Benthic macroinvertebrate survey 2022-2023 report. Coorong, Lower Lakes and Murray Mouth Icon Site

1. Executive summary
This report presents results from the annual macroinvertebrate and mudflat monitoring in the Lower Lakes, Coorong and Murray Mouth Icon site (LLCMM) as part of The Living Murray (TLM) Condition Monitoring Program. The monitoring was carried out in spring/early summer as per the TLM LLCMM Icon Site Condition Monitoring Plan (Revised) 2017 (DEWNR). The results from the current monitoring investigations are assessed against the ecological objectives and specific targets of this plan, for macroinvertebrates and mudflats. The monitoring is now in its 18th year and the data from late November/early December 2022 are compared against the long-term data series.
The monitoring occurred at a period of high flows with >1000 GL/month passing through the barrages in the months before field work. The flood peak in January 2023 exceeded 3000 GL/month. Conditions in the North Coorong were fresh to brackish (0 to 20 ppt), and salinities in the South Coorong substantially reduced (50-70 ppt) compared to previous monitoring years. This lowering of salinity was accompanied by a respective shift in macroinvertebrate distributions, with abundances of annelids and molluscs, and of macroinvertebrate biomass being highest in the North Lagoon. The prolonged lower salinities enabled a re-colonisation of the northern South Lagoon with macro-invertebrates, which had not been documented before in any of the monitoring years since 2004.
An additional autumn survey was carried out in April 2023 after the flood event had passed through the estuary, and results from the autumn survey are compared with the data from November/early December 2022 as well as earlier years with autumn monitoring (e.g. April 2016 and 2017).
The overall ecological objective for Coorong macroinvertebrates is: (I-1). Maintain or improve mudflat invertebrate communities that are of high condition relative to southern Australian estuarine mudflat ecosystems. Six ecological targets were assessed to inform progress towards achieving the overarching ecological objective. The monitoring in spring/early summer 2022 documented improvements in the distribution, abundance, and biomass for macroinvertebrates as a result of the very high flows. Most of the ecological targets were either met, partially met, or showed an improvement compared to the previous years’ monitoring.
The TLM condition monitoring ecological target 1- Macroinvertebrate species richness increases throughout the Murray Mouth and Coorong was partially met. For the North Coorong, species richness remained above reference levels. Mulbin Yerrok (site 6) in the North Lagoon had the highest number of macroinvertebrate species. Species richness increased into the South Coorong, which was most pronounced at Parnka Point (site 8) and Villa de Yumpa (site 9). Parnka Point had the highest number of macroinvertebrate taxa recorded at this site since monitoring began. Species richness in the South Coorong had the largest increase (above the reference value) for the long-term monitoring data series. Yet, species richness at the two southernmost sites in the South Lagoon was still low.
Target 2 - Macroinvertebrate occurrence extends along the Coorong into the South Lagoon was partially met in 2022. More species had a wider distribution, reaching into the South Lagoon. Some species were recorded for the first time in the South Lagoon since monitoring began. The occurrence was still confined to the northern end of the South Lagoon, which is why the target was not fully met.
Target 3 - The area of occupation for typical estuarine and marine macroinvertebrate species exceeds 60% of the sites sampled was partially met. While the distribution range had extended for many species, only six species had an area of occupation in the Coorong of >60%. Yet, this was the largest number of species to have reached the target in the long-term condition monitoring to date.
Target 4 - Macroinvertebrate abundance is maintained at, or increases above, reference levels was met in 2022. Abundances were maintained for the North Coorong, where they were similar to all monitoring years with continuous flows since 2012, but also the lower flow years of 2005 and 2006. Abundances for the South Coorong were also similar to most previous monitoring years and higher than abundances during the Millennium Drought. Looking at single sites, Parnka Point at the northern end of the South Coorong had the highest abundances yet recorded in the long-term data series. The Index of Relative Change for total macroinvertebrates was above the reference level for the South Coorong with the highest increase yet recorded, and remained above the reference for the North Coorong. For several species the Index of Relative Change increased, including Salinator fragilis in the North Coorong and chironomid larvae and oligochaetes in the South Coorong. For the pollution indicator species Capitella and oligochaetes in the North Coorong, the index value decreased. Abundance trends continue to be species-specific and different across the two main regions of this icon site.
