Environmental water needs for the Mitchell River

The Australian Government has identified water-resource development in the north of Australia as a priority for the coming years and Queensland’s Mitchell River is a focus of this interest. The Queensland Government currently regulates small amounts of water extraction from the Mitchell River, meaning that flows are largely unregulated. The catchment supports existing water users including Traditional Owners as well as significant ecological assets, including wetlands of national significance, threatened species, traditional harvests, and commercial and recreational fisheries. Information on the water needs of freshwater ecosystems and their associated biodiversity is required to guide decision-making around water planning and development.

This project examined the links between freshwater flows and floodplain inundation, aquatic plant biomass accumulation, floodplain subsidies to freshwater food webs, fish movement and fisheries production. The research had five major objectives.

1. identify the dependence of floodplain inundation and aquatic plant biomass accumulation on river flows
2. map key ‘hotspots’ of freshwater primary production on the floodplain
3. quantify riverine and floodplain connectivity to assess the likely impact of varying wet seasons and water-resource development on aquatic food webs and the movement of fish between river channels and floodplain wetlands
4. quantify the importance of river flows to estuaries by examining the flow dependence of barramundi growth, population dynamics and habitat use
5. assess the implications of flow alteration on freshwater ecological communities, including movement of fish throughout the catchment.

The Mitchell River flows west from its headwaters in the Daintree rainforest to its mouth in the Gulf of Carpentaria.

The Mitchell River’s freshwater biodiversity – and the food and habitats that sustain it – depends on upstream flows and the connectivity they provide. Flows are important for all habitats, including rivers, floodplains and coasts. They inundate floodplains, and the nutrients they deliver fuel the growth of aquatic plants and algae which supports the entire food web. Flows also connect highly productive floodplains with their rivers. Flows and freshwater habitat availability support the abundance, age structure, biomass and population of barramundi in the coastal fishery in the Gulf of Carpentaria. Fish move more easily when rivers and floodplains are connected by flows. Given the importance of flows for the Mitchell’s freshwater biodiversity, these findings have direct relevance to environmental risk assessments for future water planning in the Mitchell catchment.

Key findings:

• Inundation of floodplains and accumulation of aquatic plant biomass in floodplain wetlands were dependent on flows from upstream as well as local rainfall. Interannual variation in river flows explained approximately 60% of the variation in floodplain inundation.
• More than 90% of algal primary productivity in the floodplain region occurred in floodplain wetlands due to their vast surface area relative to the major river channels. Aquatic plants growing in floodplain wetlands provided important substrate for the algal growth that fuels the base of aquatic food webs.
• Floodplain connectivity, facilitated by inundation from river flows, ensured up to 58% of algal production in floodplain wetlands was connected to the main Mitchell River channels over the course of inundation events, boosting riverine food webs. The duration and extent of inundation were primary drivers of the connectivity between wetlands and river channels, and this connectivity would be reduced by dams and drier wet seasons. The water-resource development scenarios showed reductions of up to 26% in potential primary productivity input to aquatic food webs by reducing access to floodplain wetlands.
• Key consumers of the algae produced in the Mitchell River system, including the floodplain wetlands, were invertebrates such as dragonfly and mayfly larvae which contain high levels of essential fatty acids and are a high-quality food for fish. Fish used wet-season connections to the floodplain to access this high-quality food.
• Wetter years with larger flows support increased growth, abundance and biomass of coastal barramundi. The sequential pattern of river flow over multiple years was an important driver of barramundi population dynamics. Reduced flows as a result of water-resource development may lead to smaller fish and increased natural mortality.
• Barramundi that access freshwater habitats in the wet season had faster growth than those that stayed in the coastal zone. Changes in the inter-annual river flow patterns may impact the opportunities for barramundi to migrate and access freshwater habitat.
• Fish movement varied between species and across the catchment; however, most fish made at least one large-scale movement in their lifetime, facilitated by wet-season flows connecting drying habitats. The widespread movement across all species indicated they were most likely foraging and seeking refuge in habitats that persist over the dry season. The mainstem of the Mitchell River itself acted as an important source habitat of individuals that move around the rest of the catchment.
• The extent of fish movement to a given location is dependent on the connectivity of that location to the rest of the river network. Locations that have higher levels of connectivity had a higher proportion of fish moving to them, supporting biodiversity and ecosystem function. Predictive modelling showed that reductions in connectivity through the development of dams without mediating environmental flows will reduce fish movement across the catchment.

