In managed wetlands, the spatial scales and legacies of infrastructure disturbance impacts are controlled by hydrologic, biogeochemical, and biotic connectivity across boundaries. As hydrologic restoration ensues in the oligotrophic Florida Everglades, it is important to minimize the unintended impacts of restoring hydrologic connections. Of particular concern are sources of the limiting nutrient, phosphorus, stemming from historical enrichment of large parts of the Everglades and adjacent lands. Benthic microbial mat (periphyton) communities can provide early signals of surface and groundwater supplies and downstream transport of both newly delivered and historically sequestered (legacy) nutrients. To determine how hydrologic restoration influences nutrient mobilization, we examined datasets containing up to 25 years of limiting nutrient, phosphorus (P) concentration, biomass, and diatom composition in periphyton along 14, 100-m transects perpendicular to the eastern boundary of the Everglades. A before-after-control-intervention (BACI) analysis was applied to detect effects of restoration projects beginning in 2016. There was a steep distance-decay function in periphyton TP with an intervention x time interaction effect at near-boundary locations. Restoration decreased periphyton biomass and the abundance of oligotrophic diatoms in interventin relative to control transects. Stepwise linear regression models including water flow and plant community composition as independent variables significantly explained >80% of interannual variability in these periphyton attributes. Despite water quality improvements along some transects, legacy P continues to travel downstream along enriching transects. This is the first signal of an enrichment cascade whose reversibility will become increasingly challenging.