Seeds of Pistia stratiotes L. (water lettuce) in the paleo-sediments of Lake Annie, Florida

Pistia stratiotes L. (water lettuce) is a floating tropical macrophyte long identified and managed as a non-native species within the State of Florida and other areas of the southern United States. Macrofossil seeds from Lake Annie, Florida, however, indicate abundant presence of P. stratiotes intermixed with other locally native macrophytes from ~13,500 to ~12,000 calibrated years before present (cal yr BP). This was soon after the lake depression first began filling with water as the piezometric groundwater surface of the Florida peninsula rose in response to rising seas during the transition (~18,000 to ~11,000 cal yr BP) from the Last Glacial Maximum (LGM) to the Holocene interglacial. These macrofossil records join several other lines of evidence supporting native status of P. stratiotes in the Florida peninsula. While recent cryptic invasion of non-native Pistia genotypes into some of Florida’s freshwater ecosystems also appears likely, confirmed paleo-presence and contemporary persistence of native P. stratiotes subpopulations may have especially important management and conservation implications for Florida’s spring-fed streams. Palaeobotanical evidence of this type may be useful in further resolving the global biogeography of P. stratiotes and other cryptic aquatic plant species.