About the document
The summary and technical reports present the 2020-21 results from the Commonwealth Environmental Water Office’s (CEWO) Monitoring, Evaluation and Research (MER) Project (Flow-MER) in the Lower Murray River. Monitoring and research activities in 2020-21 included the monitoring of Hydrology, Stream Metabolism and Water Quality, Fish Community, Hydraulic Regime, Matter Transport and Coorong Habitat, Littoral Vegetation Diversity and Productivity, Microinvertebrate Assemblage, Murray Cod Recruitment and Flow-cued Spawning Fish Recruitment. Both reports evaluate the contribution that water for the environment has made to environmental outcomes in 2020-21 in the Lower Murray River, building on the monitoring and evaluation undertaken from 2014-2020.
Key findings include:
- Increased hydrological connectivity: Environmental water increased the annual flow volume by 31% at the South Australian border. The length of river and duration with water velocity greater than 0.2 metres (important for maintaining water quality) per second increased by 58%.
- Increased flowing habitat: The length of river with ‘flowing water’ habitat (>0.3 m/s) increased by 9% (for at least a four-week period).
- During spring–summer, increased water mixing and oxygen exchange (due to more flowing water) decreased the likelihood of low oxygen levels (e.g. <5 milligrams per litre, mg/L). Aquatic animals generally need oxygen levels above 5 mg/L, particularly during spring–summer, which is the main reproductive season of many species.
- The amount (density) and variety (diversity) of microinvertebrates increased (by 24% and 9%, respectively) due to Commonwealth environmental water, likely through transport from upstream environments as a result of increased longitudinal connectivity. However, the amount of preferred prey species of large-bodied native fish larvae declined by 40%.
- Substantial numbers of silver perch larvae were collected in the LMR during spring–summer. The majority of these larvae originated from spawning in the mid-Murray. The spawning of silver perch and golden perch, and downstream larval drift, were likely supported by environmental water delivery. While autumn sampling did not detect ‘recruitment’ (i.e. young-of-year age 0+ fish were not collected), the cohort may be evident in subsequent years as larger fish are more readily sampled. The presence of additional young cohorts (e.g. 2 and 3 year olds) in the golden perch and silver perch populations in 2021 suggests some recruitment occurred during recent in-channel flow years from 2015–2019 and these cohorts have contributed to population resilience.
- Murray cod recruitment was relatively low. Lotic conditions promoted by environmental water were unlikely to benefit spawning as these conditions occurred after the spawning period. However, increased flowing water habitat may have supported the survival and body condition of the limited number of new recruits.
- The current fish community in the LMR is characteristic of low river flows. However, Murray cod abundance increased, mainly driven by the survival of fish, now age 1+, from the 2019-20 cohort.
- Increased water level variability: Environmental water, in combination with weir pool manipulations, increased water level variability. Subsequent inundation of the riverbank increased the diversity of native vegetation on the bank by 42–82% and the above-ground biomass of understory vegetation by 121–292%.
- In the river channel, primary production marginally increased (by ~0–2.5%) due to increases in water level and channel width, increasing the overall area of available light to phytoplankton and aquatic plants. This low-level increase in production was mainly due to generally stable water levels in the regulated LMR. However, the rate of organic matter breakdown (decomposition) increased by 5–13% due to the delivery of Commonwealth environmental water, suggesting increased food resources to the river.
- Maintained connectivity between the river, Coorong estuary and Southern Ocean. Flows through the barrages to the Coorong were continuous throughout the year and comprised of 65% Commonwealth environmental water.
- Barrage flows during winter and spring facilitated connectivity and promoted lamprey migration. Migration between fresh and saltwater habitats is necessary for lamprey to successfully reproduce.
- Water for the environment substantially increased salt export out of the Basin (by 1.07 million tonnes), reduced salt import into the Coorong (by ~3.2 million tonnes 2017-21) and reduced salinity concentrations in the Coorong, which maintained estuarine habitats (e.g. for fish and aquatic vegetation) in the Coorong. This was crucial in maintaining species diversity and ecosystem functions.
The information from the Lower Murray MER reports is being combined with six other MER Project Selected Areas to provide a Basin-scale evaluation (led by the Commonwealth Scientific and Industrial Research Organisation (CSIRO)) of the outcomes of water for the environment. Recommendations and limitations outlined in the Flow-MER Program are used to inform future management of water for the environment, using the best available scientific knowledge.