INFLUENCE OF LITTORELLA-UNIFLORA ON PHOSPHORUS RETENTION IN SEDIMENT SUPPLIED WITH ARTIFICIAL POREWATER

Phosphorus retention was measured in sediments with growth and without the growth of the isoetid plant, Littorella uniflora, over a 30-day p ercolation period with artificial porewater. The porewater, which cont ained 100 mu M phosphate, was pumped vertically through a 12-cm layer of sediment at a rate of about 61 m(-2) h(-1) and a quantitative exami nation was made of the P added. Sediment with L. uniflora had a signif icantly higher redox potential than bare sediment owing to O-2 release from the plant roots. Sediment with L. uniflora retained all P added by porewater percolation in the first 10 days. Thereafter, retention d ecreased and reached a steady-state after 23 days of pumping, in which 43% of added P in the light and 47% in the dark periods was retained. The high steady-state retention in both light and dark was due to P u ptake by the plants. Bare sediment had a lower capacity for; retaining P, thus steady-state was reached after 7 days with retention efficien cies of 24% in the light and 3% in darkness. The higher retention in l ight, in the bare sediment, originated from P assimilation by benthic microalgae. In the sediment with L. uniflora, 63% of total recovered P was in the sediment mostly as adsorbed and iron-bound phosphate at de pths below 5 cm. In the bare sediment, only 21% of the total recovered P was in the sediment, mostly as organic-P and iron-bound phosphate o n the surface.