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 percolation period with artificial porewater. The porewater, which contained 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 examination was made of the P added. Sediment with L. uniflora had a significantly 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 decreased 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 uptake by the plants. Bare sediment had a lower capacity for; retaining P, thus steady-state was reached after 7 days with retention efficiencies of 24% in the light and 3% in darkness. The higher retention in light, 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 depths 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 on the surface.