Leachate From the River Red Gum <i>Eucalyptus camaldulensis</i> (Dehn.) Litter Inhibits the Germination and Subsequent Growth of Wetland Macrophytes

ABSTRACT The (re)establishment of macrophytes is a key objective for wetland managers across the Murray–Darling Basin, Australia. Numerous factors could influence wetland macrophyte germination and growth in forested floodplain wetlands, including allelopathic substances, particularly from the litter of fringing trees, but the role of allelopathy in wetland plant germination and growth is not well understood. In the current study, the allelopathic effects of aqueous leachate from the litter of the river red gum ( Eucalyptus camaldulensis Dehn.) on the emergence and growth of wetland macrophytes (and other phototrophs) were studied in a mesocosm experiment. Sediments were collected from two wetlands in Gunbower Forest, Victoria. One wetland (Reedy Lagoon) supports a diverse range and high abundance of wetland plants, while an adjacent wetland (Barapa Swamp) is humo‐eutrophic and has low diversity and abundance of wetland macrophytes. Replicate sediment samples from both wetlands were exposed to four concentrations of leachate from E. camaldulensis leaf litter: High (75 mg/L as C), Medium (37.5 mg/L as C), Low (18.75 mg/L as C) and a Control (containing no leachate). After 90 days of incubation germinants were identified and counted to the lowest taxonomic level practicable, percent cover of filamentous algae was estimated, and the concentrations of chlorophyll a and phaeophytin were measured. There were statistically significantly more germinants in the Control treatment than in the three leachate treatments from the Reedy Lagoon sediment, and a statistically significant greater number of germinants in the Control than in the High treatments for Barapa Swamp sediment. At higher concentrations, the development of germinants was inhibited, with the vast majority unable to be identified beyond monocotyledon or dicotyledon due to lack of growth and with no dicotyledons present in the High treatment from either wetland. There was no noticeable effect of leachate concentration on the emergence of free‐floating species or on filamentous algae. The impact on phytoplankton growth was more complex, with a higher algal biomass in the Low treatments compared to the Control but a decrease at higher concentrations.