Eurasian milfoil invasion in estuaries: physical disturbance can reduce the proliferation of an aquatic nuisance species

Biological invasions are among the most pervasive yet least understood of the con sequences of the urbanization of estuarine ecosystems. In Mobile Bay, Alabama (USA), the construction of a transportation corridor, locally known as the Mobile Bay Causeway, has been hypothesized to have modified natural disturbance regimes to the point that numerous invasive species now persist in oligohaline reaches of this estuary. Here, we provide the results of field surveys and experiments designed to determine if the causeway facilitated the proliferation of the dominant invasive species, Eurasian milfoil Myriophyllum spicatum Linnaeus, 1753, throughout the Mobile-Tensaw Delta (MTD). Field surveys showed that the composition of submerged aquatic vegetation (SAV) varies greatly with location in the MTD; SAV south of the causeway is dominated by a single native species, wild celery Vallisneria americana Michaux, 1803, while milfoil and canopy-forming native species dominate areas north of the causeway. We found no evidence that the differences in species composition were related to differences in salinity, sediment grain size composition along the causeway, or competitive exclusion of the dominant native species by milfoil. We did, however, find a strong negative relationship between milfoil biomass and maximum wave force. These results suggest the causeway functions as a breakwater, reducing the penetration of large, wind-driven waves into oligohaline embayments north of the causeway. Counter to current thinking, these findings suggest that reductions in the intensity of physical disturbances will create opportunities for invasive milfoil to proliferate in the estuarine waters of the northern Gulf of Mexico.