Effects of Nonnative Eurasian Watermilfoil, Myriophyllum spicatum, on Nekton Habitat Quality in a Louisiana Oligohaline Estuary

Submerged aquatic vegetation (SAV) provides critical habitat for estuarine nekton in the Gulf of Mexico, but habitat quality of SAV beds may change when nonnative species, such as Eurasian watermilfoil (Myriophyllum spicatum), become established. We compared the habitat value of Myriophyllum with another common native SAV (Ruppia maritima) by using field collections to document shifts in nekton community structure and a field experiment to compare growth rates of commercially important juvenile white shrimp (Litopenaeus setiferus). Similar communities were collected from both SAV species, though nekton abundance scaled with SAV biomass. The habitat quality provided by Myriophyllum for white shrimp appeared to meet or exceed that of Ruppia, with densities and growth rates of shrimp in Myriophyllum (2.2 ± 0.47 m⁻², 1.0 ± 0.07 mm TL day⁻¹, 25.7 ± 2.98 mg day⁻¹) higher than in Ruppia (1.0 ± 0.36 m⁻², 0.6 ± 0.09 mm TL day⁻¹, 11.6 ± 2.04 mg day⁻¹). Though differences were detected between SAV species, other factors, such as hypoxia and interspecific competition, likely contributed to the differences we documented between these habitat types in shrimp densities and growth. Our study provides an example of a nonnative habitat-forming species providing a viable alternative to native habitat. This result contradicts the paradigm of negative effects associated with nonnative species following their introduction into an ecosystem.