The Coorong is a unique and important wetland that provides significant cultural, environmental and economic values at local, national and international scales. Freshwater inflow reduction, along with other anthropogenic impacts, have led to a long-term ecological decline in the Coorong, with conditions exacerbated during the Millennium Drought (20012010). Over the last decade, increased inflow supported the recovery of some elements of the Coorong ecosystem, although the South Lagoon remained in a deteriorated condition with the ongoing profound impacts of hypersalinity and eutrophication (nutrient enrichment) on invertebrates, fish, waterbirds, and the entire food web.
To restore and maintain the ecological condition of the Coorong, particularly for the South Lagoon, the Healthy Coorong Healthy Basin (HCHB) program has commenced, aiming to provide evidence-based solutions to both immediate threats and future conditions anticipated under a changing climate. The Phase One Trials and Investigations (T&I) Project (2020−2022) is part of the HCHB program and involves a series of research components that will collectively provide knowledge to inform the future management of the Coorong. Investigations for ‘Restoring a functioning Coorong food web’ forms Component 3 of the T&I Project.
This report is the output of Activity 3.1 “Knowledge review and synthesis” (Deliverable 3.1.1) of Component 3 of the T&I Project, which aimed to: 1) Review and synthesise existing knowledge and information in relation to the Coorong food web, including the diets of fish and waterbird species, key food resources and environmental drivers, and food web conceptual models; and 2) Identify knowledge gaps and develop hypotheses of how the Coorong food web, particularly in the South Lagoon, may respond to key environmental drivers, including those potentially affected by management interventions. The main findings are summarised below:
Major food sources for key biota
• The diets of abundant fish species in the Coorong, in general, are well understood. Fishes in the Coorong demonstrate a variety of feeding modes. Most are zoobenthivores or omnivores and have benthic invertebrates as the main prey. The diet composition of several species (e.g. lagoon goby and bony herring) in the Coorong is unknown, and that of larval fish remains poor in this estuary, despite being an important nursery to many species.
• Current understanding of waterbird diets and their major food sources in the Coorong is predominantly based on unpublished feeding observations or limited, early recordings of the stomach contents. Literature on the feeding modes and diets of waterbirds in other geographical locations is available. With the exception of some waterfowl and shorebirds, there is poor understanding of the diet composition or the major food sources of key waterbirds in the Coorong.
Key biota – potential food resources and environmental drivers
• Ecological monitoring in the Coorong, particularly over the last two decades, has significantly advanced our foundational knowledge of the spatio-temporal dynamics of key biota (potential food resources) and their key environmental drivers.
• Freshwater inflow is the primary driver for physiochemical changes, ecological processes and biological responses in the Coorong.
• Zooplankton spatio-temporal dynamics in the Coorong reflect environmental conditions, which are affected by freshwater and marine inflows and the connectivity between and within systems. Barrage inflows not only homogenise Coorong zooplankton composition with upstream sources but affect the distribution of zooplankton in the Coorong.
• Flow, salinity, pH and water temperature are the abiotic factors with the strongest correlation with zooplankton abundance and composition. Nutrients and turbidity also likely contribute, via effects on phytoplankton density and composition. However, biotic factors may also be critical for zooplankton with bottom up and top down controls on composition and abundance, although these processes are poorly understood.
• Salinity, which is strongly influenced by barrage inflows, is the major factor influencing shifts in macroinvertebrate and fish assemblage structure in the Coorong.
• Macroinvertebrate species richness, abundance and biomass have improved in the North Lagoon post-drought under more regular barrage inflows. However, the South Lagoon has not changed as much and the assemblage remains dominated by insect larvae.
• Fish species richness and abundance have increased post-drought associated with increased freshwater inflows and connectivity. Nevertheless, species richness has remained low in the South Lagoon due to hypersalinity (>70 psu).
• Smallmouth hardyhead and sandy sprat are the most abundant prey fish species in the Coorong, with smallmouth hardyhead dominating the South Lagoon and North Lagoon, with sandy sprat being more abundant in the Murray Estuary.
• From the food web perspective, further research is needed to investigate the changes in biomass of key biota across space and time to inform quantitative modelling.
• Furthermore, investigations into the energetic and nutritional values of key biota will improve our understanding of the quality of different food resources for waterbirds and fish and help quantify bioenergetic trophic links in the food web.
Current Coorong food web conceptual models
• Our conceptual understanding of the Coorong food web, keystone species, and how it operates spatially and temporally has improved considerably over the last two decades. At least four distinct food webs have been described for the Coorong along its salinity gradient, with a general decline in the diversity of feeding guilds and food chain length with increasing salinity. Freshwater inflow, being the key driver, is fundamental in: reducing salinities and increasing the distribution of biota; transporting nutrients and food resources to increase productivity; and maintaining connectivity between environments for movement.
• Food web models that have been developed in the past for the Coorong are semi-quantitative or qualitative, and thus have limited capacity to predict responses to environmental change, including management options/interventions. Integrated, quantitative food web models can assess food web responses to various environmental changes and will be useful tools to help guide the management for food web restoration in the Coorong.
Knowledge gaps and hypotheses for the Coorong food web
The prolonged hypersaline and hypereutrophic conditions in the southern Coorong are currently constraining the reinstatement of key ecological attributes of a desired ‘healthy’ state. To restore a healthy ecological state, including a more complex and resilient food web in the South Lagoon, we hypothesise that lower salinity, reduced nutrient load and water level management are key to reinstate suitable conditions and improve ecological functions and services. This report provides a knowledge synthesis for the Coorong food web and identifies key knowledge gaps for further study. The food web investigations through T&I Project Component 3, building on existing knowledge and data, will provide critical information and an integrated quantitative food web model to support the assessment of the ecological response under different environmental conditions to potential management scenarios/options. The model will provide a decision support tool to help identify and optimise management options that will maximise the ecological outcomes in the Coorong, particularly concerning the ecological restoration in the South Lagoon