Drivers of occurrence, richness, and extent of instream plants in flow-regulated rivers of south-eastern Australia

Context Instream plants play important roles in river ecosystems, yet their ecology remains poorly understood. Improved understanding of the factors influencing their distribution is needed in flow-regulated rivers to guide effective environmental flow management and restoration. Aims We investigated instream plant occurrence, richness, and extent across seven lowland flow-regulated river systems in Victoria, Australia, testing whether patterns could be predicted by flow (discharge, frequency of high flow events and the number of zero-flow days) and non-flow variables (turbidity, riparian canopy shading, and livestock grazing). Methods Instream plants were identified to species and mapped as polygons at 44 sites. At each site we calculated extent, assessed as the length of each species polygon divided by the site length, and species richness. Flow variables were calculated from flow gauges. Non-flow variables were obtained from site-level observations and aerial mapping. Key results Many sites had either one (25% of sites) or zero (18%) species of instream plants present. Assemblages across all systems were dominated by just two species: (1) Cycnogeton procerum; and (2) Vallisneria australis. Richness was generally higher where turbidity was low. Extent had a unimodal relationship with the frequency of high flow events and declined with the number of zero-flow days. Conclusions Sites with perennial flow, intermediate frequencies of high flow events, and low turbidity appear most likely to support instream plants. Implications Environmental flows are likely to best support instream plant communities where they can improve water quality, and/or in drier systems where they can be used to reduce the duration of no flow periods.