Recent human activities have caused excess phosphorus (P) to be bound in, and at times released from, the sediments of some aquatic ecosystems.  Shallow lakes and wetlands that undergo periodic desiccation can differ in their sediment-water P exchanges from those that are permanently inundated, particularly when iron minerals sequester much of the P.  To investigate the importance of desiccation, water levels were manipulated in laboratory mesocosms containing sediments collected from permanently inundated and periodically emergent sites in a Michigan lake.  Sediments were subjected to four treatments: continuously flooded with overlying water, saturated without overlying water, moist, and allowed to dry.  Subsequently, sediments were re-flooded and overlying water chemistry was observed for 40 days.  In the periodically emergent sediment that was dried, average SRP increased greatly to 780 μgL^-1, whereas other treatments with this sediment had lower concentrations (<22 μgL^-1).  In the permanently inundated sediment, SRP concentrations ranged from 1-66 μg^-1 across all treatments, with the flooded and saturated treatments tending to have higher concentrations.  Alternative hypotheses for the large P release from the periodically emergent sediment that had been dried will be evaluated, and future research will be outlined.