Environmental Flows Rapidly Increase Zooplankton Abundance in a Regulated Lowland River

Zooplankton are important parts of the food web that support the survival of larval fish in floodplain rivers. Zooplankton abundance and community structure are sensitive to flow-driven changes in the environment, which have been heavily impacted by river regulation with dams. Environmental flows (e-flows) are managed dam releases designed to restore a degree of natural ecological functioning to regulated rivers. Native fish spawning can be triggered by e-flows, but it is not well understood whether these flows can stimulate instream zooplankton production in an appropriate timeframe to support feeding by fish larvae in their first weeks of life. This study aimed to investigate the immediate and longer-term impacts of a planned dam release during the drought of 2018 in the Macquarie–Wambuul River of New South Wales, Australia, on the abundance of zooplankton and associated environmental variables. Dissolved organic carbon, nitrogen oxides (NOx) and chlorophyll a increased significantly at the onset of the e-flow. The zooplankton community was dominated by Cladocera (also named Diplostraca). The significant drivers of zooplankton community structure were NOx, temperature and river level. The start of the e-flow coincided with a significant increase in cladoceran densities, which resulted in up to 36.2 × 10⁹ ± 1.5 × 10⁹ Cladocera d⁻¹ being exported through the study sites. Increased zooplankton exports were maintained for at least 6 weeks after the onset of the e-flow. These findings demonstrate that e-flows during drought can stimulate increased zooplankton abundance within appropriate timeframes to support the survival of native fish larvae.