We used paleoecological methods to infer environmental conditions in Water Conservation Area 2A (WCA-2A) of the Everglades before impoundment in 1961, and we compared pre-impoundment environmental conditions to present conditions. Abundances of siliceous microfossils (diatoms, chrysophyte cysts, sponge gemmoscleres, and plant phytoliths) were analyzed in two sediment cores from nutrient-enriched northern WCA-2A and in two sediment cores from unenriched southern WCA-2A. Nutrient enrichment in northern WCA-2A after 1961 was associated with an increase in relative abundance of eutrophic diatoms. A pH increase in much of WCA-2A after 1961 was indicated by a decrease in relative abundance of acidic indicators (Eunotia diatoms and the sponge Anheteromeyenia ryderi) and a decrease in richness of chrysophyte cysts. An increase in anoxia in nutrient-enriched northern WCA-2A, during about 1961–1980 when high water depth was maintained, was suggested by an increase in visible dissolution of siliceous microfossils. A decrease in palm phytolith abundance after 1961 and lack of recovery after water depth was lowered suggest either that water depth is still maintained high enough to adversely affect vegetation or that there has been insufficient time for recovery. Numbers of sponge gemmoscleres decreased to less than 1% of pre-impoundment numbers, suggesting that there has been a dramatic decrease in sponge abundance. The environmental changes discovered in this study suggest directions for future management of the Everglades.