For more than two years, researchers from Griffith University, Charles Darwin University and the University of Western Australia have been collecting samples from Kakadu’s rivers and floodplains to find out what animals are eating and where the ‘hotspots’ of aquatic productivity are.

The team has been using natural chemical tracers (stable isotopes) in the tissues of animals, to work out which habitats are most important in providing high quality food for their growth and reproduction. Stable isotope analysis works on the premise that “you are what you eat”: in other words, animals take on the chemical signatures of their diet.

Small samples are taken from aquatic animals, such as insects, fish, and crocodiles, together with samples of their prey, from different habitats to understand what they eating and from where.

Samples of fish gonads provide further information about whether specific habitats or high quality food sources are especially important to support reproduction.

Here are some of the key findings from the research in the floodplain billabongs:

• Small fish and invertebrates are dependent on local sources of production, particularly from microscopic algae on aquatic plants and in the water column, but also from decaying plant matter.
• Some predatory fish like barramundi move between habitats and derive much of their growth from feeding on the floodplain during the wet season or in marine environments.  Others show evidence of a long feeding history (months to years) within individual billabongs.
• Algae are the key energy source for these larger fish because this is the main food source of their prey on the floodplains, in billabongs, and the main river channel.
• Saltwater crocodiles in billabongs appear to be largely disconnected from the aquatic food web, instead deriving most of their food from the surrounding savanna (land-based animals).

These investigations of aquatic food webs are complemented by field measurements of algal production and satellite imagery of floodplain inundation, which allow the research team to map ‘hotspots’ of production at a regional scale during the wet season.

Apart from the obvious importance of algae as the building block of the food web, what this tells us is that larger animals like crocodiles and barramundi need to access other areas of the floodplain during the wet season to supplement their diet. This means it is important to maintain seasonal connectivity between floodplain areas and waterholes.

The work also highlights the importance of reducing the spread of exotic plants such as para grass and salvinia. These do not contribute to the aquatic food web and weed-affected floodplain habitats are associated with lower algal production.

The partnership between Kakadu staff, Traditional Owners and Northern Australia Hub researchers has allowed all involved to develop a much better understanding of the food webs in Kakadu.