Freshwater inflows play an important role in delivering dissolved organic carbon (DOC) to estuaries. Episodic inputs of DOC may support increased bacterial production. However, the role of DOC in supporting zooplankton production is widely debated. To evaluate this role we performed an in situ mesocosm experiment in the Bega River estuary, Australia. We added a DOC leachate derived from terrestrial vegetation to 400 L mesocosm bags as treatments of +1.5, +3, and +16 mg C L−1 and monitored changes in carbon, nitrogen, phosphorus, bacteria, chlorophyll a (Chl a), and zooplankton over 22 d. Bacterial biomass peaked at day 2 and was highest in the +16 mg C L−1 treatment. Chl a was not significantly different between treatments. Mesozooplankton was dominated by copepodites of Gladioferens pectinatus and Sulcanus conflictus between days 5–9 and by adults between days 9–15. Significantly higher numbers of copepods were present in the +16 treatment followed by the +3 mg C L−1 treatment compared with the controls. Stable carbon isotope signatures of copepods in the +16 mg C L−1 treatment were significantly different from the control and showed leachate carbon supported between 29.3% and 55.8% of copepod biomass. These results suggest that the impact of allochthonous DOC loading events on estuarine zooplankton occurs over short periods, and that the magnitude of response is, in part, controlled by the quantity of bioavailable DOC loaded to the system. Our findings underscore the importance of microbial dynamics stimulated by DOC loading events from freshwater inflows as a trophic path in estuarine food webs.

Following a series of fish deaths in the lower Darling River in the austral summer of 2018-19, several field trials were undertaken over two summers to determine the efficacy of various aerators to mitigate the effects of hypoxia to prevent fish deaths. The aerators evaluated included: low-powered, solar bubble-plume diffusers; high-flow, single-port Venturi systems; pontoon-mounted multi-port Venturi systems; and an ultra-fine bubble generator. The effectiveness of bubble diffusers appears related to their power, the number of units used and their running time. Pontoon-mounted multi-port Venturi aerators and high-flow, single-port Venturi aerators were much more successful in maintaining oxygen levels and may have prevented fish deaths in the Darling River in early 2019. However, the Venturi aerators were expensive to run, consuming hundreds to thousands of litres of diesel fuel per week. Trial of an ultra-fine oxygen bubble pump showed promise because it supersaturated the water with dissolved oxygen while not elevating total gas pressure to levels that would cause gas-bubble trauma in fish. Although aerators may play a role in emergency responses to hypoxic fish deaths, they have limitations regarding their area of influence (at best thousands of square metres) and their cost to deploy and operate.

Managed flow releases are increasingly being utilised in the rehabilitation of regulated rivers to improve physical habitat condition and restore spatial connectivity. However, the potential for managed flow releases to influence basal resource availability to the downstream food web has received less attention. This study investigated dissolved organic carbon (DOC) delivery from managed flow releases from Jindabyne Dam and a regulated tributary to the Snowy River; a mixed rainfallsnowmelt river in south-east Australia. DOC concentration and load were monitored downstream of Jindabyne Dam during two high-flow dam releases and 2 month-long tributary releases provided by temporarily suspending tributary weir diversions. DOC chemical composition in the downstream Snowy River was characterised using fluorescence spectrophotometry. A negligible change or decrease in DOC concentration occurred at all monitored sites during both dam releases. In contrast, pulsed increases in DOC concentration concomitant with natural high-flow events were observed in the Snowy River during the tributary releases. The estimated DOC load delivered by the larger dam release increased 1.5-fold between sites 2 and 22 km downstream of the Jindabyne Dam. Reservoir release waters contained both humic-like and protein-like DOC fluorophores, whereas tributary releases contained only humic-like DOC fluorophores. Collectively, these results suggest that changes in DOC quantity and composition during managed dam releases reflect localised wetting and DOC mobilisation from the riparian zone whilst tributary releases deliver storm-associated pulses of terrestrial DOC flushed from the catchment. The unique DOC regimes associated with dam and tributary-sourced water releases may influence ecosystem functioning in the downstream river.