Key implications for water planning and river management:

• The biodiversity and ecosystem function of the Mitchell River catchment is dependent on flow-mediated connectivity. Floodplain primary productivity, which supports aquatic food webs as well as birds and other animals, relies on overbank flows that result from upstream flows and local rainfall.
• The extent of movement of fish around the catchment, particularly those that come from the Mitchell River mainstem, indicates that planning of water-resource development needs to consider the entire catchment. The Palmer, Walsh and upper Mitchell rivers are not separate ecological systems but rather interconnected by flows and fish movement.
• Floodplain inundation is a crucial process with considerable evidence showing that aquatic food webs rely on access to inundated areas including wetlands and floodplain creeks and rivers. While local rainfall is important, catchment flows from upstream are vital for inundation. Any reductions in wet-season flows are likely to impact floodplain inundation, particularly in years with limited cyclone activity in the Gulf of Carpentaria.

The findings and implications have been summarised in three conceptual diagrams that depict the ecological processes in the dry season and wet season, and the predicted impacts of changes to the catchment.

Conceptual model of Mitchell River ecosystem function in the dry season.

Dry season

• The mainstem of the Mitchell River flows year-round, while the Walsh (below the irrigation scheme), Lynd, and upper Palmer rivers cease to flow in the dry season and become a series of disconnected pools. Floodplain inundation recedes after wet-season floods and water only remains in depressions in the landscape (some wetlands and creeks) until the next wet season. These variations in flows throughout the catchment create habitats with different degrees of connectivity to the rest of the river network. Connectivity changes across the catchment and with longitudinal landscape changes (e.g. elevation, geology).
• The Mitchell Falls on the Mitchell River are a natural barrier to upstream fish migration. Below this barrier, fish in the main channel of the Mitchell can move freely within this reach during the dry season, while other fish in the catchment find refuge in wetlands or pools. Off-channel wetlands contain functionally unique fish assemblages, with a higher proportion of fish that do not need to move to spawn than at sites in the main channel, making them important habitats for the biodiversity of the catchment.
• As the dry season progresses and habitats contract, fish become more reliant on local food sources to sustain them through the dry season.

Conceptual model of Mitchell River ecosystem function in the wet season.

Wet season

• All rivers flow in the wet season and the floodplain is periodically inundated, connecting floodplains with river channels. Connectivity and its seasonal changes determine the habitat and food available for fish, and these vary across the catchment. High turnover of species occurs in the headwaters and between adjacent sites of low connectivity, indicating that species composition is influenced by opportunities for movement between sites.
• Floodplain inundation and aquatic plant biomass accumulation are dependent on upstream flows and rainfall. At the end of the 2017–2018 wet season, most of the algal primary productivity in the floodplain region occurred in wetlands outside of the major river channels. Floodplain wetlands support macroinvertebrates that are an important source of high-quality food for fish. Macroinvertebrates feed on epiphytic algae, concentrating lipids and polyunsaturated fatty acids, and are then eaten by fish. Fish use wet-season connections to the floodplain to access this rich food source. Inundation periods can be short, and many of the fish that move into floodplain wetlands do not have the opportunity to move back to the river until the following wet season. Sufficient wet-season flows are required to ensure floodplain wetland habitats persist through the dry season to support this biodiversity.
• Fish movement varies between species and across the catchment, and most fish make at least one large-scale movement in their lifetime, showing that even non-reproductive individuals access different parts of the catchment. Most fish caught in the Walsh and upper Palmer rivers had moved there from other areas of the catchment. Short-term fish movement depends on the connectivity of river reaches, with more movement occurring through more connected areas.
• Barramundi that access freshwater habitats in the wet season have faster growth than those that stay in the coastal zone. Wetter years with larger flows support increased growth, abundance and biomass of coastal barramundi.

Conceptual diagram of possible threats from water-resource development to Mitchell River ecosystem function.

Possible threats from water-resource development to Mitchell River ecosystem function

• Dams impact connectivity along the river as well as across the floodplains, reducing the potential for fish movement and access to primary productivity in floodplain wetlands.
• Reduced flows under water-resource development scenarios are predicted to affect the growth of barramundi in the coastal fishery, with younger fish likely to be most impacted.
• Connectivity changes characteristics of the environment which in turn influences fish species composition and functional diversity. Reduced connectivity will alter fish assemblages, impacting catchment biodiversity.
• Impoundments created from dams and weirs are ideal habitat for invasive tilapia.
• Many locations in the catchment, including floodplains, are phosphorus limited. Eutrophication, harmful algal blooms and ultimately oxygen depletion and fish kills would be a risk if agricultural run-off resulted in increased phosphorus levels.