Non-pollen microfossils in Everglades sediments

Abstract

Modifications of the hydrology of the Florida Everglades began in the 1880s. Impacts due to changes in the hydrological regime, as well as nutrient enrichment are clear in the northern Everglades. Although these changes were more subtle in the southern Everglades, altered hydroperiods and plant communities are targeted for restoration. Identification of restoration goals requires knowledge of past conditions, but detailed vegetation surveys prior to the pre-drainage era are lacking. Palynological studies can help fill this void. Interpretations based upon pollen and plant spores alone are limited by low pollen production of the dominant marsh species and low plant diversity in the ecosystem. In some regions the occurrence of other microfossils present in palynological preparations has been used to help interpret hydrological regimes and nutrient status and here we explore that potential for paleoecological records from the Everglades. In this study we report the distribution of various microfossils found within pollen zones identified in sediments cored from three tree islands in Shark River Slough, Everglades National Park. Using published modern pollen analogues we classified paleovegetation zones as slough, sawgrass marsh, tree island or tree island edge and documented the abundances of microfossils of algae, fungi, cyanobacteria and testate amoebae. Tree island zones, indicative of relatively short hydroperiods, were characterized by highest abundances of fungal material, testate amoebae and the base of the deposits, by the cyanobacterium, Gloeotrichia. Much of the fungal material appears to be associated with plant roots, and the mycorrhizal Glomus is locally abundant. The taxonomic affinity of the testate amoebae has not been determined, but some were arcellaceous forms that included charcoal particles in their tests. Gloeotrichia also is found in marsh and slough deposits. Its occurrence in the latter probably signals minimal water depth. Botryococcus and Zynemataceae zygospores are found in all deposits, but are less abundant in the tree island deposits. Both groups are probably part of the periphyton flora, but Botryococcus has not been recorded in phycological studies of the Everglades. Observation of these algae in the seasonally dry Everglades is consistent with other studies where they have been reported to represent shallow, slow moving waters, but inconsistent with the assumptions of meso- or eutrophic conditions commonly made in other studies. Our observation of the alga Botryoccocus and the meroplanktonic cyanobacterium Gloeotrichia in the historically oligotrophic Everglades challenges their status as indicators of nutrient enrichment.