The specific activity of the ('32)P labeled sediment-P taken up by submersed macrophytes was shown to be identical to the specific activity of the sediment mobile P, as measured by isotopic dilution. The mobile P therefore represents the sediment-P available to aquatic macrophytes. / The ability to accurately measure the specific activity of the available sediment-P was applied to problems pertaining to the role of macrophytes in P cycling. The relative contribution of water and sediments in supplying P to macrophytes was measured by growing macrophytes in situ, rooted in ('32)P labeled sediments, and with the shoot in free contact with the unlabeled overlying water. Macrophytes grown in mesotrophic and eutrophic sites derived more than 95% of their P from the sediments alone. When grown in a hypertrophic site, the sediments still supplied 70% of the P. / Rates of P release by macrophytes and significance to their periphyton and surrounding phytoplankton were estimated by using fully labeled plants. The periphyton derived only 6.5% of its P form the macrophytes. Myriophyllum released 0.32 ug.g('-1).h('-1) P, most of which being readily available to phytoplankton. / The high vertical mobility of the available sediment-P was demonstrated both by experimental manipulations and direct observations.

No description available.