Environmental factors affecting the distribution of native and invasive aquatic plants in the Atchafalaya River Basin, Louisiana, U.S.A.

Over the last century, the aquatic macrophyte community of the Atchafalaya River Basin (ARB) has become increasingly dominated by invasive species. I used digital photography and GIS software to determine ARB macrophyte community composition by measuring percent cover of each plant species within five 0.25-m2 quadrats sampled from 108 sites in 2005 and 103 sites in 2006. Macrophyte community data and measurements of soil and water physicochemistry at each site were used to investigate environmental associations of the macrophytes inhabiting bayou, lake, excavated canal, and swamp habitats. Analyses indicated substantial differences in physicochemical conditions among habitats, but none of the 20 macrophyte species collected during the study exhibited consistent preferences for specific habitat types. Percent coverage of invasive plants was greater than native plants across all habitat types in both years, with invasive plant richness increasing in 2006 with the arrival of giant salvinia (Salvinia molesta). Common salvinia (Salvinia minima) appeared to have the greatest impact on the native plants, exhibiting inverse abundance relationships with six of fourteen species (43%). Comparisons of slopes from areal coverage – dry weight regressions based on macrophytes that were photographed, collected, and dried from quadrats sampled at 26 sites in 2006 suggested that invasive species accumulated more biomass per unit area than ecologically similar native taxa. In both years, terrestrial plants were observed in association with floating mats of other macrophyte species, apparently taking advantage of the mats as “terrestrial†substrate. Submerged plants exhibited few significant differences in abundance among the four habitats, although they did tend to occur where floating plants were not abundant. There were few physicochemical differences among vegetated and non-vegetated sites for native or invasive plants, although pH was lower at vegetated sites (versus non-vegetated) for both native (2005) and invasive (2005 and 2006) plants. Canonical discriminate function analysis revealed substantial changes in plant community composition and physicochemistry between the two years at approximately 25% of the study locations. These changes highlight the dynamic nature of the littoral zone and the multiplicity of deterministic and stochastic factors that likely affect the composition of the resident macrophyte community in the ARB