Seagrass losses are occurring worldwide with about 30 percent loss since the 1980s. Florida has had some areas with almost complete loss of seagrasses in enclosed areas such as the Indian River Lagoon and Biscayne Bay. Most of Sanibel’s waters have a good water exchange with the Gulf of Mexico, but areas with less exchange or that are surrounded by land are shifting to benthic algae cover.
In the Caloosahatchee, Polysiphonia subtilissima, a branching red algae, has been growing densely in the shallows where tape grass and widgeon grass used to thrive. In Matlacha Pass and other areas in Charlotte Harbor, Caulerpa fastigiata is a dominant species and its proliferation has coincided with a drop in seagrass density, as well as large-scale hypoxic events for the last two summers. Species of Caulerpa have been associated with significant ecosystem shifts in Europe and California.
They can form impenetrable mats that don’t allow seagrass blades to poke through, and they can suffocate clams and other infauna in the sediments beneath. When they create hypoxic events, then the plankton and small fish in the water column can suffocate as well. Excessive nutrient loading, and a starter culture of algal gametes, are all it needs to quickly spread. Among other critical functions, seagrasses feed manatees and green sea turtles. Through funding from a U.S. Environmental Protection Agency grant, SCCF is collaborating with Florida Gulf Coast University to document this recent shift in Matlacha Pass.