Background/Question/Methods
The submersed aquatic macrophyte Utricularia inflata (inflated bladderwort) has invaded lakes in the Adirondack Mountains of New York State, thereby threatening native isoetids.  Isoetids often dominate and strongly influence oligotrophic and softwater lakes due to their extensive root systems that oxidize sediment, but U. inflata can effect the demise of the widespread isoetid Eriocaulon aquaticum.  Two greenhouse experiments investigated interactions among U. inflata, E. aquaticum, and water column chemistry.  The first experiment tested the hypothesis that, in the presence of E. aquaticum, U. inflata not only exerts profound influences on sediment chemistry (previously shown), but also manifests increases in water column ammonium and iron concentrations.  The second experiment evaluated the effect of the absence of E. aquaticum, as can occur with bladderwort invasion, on U. inflata growth as a correlate of enhanced NH₄⁺ efflux from the sediment. 

Results/Conclusions

The sediment-oxidizing E. aquaticum reduced sediment porewater concentrations of NH₄⁺ and Fe (P<0.001 for both solutes) as well as water column concentrations (P<0.05 for both solutes).  When a dense cover of U. inflata was imposed on E. aquaticum, however, sediment redox potential declined (P<0.001), porewater concentrations of NH₄⁺ and Fe and the water column concentration of Fe increased, in accord with our hypothesis.   In contrast, water column NH₄⁺ concentrations decreased in the presence of U. inflata, presumably due to nutrient uptake.  In our second experiment, U. inflata did indeed show greater relative growth rates when grown over bare sediment compared to sediment with E. aquaticum (P<0.05), perhaps due to the greater release of NH₄⁺ from sediment into the water column.  If U. inflata causes a decline of native isoetids in Adirondack Mountain lakes, changes to sediment and water column chemistry may then create a positive feedback loop further escalating the impact of this invasive species.