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Lower Murray River Project Updates – October 2020

Written by PIRSA-SARDI

Image: River Murray near Waikerie (Photo credit: SARDI)

Winter is the quiet season for fieldwork, as most field sampling trips are wrapped up by late summer- early autumn. During this quarter, our researchers concentrated on laboratory analyses, crunching numbers, performing statistical analyses, and interpreting some results for reporting before the new field season starts.

SARDI staff preparing samples for laboratory analysis (Photo credit: SARDI)

Here are the updates for the Lower Murray individual indicator investigations.

Hydraulic Regime

Flow and environmental water data for 2019-20 have been received from the Murray-Darling Basin Authority modelers, including the sources of water. We will soon start running the hydrodynamic models to simulate conditions (i.e. water velocity and water level) with and without Commonwealth environmental water. This information will then be used by other indicators in evaluating the contribution of environmental water to other ecological outcomes.

Matter Transport and Coorong Habitat

The flow and environmental water data will also be used in the hydrodynamic-biogeochemical modeling to assess on the influence of Commonwealth environmental water on matter transport including salt, nutrients and phytoplankton (as Chlorophyll).

The 2D CLLMM model will be used to simulate water level and salinity from the Murray Estuary, North Lagoon and South Lagoon of the Coorong. The outputs of the model will assist us in determining to what extent Commonwealth environmental water contributes to the improvement of Ruppia and fish habitat in the Coorong.

Stream Metabolism

The last season data collected from our water quality loggers in the Lower Murray River has been processed and is ready for analyses. Using the newly available flow and environmental water data we will be able to provide further insights into the influence of environment flows on primary production and ecosystem respiration in the main channel of the Lower Murray River.

Stream metabolism site downstream of Lock 6 (Photo credit: SARDI)

Microinvertebrates

The data from microinvertebrate samples collected between October 2019 and January 2020, have been collated. Analysis and reporting will start soon when Deborah Furst, our microinvertebrate task leader, re-joins the team after returning from maternity leave. Meanwhile, Russel Shiel is working through identifying microinvertebrates present in the stomachs of Murray cod larvae to determine the important prey items.

Vegetation

Analysis of vegetation data collected during December 2019 has shown that plant communities responded well to the spring flow pulse supported by water for the environment. For instance, native species richness (number of species) increased post water delivery at all monitoring sites (See full story here).

Male (top) and female (bottom) copepods collected Downstream of Lock 6 (Photo credit: SARDI)
Littoral zone vegetation monitoring (Photo credit: SARDI)

Murray cod

Our fish team have concluded ageing of larvae and juvenile Murray cod (less than 150 mm) collected from November 2019 to April 2020 by counting daily increments in otoliths (ear stones). The results confirmed spawning dates between late October and early November 2019. The estimated ages will also be used to develop length-at-age relationships and calculate seasonal growth rates. For the same fish, daily increment width analysis is currently underway. This enables assessment of instantaneous otolith growth (a proxy for body growth) that may be related to flow.

Fish assemblage

Fish assemblage data collected by boat electrofishing and small-meshed fyke nets, in March-April 2020, have been processed and saved to a centralised database. This data set will be used to investigate changes in fish assemblage through time by the Flow-MER Selected Area and Basin-scale evaluation teams.

A subset of golden perch collected through general and targeted sampling in Autumn 2020 were kept for aging (annual rings on otoliths) to assess recruitment. Similar to recent years, results showed a lack of young-of-the-year (age 0+) recruitment in 2019-20. The dominant cohorts in the population were 7 to 10 years old, and originated from spawning that occurred in 2009-10 (high flow year in the Darling River), 2010-11 (flood); and 2011-12 and 2012-13 (high flow years).

In addition, bony herring collected in June 2020 were dissected and fin clips sent to the Basin-scale fish research team. These samples form part of the research project investigating movement using a combined genetics and otolith microchemistry approach.

Juvenile Murray cod larvae collected in May 2020 in the Lower Murray River, and retained for daily ageing (Photo credit: SARDI)
Basal food items grinded to fine powder (Photo credit: SARDI)

From primary productivity to Murray cod recruitment

Our research project aims to investigate the influence of the food quality, food source and pathways of energy transfer through the Lower Murray food web on the recruitment of Murray cod, and how this process is affected by flow. It has a specific focus on the region downstream of Lock 4. A component of the project involves assessment of the diet of Murray cod at four stages during early life (collected in November, January, February and April/May) through molecular (e-DNA) and microscopy approaches.

