Positive feedback favors invasion by a submersed freshwater plant.

The submersed macrophyte Utricularia inflata has invaded lakes in northern New York State, thereby threatening native isoetids such as Eriocaulon aquaticum. Isoetids often dominate and modify softwater lakes due to their capacity to oxidize sediment and thus influence solute mobilization. Greenhouse experiments tested the hypotheses that U. inflata invasion could result in higher porewater iron (Fe) concentrations and greater ammonium (NH4 (+)) and Fe release from the sediment into the water column, and that this mobilization would stimulate further U. inflata growth. In the first experiment, three levels of U. inflata impact on E. aquaticum were imposed using sediment cores overlain by lake water: E. aquaticum alone, E. aquaticum with a cover of U. inflata, and bare sediment--the latter to simulate local extirpation of the isoetid by the invasive. After 16 weeks, sediment porewater NH4 (+) and total dissolved Fe concentrations were significantly higher (P < 0.05) for the U. inflata and bare sediment treatments. Water column concentrations of these solutes were five-fold higher (P < 0.05) for the bare sediment treatment than E. aquaticum alone, indicating that isoetid extirpation by U. inflata can compromise water quality. A second experiment demonstrated that U. inflata grew faster over bare sediment than over sediment with E. aquaticum (P < 0.05), likely due to greater solute mobilization in the absence of E. aquaticum. Where U. inflata causes a decline of native isoetids in Adirondack Mountain lakes, changes to lake sediment and water chemistry can create a positive feedback loop further escalating the impact of this invasive species.