Alteration of Sediment Porewater Associated with a Eurasian Watermilfoil Invasion

Competitive advantages possessed by invasive species encourage aggressive growth that ultimately results in loss of native species diversity and richness. Eurasian watermilfoil, Myriophyllum spicatum L., is a non-native invasive macrophyte that has altered native species composition on several trophic levels. In this study, we examined changes in the nitrogen and phosphorus porewater content within sediments underlying areas dominated by M. spicatum and adjacent sediments supporting native macrophytes. Total dissolved phosphorus (TDP) concentrations were significantly greater early in the growing season in porewater underlying an M. spicatum canopy and declined more rapidly early in the growing season under areas dominated by M. spicatum than under native macrophytes. Rapid, early season uptake of TDP may provide a competitive advantage for M. spicatum. Throughout the growing season, total dissolved nitrogen (TDN) concentrations were on average 2 to 3 times greater in porewater from the native macrophyte areas than the M. spicatum dominated areas. A horizontal transect showed similar TDN profiles within 10 m of the M. spicatum bed center. Profiles of TDN at the edge and in the native area were significantly different from all other samples. We demonstrated that in Lake George, an oligotrophic lake, M. spicatum influences porewater TDP on a seasonal basis, while porewater TDN concentrations appear to be altered on a longer time scale. The increased microbial activity associated with macrophyte biomass and the possible strengthened nitrification-denitrification coupling may help explain the TDP and TDN profiles, respectively.