2008-09 Condition Monitoring at Hattah Lakes- M/BUS/87-2,3,4 BW283
MDFRC Technical Report
This report details the condition monitoring undertaken at Hattah Lakes Icon Site as part of the 2008/09 Living Murray Condition Monitoring Program. The Hattah Lakes is part of the 48,000 ha Hattah-Kulkyne National Park located in the north west of Victoria. It contains 18 freshwater lakes (12 of which are Ramsar-listed) connected by a series of floodplain channels fed by the Murray River during periods of high flow. River regulation has threatened the ecological values of floodplain areas at Hattah Lakes due to changes in the timing, duration and frequency of wet and dry phases in the lake system. Storage and extraction of water from the Murray River upstream of Hattah Lakes have resulted in the mean discharge in the Murray River near Hattah Lakes being c. 50% of natural and natural overbank flooding has occurred on only one occasion in the past eleven years. Due to the reduced frequency of flooding, pumping has become a necessary management intervention to achieve inundation of the lakes in the current regulated and drought conditions. Intermittent pumping of water into Hattah Lakes from the Murray River has occurred from April 2005 to December 2006 and in June 2009 as an emergency measure to maintain existing River Red Gum communities. Pumping is also expected to remain a long-term management intervention for the Hattah Lakes Icon Site. The Living Murray (TLM) initiative was developed in 2002 in response to evidence of the degraded condition of the Murray River system. The First Step of this program focuses on protecting and improving the ecological condition of six Icon Sites along the Murray River, including Hattah Lakes. The program aims to return the Murray River system to a healthy working river through the accrual and release of an average of 500 GL.y-1 of recovered water and the implementation of a capital works and measures program. The Outcomes Evaluation Framework (OEF) stipulates that measurable management targets be created to unambiguously assess whether a pre-determined level of ecological condition has been achieved. At the time of writing, such targets had not been developed for Hattah Lakes. Therefore, in the absence of defined targets, this report focuses upon the ecological objectives specified in the First Step decision, the Outcomes Evaluation Framework and the Hattah Lakes Icon Site Environmental Management Plan. Where possible, results are compared with those of previous years to determine any directional change in ecological condition. The ecological components monitored at Hattah Lakes in 2008/09 include River Red Gum, Black Box, wetland vegetation, floodplain vegetation, Lignum, Cumbungi and fish. River Red Gum (RRG) at Hattah Lakes is distributed within three previously defined vegetation classes (WRCs) including Red Gum Forest (RGF: 341 ha), Fringing Red Gum Woodland (FRGW-2739 ha) and Red Gum with Flood Tolerant Understorey (RGFTU: 1,533 ha). Achievement of the ecological objectives for RRG at Hattah Lakes requires the maintenance of sustainable RRG populations. Progress towards achievement of this objective was assessed by examining RRG distribution, condition, age-structure and relative abundance across the the three vegetation classes. Low Foliage Vigour (generally less than 30%) scores were common to RRG in all WRCs indicating that condition of RRG within all three communities at Hattah Lakes is generally poor. Analysis of Foliage Vigour data collected in 2007/08 and 2008/09 indicated that an increase in tree condition occurred at 24 of the 27 sites. However, the between year changes in Foliage Vigour detected in the analyses were not supported by photo records and may be attributed to the subjective nature of tree condition assessments. The size-class distributions associated with sampling transects were compared to those considered ideal for population maintenance and sustainability (i.e. those approximating an ‘inverse j-curve' shape or log-normal function). Distributions of RRG in all three WRCs approximate smooth inverse j-curve shapes indicating robust recruitment. Each WRC is well represented within the smallest size class (DBH = 0-15cm) where the proportion of trees was 63.8%, 49.1% and 58.5% in RGF, FRGW and RGFTU respectively. Black Box (BB) at Hattah Lakes is distributed within the two previously defined vegetation classes (WRCs) Riverine Chenopod Woodland (RCW: 6,073 ha) and Black Box Swampy Woodland (BBSW: 339 ha). Achievement of the ecological objectives for BB at Hattah Lakes involves the maintenance of sustainable BB populations. Progress towards achievement of these objectives was assessed by examining BB distribution, condition, age-structure and relative abundance across the two vegetation classes. Analysis of Foliage Vigour data collected in 2007/08 and 2008/09 indicated that an increase in tree condition occurred at all 18 sites. However, mean Foliage Vigour scores were low (generally less than 30%) in both 2007/08 and 2008/09. Low Foliage Vigour scores were common to RCW and BBSW indicating the condition within both BB communities at Hattah Lakes is generally poor. The survival of BB indicates the resilience of this species under stressful conditons, since the Hattah Lakes system has not been flooded for more than ten years. Examination of photo point images taken at each of the sites at the time of each survey indicates that any change in Foliage Vigour between years was minimal. The size class distributions associated with sampling transects were compared to those considered ideal for population maintenance and sustainability (i.e. those approximating an ‘inverse j-curve' or log-normal function). Distributions with both WRCs approximate smooth inverse j-curves, indicative of robust