Effects of bioturbation in controlling turtlegrass ( Thalassia testudinum Banks ex König) abundance: evidence from field enclosures and observations in the Northern Gulf of Mexico

Abstract

Bioturbation by stingrays and sand dollars can limit the distribution of temperate zone seagrass (eelgrass, Zostera marina Lamarck) meadows through the disruption of the root-rhizome matrix. The effects of bioturbation by these organisms on tropical or subtropical seagrass (turtlegrass, Thalassia testudinum Banks ex König) meadows are less well known. Stingray enclosure studes in St. Joseph Bay, Florida (29°N, 85.5°W) found that rays were unable to create unvegetated patches within continuous turtlegrass and that only large (disc width ≥0.9 m) rays ( Dasyatis americana Hildebrand & Schroeder) can damage rhizomes at the turtlegrass/sand flat interface habitats. Field surveys found large rays to be abundant in turtlegrass habitats only during July–August. These data indicate that stingrays are not responsible for the widespread occurrence of unvegetated patches within St. Joseph Bay. Additional exclosure experiments found that sand dollars ( Mellitta quinquiesperforata Leske) did not affect turtlegrass colonization of unvegetated sand flats. Comparisons of sand flat perimeters enclosing sand dollars at ambient densities (5–15 individuals/m 2) and those from which sand dollars were removed showed no inhibition of turtlegrass colonization of unvegetated sand patches after a 2-yr period. However, stone crab ( Menippe spp.) burrow construction at the seaward edge of turtlegrass habitats was found to cause significant, previously unreported, losses of turtlegrass habitat (average seagrass recession > 1 m/7 months). We conclude that bioturbation has less impact on turtlegrass than eelgrass habitats due to thicker, deeper turtlegrass rhizomes, and coarser sediment grain size.