Herbivory impacts Vallisneria americana recovery in the lower St. Johns River, Florida

Abstract

Tape grass (Vallisneria americana) once formed large, dense meadows of long (up to 1 m) plants in the littoral zones of Lower St. Johns River (LSJR) and its associated lakes and tributaries (Sagan 2007). Tape grass and most other nonalgal submerged aquatic vegetation (SAV) vanished from much of the river basin after an extended period of high, tanninstained water and increased turbidity due to flooding from Hurricane Irma in 2017 (Goldberg and Trent 2020, Lundy et al. 2022, K.J., unpub. data). Flood events in the LSJR commonly cause temporary SAV die-offs (Lacoul and Freedman 2006, Bornette and Puijalon 2011). In 2004 prolonged conditions of low light availability caused by strikes from three major hurricanes (Charley, Frances, and Jeanne) were followed by extensive SAV loss throughout the river basin. Tape grass and other SAV re-emerged within 3 yr as the river returned to baseline hydrologic and water quality conditions. By contrast, SAV in the LSJR did not fully recover in the 5 yr between when Hurricane Irma made landfall in 2017 and 2022, despite a return to background water conditions. The tape grass plants that remain as of 2022 when data were collected for this study are sparse and have canopy heights below 10 cm (Goldberg and Trent 2020, Lundy et al. 2022, K.J., unpub. data). Herbivory commonly limits tape grass recovery in freshwater and estuarine habitats, including Kings Bay and other Florida waterways, but the extent to which it impacts SAV in the LSJR is unknown (Carter and Rybicki 1985, Hauxwell et al. 2004b, Johnson et al., 2019). Recent efforts to restore tape grass upstream of the LSJR in Lake George and Silver Glen Spring by using wire fenced enclosures to prevent grazing by herbivores yielded canopy heights approximately 10fold higher than surrounding unenclosed plants. When these protective fences were removed or breached the plants were grazed by cooters (Pseudemys spp.) within days to canopy heights equal to the surrounding SAV (, 10 cm). Additionally, blue tilapia (Oreochromis aureus) were observed uprooting unprotected plants near the restoration area (D.K., unpub. data). The Florida Fish and Wildlife Conservation Commission (FWC) has received multiple reports indicating that the tape grass beginning to re-establish areas of the LSJR appear similar to the heavily grazed plants in Lake George and Silver Glen Spring. This study was conducted to determine the effect of herbivory on the growth of tape grass in the Lower St. Johns River.