Links between Gulf rivers and food for migratory shorebirds

Migratory shorebirds travel thousands of kilometres, from as far away as Siberia, before arriving at sites in the southern Gulf of Carpentaria. Our research has shown that the shorebirds aren’t stopping in the Gulf just because it’s convenient – the area also provides a vital place to feed and recharge after their long migration and some stay here for the whole summer. From spring to autumn, shorebirds use the intertidal mudflats and sandflats for feeding on the invertebrates that live in these environments.

The intertidal flats of the Gulf of Carpentaria are a rich source of food for migratory shorebirds.

The Queensland Government has identified three river systems – the Flinders, Gilbert and Mitchell rivers in the south-eastern Gulf of Carpentaria region – where water extraction for irrigated agriculture is already occurring and is likely to increase. Future decisions about water allocation and management should include improved knowledge about the contribution of these river flows to the Gulf resources needed by iconic and threatened species such as migratory shorebirds. Safeguarding the availability of these resources will help to ensure Australia continues to meet its international obligations to protect migratory shorebirds that use the East Asian–Australasian Flyway, which connects Australia to the birds’ breeding grounds in Russia and Alaska.

This research took place in the Mitchell, Gilbert and Flinders rivers in the south-eastern Gulf of Carpentaria.

Each year, millions of shorebirds fly along the East Asian–Australasian Flyway which connects their breeding grounds in the northern hemisphere with their feeding grounds in the southern hemisphere summer.

Previously, we knew little about the numbers and species of shorebirds using the estuaries of these three rivers in the Gulf of Carpentaria, and the quantity and type of food they eat. Shorebirds probe the sediments of the sand and mud flats for food including worms and bivalve molluscs such as clams and pipis, collectively known as the macrobenthos. This project examined links between freshwater flows and the productivity of Gulf estuaries and coasts to understand how flows impact the food supply for migratory shorebirds in these environments.

The project characterised shorebirds and their food sources (quantity and type) in the south-eastern Gulf of Carpentaria by:

• measuring the amount and growth of coastal/estuarine algae and macrobenthos in three Gulf rivers to build a picture of resources available for shorebirds
• quantifying the importance of a range of river flows, and determining how these flows affect food sources for threatened migratory shorebirds
• collaborating with the Queensland Wader Study Group, the peak Queensland shorebird organisation, to conduct shorebird surveys in the Gulf of Carpentaria and assess the relative numbers of birds in the Flinders, Gilbert and Mitchell estuaries and nearshore shorebird habitats
• collaborating with rangers from the Carpentaria Land Council Aboriginal Corporation to understand the seasonal changes of shorebird numbers in the Flinders estuary
• contributing to the understanding of distribution and management of endangered and critically endangered shorebirds.

• River flows at the start of the wet season are critical for contributing nutrients that fuel productivity.
• Maintenance of river flows in low- and medium-flow years is critical for ensuring that estuaries and nearshore environments receive vital nutrients that boost productivity and provide sufficient food for shorebirds.
• Water extraction following years of little or no flow could have major impacts on productivity.
• On-ground surveys of shorebirds in the Gilbert and Mitchell estuaries – the first of their kind for these locations – found significant numbers of shorebirds, including endangered and critically endangered migratory species.
• Seasonal shorebird monitoring in the Flinders estuary revealed that great knots and black-tailed godwits peaked in January, suggesting that they are resident over summer, while other species, e.g. red knot, appear to be using the Gulf as a staging area prior to further southward migration.
• Water development should be managed so that intertidal habitat productivity is maintained – a critical step in ensuring Australia can keep meeting its international obligations to protect migratory shorebirds.

Endangered great knots in flight. Photo: Amanda Lilleyman.

Freshwater flows had several impacts on the algae that grow on the intertidal flats in the Flinders, Gilbert and Mitchell River estuaries and nearshore environment, which ultimately could impact the food supply for the shorebirds. Wet-season flooding lowers water salinity and as a result, chlorophyll a concentrations (indicating the number of microalgae, an important primary producer) in the water column and intertidal flats also decrease. This is because low salinity stresses the marine microalgae living in estuaries. However, this low-salinity water also contributes significant nutrient loads to estuaries and the nearshore environments.

Our experiments showed that the addition of nutrients stimulated growth of algae on the intertidal flats at multiple sampling times, indicating chronic nutrient limitation. This suggests that catchment nutrients are critical to promoting primary productivity across all three estuaries, even though algal growth is initially suppressed by the wet-season flows.

The Flinders estuary was the most productive of the three estuaries, even though it typically has the shortest period of freshwater flow each year and more variable flows from year to year. This makes the Flinders highly vulnerable to excessive water development.

The far eastern curlew is one of several critically endangered species that use the Gulf to feed and fatten up. Photo: Northern Australia Hub.

Macrobenthic animals had strong seasonal responses to freshwater flows in the Flinders estuary and nearshore environment. The numbers of macrobenthos were lowest during the post-flood period when salinity was also low, and numbers were highest in the late dry season. Polychaete worms were the dominant group in the estuarine intertidal flats. In contrast, bivalves dominated in nearshore areas in both the early and late dry season. Bivalves were almost completely absent from the nearshore intertidal flats in post-flood conditions while polychaetes were less affected by the freshwater inputs, probably because they can burrow deep into sediment. Although flow had short-term negative effects on microbenthic abundance, numbers quickly returned in the dry season.

This study also compared dry-season macrobenthos assemblages across the three estuaries. All three estuaries had high densities of macrobenthos compared with many other Australian tropical estuaries. This work showed that the dry-season river flow regime dictates the salinity regime of each estuary, which in turn appears to influence benthic community composition, but not the abundance of animals. All three estuaries had similar numbers of animals.

This project also investigated shorebird numbers and diversity. One of the studies, led by the Queensland Wader Study Group, compared the shorebirds at and near the three estuary mouths. Birds were counted at low- to mid-tide to coincide with when they feed on the exposed intertidal flats. All three river mouths had thousands of shorebirds, including endangered and critically endangered species such as the great knot and far eastern curlew. This provides important new information for management and protection of these species, particularly for the understudied Gilbert and Mitchell estuaries.

The black-tailed godwit was present in the highest numbers compared with other species in all three systems. Surveys by the Queensland Wader Study Group suggested that the Gilbert and Mitchell river estuaries and adjacent shorelines had more than 1% of the estimated flyway population. Total numbers of all shorebird species in the Flinders region in March 2019 were relatively low compared with other sampling times, possibly due to the major flood in this river system which washed away much of the food supply. At this time, numbers of great knot declined, which may be due to a major reduction of bivalves, their preferred food source.

The Carpentaria Land Council Aboriginal Corporation undertook a seasonal comparison of shorebirds in the Flinders system by surveying high-tide roost sites. These surveys revealed there were large numbers of great and red knots, and black-tailed godwits. The highest numbers of great knot and black-tailed godwits were in mid-summer (January), compared with August/September, suggesting that they are resident over summer rather than using this region as a staging area for southward migration. Other species, e.g. red knot, had higher numbers in August/September, suggesting that these species use the Flinders region as a staging area before migrating further south for the Australian summer.

Normanton ranger surveying shorebirds in the Gulf. Photo: Carpentaria Land Council Aboriginal Corporation.