Persistence of legacy soil P and elevated background water P concentrations in Water Conservation Area 2A, a northern Everglades wetland

Abstract

Upstream source control and Stormwater Treatment Areas (STAs) have reduced phosphorus (P) loads to Water Conservation Area 2A (WCA-2A), a northern Everglades wetland, by three quarters since year 2000. Nevertheless, large storages of P remain in enriched peat soils and it is unclear how legacy stores will impact spatial and temporal scales of recovery. We remeasured soil P enrichment along a well-studied eutrophication gradient in WCA-2A and applied a profile modeling approach with uncertainty analysis to assess changes in longitudinal soil P gradients 13 years after load reductions. We then analyzed existing internal water P data, using a novel data screening approach, for evidence of lowest possible water P concentrations independent from inflows. We interpret such water P limits as evidence of the strength of internal loading at a location. Results indicate that soil P enrichment persists in the ∼7.5 km long "impacted" zone, with no significant evidence of net advancement or recession, while a large pool of labile P in the flocculent layer consolidated and diminished. There is indeed evidence, both spatial and temporal, that this extensive zone of enriched soil P continues to elevate lowest achievable water P concentrations. The corresponding gradient of elevated water P limits is both receding and diminishing since load reductions, thus providing further evidence toward recovery. However, results also suggest that these "transitory P limits" due to internal loading are likely to persist for decades above water quality targets. These results advance our understanding of recovery in impacted wetlands and are relevant to Everglades restoration.