Warrego and Darling rivers Selected Area 2022-23 - Appendix I Dissolved organic matter, microbial communities and productivity

Previous monitoring in the Warriku-Baaka (Warrego-Darling) Selected Area has indicated that both systems are strongly heterotrophic – that is more carbon is consumed than produced in the rivers - and that water quality and productivity differ between the Warriku and Baaka Rivers. Although both systems consume more carbon than is produced, productivity overall is still high, and both river systems support diverse invertebrate and vertebrate communities. The Warriku and associated floodplain (the Western Floodplain) have been observed to support consistently higher phytoplankton productivity (as measured by chlorophyll a concentrations), than Baaka River sites. This study aimed to investigate the processes which might regulate productivity in these systems and understand why it differs.

To do this we investigated dissolved organic matter (DOM) in both systems and conducted a detailed investigation of metabolism in the Warriku (Warrego). DOM forms the basis of aquatic food webs, and its concentration, type, composition, source and bioavailability influences the composition of microbial communities including algae, bacteria and fungi, both directly (as a food source) and indirectly (through light attenuation and nutrient binding). This in turn influences the overall productivity of a system, with impacts that may flow through to higher order consumers such as fish and birds, through influences on trophic dynamics in the basal food web. We also used eDNA techniques to describe the microbial community in each system, and identify links between microbial community composition and function, and DOM sources.

We found that concentrations of DOM (measured as dissolved organic carbon [DOC]) did not differ between systems but that composition did. The Baaka (Darling) River had higher amounts of terrestrial (or allochthonous) DOM indicating a stronger influence of transported terrestrial litter on carbon sources in this system; whereas the Warriku (Warrego) River and Western Floodplain had higher amounts of microbially derived (authochthonous) DOM. Microbially derived carbon can come from either bacterial degradation of organic matter or algal production but in either case represent a stronger reliance on in-stream carbon production/processing than terrestrial inputs. Different sources of DOM resulted in significantly different microbial community composition among systems.

Metabolism experiments conducted in the Warriku (Warrego) showed that water column (pelagic) and benthic productivity differed among sites, with benthic production and respiration exerting the strongest influence on metabolism in most sites. This study also captured a mixing event in Ross Billabong which was influenced by flows from both the Baaka (Darling) and Warriku (Warrego) channels in this water year. This site recorded the highest autotrophic and heterotrophic productivity, and the most diverse microbial community. This result suggests that a mixed DOM profile, resulting from mixing of the Baaka (Darling) and Warriku (Warrego) waters likely provided an improvement in productivity which supported a more diverse microbial community than either of the systems individually.

This study investigated sites in channels upstream of the confluence of the Baaka (Darling) and Warriku (Warrego) Rivers to facilitate differentiation and comparison between systems. However, it is likely that the increased productivity observed in Ross Billabong which is attributed to the mixed DOM pool resulting from mixed river flows, would also be observed in the Baaka (Darling) River downstream of the confluence when the channels are connected. The benefit appears to come from a mix of terrestrial and aquatic sources of DOM providing a more complete suite of basal resources than either type alone. This suggests that increased connectivity between these river systems will benefit productivity in the Baaka (Darling) through greater contributions of aquatic DOM from the Warriku (Warrego). Similarly, productivity in the Warriku (Warrego) may also be improved if overbank flows along the length of the channel were able to incorporate more terrestrial litter.