FAUNAL COMMUNITY SHIFTS IN FRESHWATER SPRINGS AND RIVERS: USING FOSSIL, DEAD, AND LIVE MOLLUSKS TO TRACK RESPONSES TO LONG- AND SHORT-TERM DISRUPTIONS

The freshwater springs and rivers of Florida are increasingly threatened by human impacts, climate change, and environmental disturbances. In tidally influenced rivers, rising sea levels and amplified storm surge events may increase salinity, causing shifts in freshwater vegetation and invertebrate communities. These events may further contribute to the upstream retreat of freshwater species and their displacement by both introduced and brackish-tolerant taxa. While these systems respond to dramatic, short-term disturbances such as hurricanes, they have also undergone protracted community restructuring.

Hurricane Michael made landfall in Florida on 10 October 2018, bringing more than three meters of storm surge up the Wakulla River and inundating portions of the river with brackish water. Using death assemblages and live mollusk records collected before and after the storm, we documented the response and recovery of Wakulla’s molluscan community. We also compared recent communities with fossil (late Pleistocene-Holocene) assemblages. A total of 123 live, 38 dead, and 32 fossil samples were collected between 2017-2023, yielding a total of 30898 specimens representing 23 taxa. Thirteen taxa (all native) were identified from fossil deposits. Before the storm, modern species richness had dropped to ten taxa, including two introduced species and one brackish-tolerant gastropod. This drop reflects the local extirpation of multiple native freshwater species from the Wakulla River. Of the extant native species, half were found at severely diminished abundances (< 20 individuals). Following the hurricane, species richness did not change but the relative abundance and distribution of species shifted. Introduced species prospered in the short-term (two-months post-storm), while brackish-tolerant taxa saw a large range expansion upriver in both the seven-months and five years post-storm surveys. Native species saw relative decreases, though less severely in the protected Edward Ball Wakulla Springs State Park surrounding the headspring and upper river. These trends suggest that protected upstream regions likely act as a refugia and are critical for maintaining biodiversity as rivers are increasingly at risk from storm and climate disturbances.