Environmental assessment of vegetation and hydrological conditions in Everglades freshwater marshes using multiple geochemical proxies

Abstract

Paleoecological reconstructions of environmental changes provide important information for Everglades restoration targets. Traditionally this has been achieved using a combination of biological and physical indicators. However, as microfossils may be sporadically abundant in Everglades soils, organic geochemical methods can provide information at the molecular level. To reconstruct vegetation trends over the last century, soil cores from Shark to Taylor Sloughs, the primary flowpaths of the southern Everglades, were examined using several geochemical proxies. The n-alkane derived biomarker Paq effectively distinguished organic inputs from sawgrass and slough habitats. Other proxies examined include Kaurenes, cyclic diterpenoids unique to sawgrass roots; biomarkers of algae (highly branched isoprenoids (C20HBIs) and Botryococcenes); lignin phenols as vascular plant indicators; and macrofossils. At all sites, soil profiles from sawgrass marshes showed vegetation had shifted over the last 100 years, from sloughs to sawgrass-dominated marshes, reflecting decreased water levels (shorter hydroperiods) induced by water management. Paleo-assessments of modern sloughs, however, indicate these habitats remained deeper water habitats throughout the period of record, though shifts toward shorter hydroperiod vegetation were observed. In Taylor Slough, evidence of increasingly dry conditions in sloughs was confirmed by seed inputs from woody species. At 3 of the 5 sites, recent increases in C20HBIs and Botryococcene concentrations indicated greater periphyton abundance, coincidental with increased mineral concentrations observed in surface waters during the mid-20th Century. Bulk proxies such as organic content and carbon:nitrogen ratios also supported findings of changes in relative contributions of microbial and higher plants in this ecosystem.