Thesis:  Effects of Pre-exhisting Submersed Macrophytes on the Invasion Success of Hydrilla verticiliata
Todd B. Chadwell, M.Sc., 2004

I evaluated (1) the effects of a native submersed aquatic plant species, Vallisneria
americana, on the colonization success of a non-native and highly invasive submersed
aquatic species, Hydrilla verticiliata, through field and greenhouse experiments; and (2)
the effects of H verticiliata propagule density on its colonization success of patches
dominated by the native species. Results of the field study, located in a tidal freshwater
region of Chesapeake Bay, suggest that pre-existing vegetation did not have significant
negative effects on H verticiliata colonization. However, pre-existing H verticiliata
biomass and H verticiliata colonization success were strongly correlated. In contrast,
results from the greenhouse study showed that V. americana had a strong inhibitory
effect on H verticiliata colonization by fragments and increasing H verticiliata fragment
density again increased colonization success. Reduced water column nutrient
concentrations, resulting from V. americana growth, appeared to negatively affect
successful rooting and subsequent colonization by H verticiliata.