recruitment. However, representation in the smallest size class (DBH = 0-15cm) was slightly higher in RCW (51.0%) than it was in BBSW (40.3%) indicating marginally higher recruitment rates in the less frequently flooded BB communities. However, the monitoring has occurred over a relatively short time frame and consisted of only two samplings and long term trends in recruitment and condition change are yet to be ascertained. Wetland vegetation was surveyed at nine wetlands at Hattah Lakes, all of which were dry during the 2008/09 survey. A total of 89 plant species (76 native) were recorded during this survey, six of which are listed as rare or threatened in Victoria by the Department of Sustainability and Environment. Recorded plant species were classified into functional groups and differences in community composition and abundance were analysed both for functional groups and individual species. There is a clear separation of wetlands at Hattah Lakes into three groups: (i) large, relatively deep wetlands that received environmental water in 2005/06; (ii) small, relatively shallow wetlands that received environmental water in 2005/06; and (iii) wetlands that remained dry during the 2005/06 pumping events. Species and functional groups both showed significant differences in community composition and abundance between 2007/08 and 2008/09. Terrestrial damp and amphibious species appeared at lower elevations, decreased in abundance or disappeared from the deep lakes such as Bulla and Hattah in 2008/09; terrestrial damp species also decreased in abundance or disappeared from Lakes Yerang and Little Hattah (shallow lakes that received water in 2005/06); Lakes Nip Nip and Boich, which have remained dry, were dominated by terrestrial dry species in both 2007/08 and 2008/09. Floodplain vegetation was surveyed at six sites, each containing ‘often', ‘sometimes' and ‘rarely' flooded ‘sub-sites' determined from historical flood-return frequency data. A total of 65 plant species (62 native) were recorded in 2008/09, six of which are listed as rare or threatened in Victoria by the Department of Sustainability and Environment. Sites fell into four groups based on vegetation type at the various flood-return frequencies: (i) site 1,‘often' and ‘sometimes' dominated by RRG Woodland with a sparse understorey; (ii) sites 2 and 3, ‘often' and ‘sometimes' dominated by RRG Woodland with a high cover of leaf litter in the understorey; (iii) sites 5 and 6, mixed RRG and BB Woodland at ‘sometimes'; and ( iv) site 4, mixed RRG, BB and River Cooba Woodland at ‘sometimes' and a sandhill at the ‘rarely' ‘sub-site'. The majority of floodplain sites have not been flooded for more than 12 years. In 2008/09, more than 90% of the species recorded in all six sites were terrestrial and only two species, recorded at very low abundance, were classified as amphibious. The absence of flooding and water in the wetlands and on the floodplain at Hattah Lakes is preventing the ecological objectives relating to sustainable wetland and floodplain vegetation from being met. The ecological objective relating to Lignum at Hattah Lakes is restoration of the macrophyte zone around at least 50% of the lakes. Lignum condition was monitored at five sites in 2006/07, 2007/08 and 2008/09 using the Lignum Condition Index (LCI) scores for Viability and Colour. Assessments of the change in Viability and Colour of Lignum across the various flood regimes indicated a decline in condition of Lignum between 2006/07 and 2007/08 and between 2007/08 and 2008/09 at all sites except one, where there was a slight increase in the mean Viability score from 2007/08 to 2008/09. The Viability and Colour scores indicate that the ‘often' flooded populations are in better condition than the ‘sometimes' flooded populations followed by the ‘rarely' flooded populations, demonstrating the significance of flooding for the survival of Lignum. Reduced frequency of flooding is a threat to the achievement of the ecological objective of a healthy and sustainable Lignum community at Hattah Lakes. Cumbungi (Typha spp.) was detected in only two small stands during surveys of Hattah Lakes in 2008/09. A near-absence of Cumbungi has been recorded for all three consecutive surveys (2006/07, 2007/08 and 2008/09), with the existing stands decreasing in extent due to grazing pressure and a prolonged drying phase. The near absence of Cumbungi from Hattah Lakes appears typical for this area and is not considered a threat to the achievement of ecological objectives relating to sustainable vegetation communities. When the fish surveys were undertaken in November 2008 at Hattah Lakes, only three of the lakes (Bulla, Hattah and Mournpall) had surface water. A total of 2,128 fish from five species (three native) were sampled with fyke nets. Two native fish species, Carp Gudgeon and Australian Smelt comprised over 94% of the total catch. Among non-native fish, 109 Goldfish and 17 Common Carp were sampled. These fish populations developed from the series of four pumping events from April 2005 to December 2006. As the lakes dried, these fish populations perished in summer 2008/09. In the Murray River, 126 fish from nine species (seven native) were sampled with electrofishing. Native fish populations comprised 93% of the sample, in which Carp Gudgeon (50.8%) and Fly-specked Hardyhead (36.5%) were dominant. Crimson-spotted Rainbowfish, Golden Perch, Murray Cod, Bony Bream and Australian Smelt each contributed <2% of the total catch. Among non-native fish populations, seven Common Carp and one Goldfish were sampled from the Murray River. Prolonged drought and continued disconnection of these wetlands from the Murray River are a threat to the achievement of TLM objectives relating to maintenance of sustainable native fish communities at the Hattah Lakes Icon Site.