Target 5 - Macroinvertebrate biomass is maintained at, or increases above, reference levels was met. For the North Coorong, biomass was lower but within the range of values from previous monitoring years, and higher than during the Millennium Drought. For the South Coorong, biomass increased and was above the reference value for the Index of Relative Change. The increase in biomass was most pronounced at Noonameena in the southern North Lagoon.
Target 6 - Macroinvertebrate communities are similar to those occurring under intermediate continuous flows was met. Long-term community patterns indicated that macroinvertebrate communities in November/December 2022 were similar to recent monitoring years for both the North and South Coorong. The more diverse and abundant macroinvertebrate community in the North Coorong shifted into a new state with the continuous flows, with salinities <25 ppt explaining the pattern. The macroinvertebrate communities in the South Coorong, which is characterised by low diversity, abundance and biomass, continued to be less well defined, and explained by salinities >58 ppt. Higher flows enabled more homogenous communities with less variability between sites throughout the Murray Mouth and Coorong.
The overall ecological objective for Coorong mudflats is: Maintain or improve habitable sediment conditions in mudflats (M-2 and M-3). Three ecological targets were assessed to inform progress towards achieving the ecological objective. The targets for mudflat habitat conditions were partially met or not met during the monitoring in spring/summer 2022.
Target 1 - Habitable sediments are occurring along the Coorong into the South Lagoon was partially met. Mudflats at several sites contained muddier sediment than in previous monitoring years, with high sediment organic matter loads. No filamentous algal mats covered mudflat surfaces, and seagrass was more commonly found throughout the Murray Estuary. The mudflats in the estuary remained submerged from prolonged periods of freshwater from the high River Murray flows. The high flows lowered salinities which improved habitable conditions for macroinvertebrates in the Coorong lagoons.
Target 2 - Sediments are maintained as fine to medium sands and are mostly moderately well sorted was partially met. The sediments were fine to medium sands, meeting the target, but more poorly or moderately sorted, which did not meet the target. The Index of Relative Change for median grain size was not different to the reference, but significantly below the long-term reference value for the sorting coefficient.
Target 3 - Sediment organic matter is maintained was not met. The sediment organic matter increased above reference values and the Index of Relative Change showed a significantly higher deviation from long-term reference levels.
Based on the condition monitoring in spring/early summer 2022, the objective I-1 Maintain or improve mudflat invertebrate communities that are of high condition relative to southern Australian estuarine ecosystems was partially met. The condition monitoring objectives M-2 and M-3 combined Maintain or improve habitable sediment conditions in mudflats were partially met.
The high River Murray flows combined with the unregulated flow peak lowered salinities in the Coorong lagoons which enhanced habitable conditions for macroinvertebrates and enabled a re-colonisation of the northern parts of the South Coorong. The autumn survey documented the beneficial effects of the prolonged period of continuous and high flows followed by a flood peak for the Coorong, with highest species richness, abundance and biomass recorded between Noonameena and Villa de Yumpa in April 2023. This was the first time in the long-term monitoring that such a shift of macroinvertebrate populations into the South Coorong occurred. Yet, the autumn survey also revealed an impact from the flood for macroinvertebrates in the Murray Mouth, with flood related water quality and/or sediment deposition as possible causes for a decrease in the species richness, abundance and biomass. The populations from the Coorong lagoons could now function as source populations for the re-colonisation of the Murray Mouth, when continued high flow can maintain the lowered salinities. Future monitoring will be able to evaluate whether the continuous and higher flows have increased the resilience of the macroinvertebrate communities. This monitoring and the comparative analyses over time also illustrate the value of long-term monitoring data, especially when different climate and flow conditions occur over the monitoring years.