Wildfire‐Driven Changes in Terrestrial Subsidies Shift Freshwater Microbial and Zooplankton Communities to New Compositional States

Wildfire frequency and intensity are increasing globally, impacting terrestrial and aquatic ecosystems. Deposition of burned materials into aquatic environments can affect biotic communities and nutrient cycling. We investigated how post-fire terrestrial deposition shapes microbial and zooplankton community composition and function across time by manipulating plant material amount (loading; 0-400 g) and chemical composition (burned vs. unburned) in 400 L experimental mesocosms over four months. Burning treatment had minimal effects (1.4%), while loading (6.6%) and time (19.2%) contributed significantly to free-living microbial community variation. Dramatic changes in environmental conditions and microbiome composition occurred at a 50-100 g loading threshold within 30 days. High-loading mesocosms showed hypoxia, increased dissolved organic carbon and aromaticity, elevated bacterial density, and shifts in bacterial community function relating to enhanced carbon degradation, suggesting efficient microbial use of carbon resources despite low oxygen and increased water colour. Zooplankton communities were primarily influenced by time (24.9%), with loading (10.3%) and burning (2.3%) having weaker effects. Zooplankton community composition shifted at a 100 g-150 g threshold that persisted over time, with crustaceans declining and mosquito larvae dominating at higher loading levels. Zooplankton- and plant detritus-associated microbiomes were distinct but showed minimal treatment effects after four months, indicating greater environmental filtering for these microhabitats relative to horizontal transmission from treatment-altered water microbiomes. In contrast, free-living microbiomes maintained loading-driven compositional differences, while predicted genome traits and functions converged across treatments. These results suggest that post-wildfire deposition drives zooplankton and microbial communities into distinct compositional states punctuated by abrupt transitions, but microbiomes may recover community-level functionality over time.