This study investigated phosphorus (P) concentrations in Donana seasonal wetlands in southwestern Spain as the wetlands started to fill. following storm events after a drought. Both soluble and particulate P flowed into the Donana wetlands through runoff. A gradient in the soluble reactive phosphorus (SRP) concentrations was found from the littoral to the pond center following a 108.5-mm storm. The SRP concentration was 100 times higher 2 m from the shoreline than it was at the pond center. Neither the incoming. SRP nor the total P concentrations were related to the magnitude of storm events. They were always highest during the initial filling periods (first tyoweeks) and lower after subsequent, even heavier storms (>100 mm) a month later. Deposited sediments were a net P sink during the initial filling period of ponds. Samples of fresh vegetation, soil, and manure from a wetland watershed were washed in distilled water for 19 h, and the SRP in the leachates was measured in the laboratory. The highest SRP concentration (1.5 mg gDW(-1)) was leached from the fine (0.1 mm), organic-matter-rich (25% OM) soil diffraction from uplands. The SRP concentration found in pasture soil was 1.3 rag gDW(-1), followed by fresh scrub (1.25 mg gDW(-1)), and manure (0.9 mg gDW(-1)). Only during periods of heavy rainfall does runoff drain from the uplands to the basin of the wetland watersheds. Therefore, we conclude that incoming P concentrations to Donana wetlands vary greatly from year to year according to changes in hydro-meteorological conditions.

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