Microbial community dynamics during historic drought and flood in Great Salt Lake.

Understanding human-driven environmental impacts on microbial community distribution, abundance, and function remains a central challenge in microbial ecology. In particular, the drivers of temporal succession in community membership following perturbation remain unclear. Great Salt Lake, Utah, bears clear hallmarks of human disturbance, including a rock-filled railroad causeway that sequestered its northern arm from freshwater river influx, leading to localized hypersalination and food web collapse. Following decades of riverine water diversion, the southern arm of this terminal lake reached a historic low elevation during a time of increased climatic change, placing strong environmental pressure on the robust saline ecosystem. Here, we use molecular methods to report microbial community composition during this severe drought year at sites across the lake, including where the north and south arm waters contact. At sites of hypersaline water intrusion, we observe surprising stability in the north arm community composition, in contrast with strong perturbation in the south arm community structure. We use hydrodynamic modeling to pinpoint physical water flow dynamics as a key driver of these community shifts. At hypersaline north arm sites far from hyposaline water intrusion, abundance shifts were detected in predatory and parasitic taxa, a discovery that reveals surprising ecological dynamics in saturated hypersaline systems. In sum, this study demonstrates drastic hypersaline microbial community shifts during salinity and extreme weather perturbations. IMPORTANCE: The Great Salt Lake ecosystem is rapidly drying due to climate warming and human consumptive water use that diverts riverine input to the lake's south arm. In 2022, the lake dried to reach its lowest elevation in recorded history, dehydrating microbial mats and exposing the toxic dust of the lakebed. This endangers food webs, industry, and human health. In response, Utah agencies are regulating Great Salt Lake salinity with ongoing adaptive structural engineering interventions. We hypothesized that these interferences would lead to dynamic mixing of microbial communities across a salinity gradient. We find that physical mixing drives dynamic, ongoing diversity loss and food web disruption at these sites of intervention, providing insights into how salinity regulation could be improved.