Since the 1970s, nutrient-enriched agricultural drainage water has been pumped into the Everglades Water Conservation Areas, markedly changing plant communities and oxygen regimes. The sawgrass plains and diverse aquatic sloughs that once dominated the area are gradually being replaced by more heterotrophic cattail stands. An oxygen budget study in Water Conservation Area-2A revealed diurnal oxygen levels, and variations were greatest in slough and sawgrass areas and minimal at the nutrient-enriched cattail site. Sediment oxygen demand was the major oxygen sink at all sites, while reaeration (sawgrass and cattail sites) and benthic algal production (slough site) were the major sources of oxygen. The slough algal mat areas represent important sites for marsh primary production and detrital processes. These areas function as feeding areas for fishes, invertebrates, and waterfowl, as well as important oxygen sources for adjacent sawgrass areas. Their elimination, therefore, could have serious ecological consequences.