Comparison of iron and phosphorus mobilization from sediments inhabited byLittorella uniflora and Sphagnum sp at different sulfate concentrations

Atmospheric deposition of sulfur may cause both acidification and elevated sulfate concentrations in oligotrophic softwater lakes. This change is ofte n followed by rapid expansion of mosses and concomitant suppression of isoe tid plant species. Sediment columns inhabited by isoetids (Littorella unifl ora) and mosses (Sphagnum sp.) were percolated with artificial porewater co ntaining low (14 mu M) and high (182 mu M) sulfate concentrations to analys e the effect of vegetation and sulfate on iron (Fe) and phosphorus (P) mobi lization from sediments. Due to oxygen release from plant rests, the redox potential (E-h) was high (400 to 500 mV) in sediments with L. uniflora and there was no consumption of sulfate in any of the sulfate treatments indica ting an insignificant sulfate reduction rate. During the 13-week percolatio n period, low amounts of Fe (<2 mmol m(-2)) and P (similar to 1 mmol m(-2)) were mobilized from the sediment to the water. For sediment with Sphagnum, Eh was lower (<200 mV) than for sediment with L. uniflora, however, E-h wa s significantly higher in the low sulfate than in the high sulfate treatmen t. The consumption of sulfate was 0.36 and 3.43 mmol m(-2) d(-1) for cores with low and high sulfate, respectively, but no significant influence of su lfate consumption rates on Fe and P mobilization was found. For cores with Sphagnum, the mobilization of Fe was about 300 mmol m(-2) and for P it was about 20 mmol m(-2) which were one to two orders of magnitude higher than o bserved for cores with L. uniflora. Analysis of solid sediment fractions of Fe and P before and after the experiment indicated that the high mobilizat ion of Fe from sediment with Sphagnum originated from the redox sensitive F e fraction whereas P originated from mineralization of organic P compounds in the surface of the sediment.