Habitat modifications are among mankind's most pervasive alterations of our nation's estuarine ecosystems. When such modifications are extensive, as is the case for the Mobile Bay Causeway, they can alter patterns of natural hydrography. Among the possible consequences of the Causeway is the reduction of water exchange between the fresh water in the lower reaches of Mobile-Tensaw Delta, and the saltier waters of the Gulf of Mexico. If true, this barrier may have created persistent low salinity conditions that local conservationists believe have provided refuge for an exotic species of submerged aquatic vegetation, the Eurasian Milfoil (Myriophyllum spicatum) to survive in during periods when salinity is high throughout this estuary.
When salinity is low, milfoil fragments from these freshwater refuges end up in nearby estuarine grassbeds, where they subsequently outgrow and competitively displace native submerged grasses.
To test these hypotheses, Dr. John Valentine and Marine Technician Susan Sklenar, both of the Dauphin Island Sea Lab (DISL) in Alabama, are currently comparing the results of salinity tolerance experiments they have conducted using milfoil at the DISL with two years of field data which document salinity patterns within the upper reaches of Mobile Bay.
The preliminary results of these experiments suggest that only the most extreme salinities, those observed during hurricane landfalls in the northern Gulf of Mexico, are lethal to milfoil.
"Right after Hurricane Katrina, we noticed that milfoil was not as abundant in those places where it used to be plentiful," recounts Dr. Valentine. "Whether it was the turbidity from the storm or the salinity from waters crashing over the MBC, we're hoping these experiments will be able to help determine the cause."
In the coming year, Dr. Valentine and his colleagues will be conducting additional field experiments to determine if in fact milfoil will outcompete native grasses for habitat within this estuary. It is hoped that these experiments, when completed, will allow DISL to make data-based recommendations for habitat restoration later next year.
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