Comparison of iron, manganese, and phosphorus retention in freshwater littoral sediment with growth of Littorella uniflora and benthic microalgae

Sediment columns from an oligotrophic lake were percolated with artificial pore-water in two 46-day experiments to examine the effects of Littorella uniflora and benthic microalgae on retention of phosphorus (P) by either iron (Fe) or manganese (Mn). Cumulative retention of P, Fe, and Mn was 2-5 times higher in sediment with L. uniflora than in sediment with microalgae, because of higher P uptake and more efficient Fe and Mn oxidation by L. uniflora than by microalgae. Thus 34% and 21% of added P was retained in L. uniflora inhabited sediments as metal-oxide bound P compared to 11% and 2% in microalgae inhabited sediments, in experiments supplied with Fe and Mn, respectively. The atomic ratio of Fe/P precipitation was about 1 and for Mn/P precipitation it was about 5. These ratios indicate precipitation of Fe(III)-phosphate (strengite) and metastable Mn(IV)-compounds containing phosphate and hydroxide ions in variable amounts. In addition to metal-oxide P precipitation, increased P retention in the vegetated sediment was also caused by the presence of humic acid compounds, which accounted for about 26% of total retained P.