Effects of water-level fluctuations on phosphorus dynamics of two subtropical wetlands in Doana National Park, Spain, and the Everglades, Florida, United States

Water-level fluctuations in wetlands result in alternate flooding and draining of soils, which greatly influences the processes regulating phosphorus retention and release to water. Drawdown can degrade or improve water quality in wetlands depending on soil conditions. It can enhance organic matter decomposition and mineralization due to increased microbial and photolytic activity in aerobic soils, promoting release of labile P to the water upon reflooding. It can also reduce P flux from soils by consolidating newly accreted soil, favoring precipitation reactions, and decimating bacterial populations.; Phosphorus dynamics were compared for two subtropical wetland ecosystems. A temporary pond in Doñana and a treatment wetland in Everglades were chosen to illustrate processes in each ecosystem. Doñana wetlands are hypereutrophic, have mineral soils, and are frequently exposed to drawdown. Everglades wetlands are oligotrophic, have organic soils, and are usually flooded. Constructed wetlands treat agricultural runoff by remaining flooded.; Phosphorus release and retention were investigated in soils exposed to varying flooding conditions. Phosphorus flux experiments showed how phosphorus release processes varied along a natural gradient of increasing drawdown exposure in Doñana ponds. In the Everglades treatment wetland, experiments evaluated the effectiveness of short-term drawdown for reducing phosphorus flux from soils.; In Doñana ponds, drawdown increased release of labile P to water upon flooding. In contrast, a 30-day drawdown of Everglades treatment-wetland soils markedly reduced P flux to the water. Mechanisms regulating P dynamics in Doñana ponds are release and uptake from phytoplankton in pond basins and rooted macrophytes-epiphytes in littoral and floodplains; complexation with iron/aluminum, calcium, and humic substances; and high organic matter decomposition rates. In the Everglades marsh, they are macrophyte-epiphyte removal and leaching, precipitation with calcium, and peat accretion.; The warm subtropical climate stimulates growth of aquatic biota, promoting high organic matter deposition. The functioning of Doñana ponds is characterized by erratic pulses of water and nutrients followed by explosive biological production, and high loss of organic matter via mineralization. Everglades marshes exhibit slower and more constant ecosystem processes under prevalent flooding, marked by limited linkage to watershed hydrodynamics, sustained high macrophyte-based production, slow detritus decomposition, and long-term accumulation of organic matter.