dry prior to the first pumping event in April 2005, this community has developed from fish larvae, eggs or juveniles pumped into the lakes. The sampling of only two exotic fish species (goldfish and only one common carp) demonstrates that pumping is a filter upon the fish community, with common carp and eastern gambusia in particular being excluded with this water delivery mode. Macrophyte seed bank trials demonstrated that there is a high potential for diverse native macrophyte communities to develop at most lakes and therefore achieve the objectives listed in the Hattah Lakes Environmental Management Plan. Macrophytes were able to establish on the lakebeds under the managed flooding regime, although with varying levels of success between lakes. A total of 115 macrophyte taxa belonging to 34 families were identified. Distinct macrophyte communities occurred at each studied lake. The highest abundance and diversity of macrophytes occurred at elevations +30 and +60cm above the waterline. These elevations occurred in areas that were initially inundated following managed flooding but were exposed as lake water levels lowered. Aquatic macrophyte establishment at or below the waterline was initially quite poor at most lakes, however, macrophyte cover increased over the sampling period. It is difficult to ascertain whether the patterns in macrophyte development are a function of water quality (e.g. low dissolved oxygen, high turbidity) or the time since inundation. Despite some water quality concerns (e.g. blue-green algal blooms), pumping and ponding allowed diverse and abundant macroinvertebrate and zooplankton communities to develop and be sustained in the longer term, which will aid in achieving The Living Murray objectives. Sediment assessments at eight lakes and Chalka Creek showed a low risk of sulfidic sediments posing a management problem within the Hattah Lakes system. Overall, the managed flooding event achieved by the pumping and ponding of water has been a successful management intervention for improving river red gum health and allowing diverse aquatic ecosystems to develop at the Hattah Lakes icon site. In the current drought period, this has created a productive wetland system that is providing important ecosystem services as a drought refuge. The lake ecosystems are dynamic and both water quality and the condition and composition of biological communities will continue to change as the lakes dry. Intervention monitoring will occur in 2007/08 for continued investigation of river red gums, lakebed macrophytes, fish and some water quality variables. This additional intervention monitoring will complement the existing data by allowing the communities within the Hattah Lakes system to be monitored over a longer term. In the likely absence of seasonal flooding or further pumping of environmental water at the Hattah Lakes in 2007/08, this monitoring will occur as the lakes undergo their drying phase Macroinvertebrate have life histories linked to the natural flow regime and the loss of this will interfere with their lifecycle and habitat requirements.