Assessment of diet will be supported by the understanding of stream metabolism, and ambient prey assemblages (e.g. zooplankton, macroinvertebrate abundance). In addition, we will investigate the basal carbon sources (e.g. phytoplankton, aquatic plants, terrestrial plants/trees) that ultimately support Murray cod recruitment, using a technique called compound-specific stable isotope analysis (CSIA). This technique determines the carbon isotopic ratios of essential amino acids – these amino acids can only be synthesised by primary producers and thus the signatures of given primary producers are conserved through the food web up to consumers. This component involved the collection of various basal food sources (algae, aquatic plants and tree leaves) and muscle tissue (from Murray cod and shrimp), before freeze-drying and grinding to powder. The processed samples have been sent to CSIRO lab in Tasmania for CSIA.

The results of this study will allow us to assess diet in relation to what is available in the ambient environment and the basal carbon sources supporting juvenile Murray cod, and ultimately, how these processes vary with flow.

Recent rainfall and spring – early summer outlook

Rainfall was below average for the south part of the Southern Basin and above average or well above average for the Northern Basin and southeast part of the Southern Basin, for the period of June to August 2020. The rainfall outlook (October to December) for the Murray-Darling Basin (MDB) looks promising, the Bureau of Meteorology (BoM) is forecasting an above average rainfall for the eastern 2/3 of Australia.

Cooling in the Tropical Pacific Ocean and warming waters northeast of Australia has been indicating the occurrence of La Niña for some time. The Bureau’s ENSO Outlook has now declared that La Nina is established in the Tropical Pacific and is predicted to persist until at least January 2021. There is a good chance we will get above average rainfall for the period of late October to December 2020.

Australian map showing rainfall for the period of March to May 2020. Image credit: BoM
Australian map predicting chance of above average rainfall. Image credit: BoM

Flow outlook – Great News

According to the MDBA, the flow outlook for the 2020-21 year is looking more positive compared to last year. This is mainly because of the trifecta we have been hoping for:

  1. catchments are primed after the wet autumn.
  2. there is more water in storages compared to the same time last year.
  3. the Bureau of Meteorology is forecasting higher than average spring rainfall, due to La Niña.

Near median flow and low flow is forecasted for about 80% of the MDB streams, but there are some places where high flow is expected. Our fish researchers are hoping that the flows to the Lower Murray exceed 20,000 ML/day during spring-early summer. If this occurs, we will undertake further investigations on the dynamics of golden perch spawning, recruitment and natal origin and understanding their responses to the flow pulses and the influence of environmental water delivery.

What’s next?

With the Lower Murray flow and environmental water data in hand, our scientists are now running models, assessing the contributions of environmental water towards ecological outcomes of hydraulic regime, stream metabolism, matter transport and Coorong habitat. For other indicators (vegetation, microinvertebrates and fish), we will continue with data analysis and reporting to evaluate the ecological outcomes from environmental water delivery during 2019-20. We aim to complete the draft report by December/January.

Meanwhile, the field teams are preparing for the season ahead. For stream metabolism indicator, this has already started with water quality loggers set up in the river, while other indicators are starting this month. The field season is expected to be very busy due to the monitoring of multiple indicators, continuing of integrated research and potential contingency monitoring for golden perch.

Our engagement and communication activities will continue, our attention and energy will continue to focus on promoting our activities. We are excited to release an animation video in coming weeks, and we will produce a series of short videos showing our “in field” sampling activities. Keep an eye out for them!!!

Our work in the Lower Murray River Selected Area

The Lower Murray River is at the end of the Murray Darling Basin (MDB) system and includes the only estuary of the MDB, which connects to the Southern Ocean. Annual flow is variable, being influenced by inputs from the southern and northern basin, and rainfall and water extraction experienced in these regions. The Lower Murray River is complex, and includes the main river channel, anabranches, floodplain / wetlands, billabongs and stream tributaries. Being towards the end of the system, the Lower Murray River is wide and deep relative to upstream reaches, and significant flow is required for floodplain inundation. Our study area covers four different zones: floodplain, gorge, swamplands and the Lower Lakes and Coorong.

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