EFFECTS OF BIOTURBATION IN CONTROLLING TURTLEGRASS (THALASSIA-TESTUDINUM BANKS EX KONIG) ABUNDANCE - EVIDENCE FROM FIELD ENCLOSURES AND OBSERVATIONS IN THE NORTHERN GULF-OF-MEXICO

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 biot urbation by these organisms on tropical or subtropical seagrass (turtl egrass, Thalassia testudinum Banks ex Konig) meadows are less well kno wn. Stingray enclosure studies in St. Joseph Bay, Florida (29-degrees- N, 85.5-degrees-W) found that rays were unable to create unvegetated p atches within continuous turtlegrass and that only large (disc width g reater-than-or-equal-to 0.9 m) rays (Dasyatis americana Hildebrand & S chroeder) can damage rhizomes at the turtlegrass/sand flat interface h abitats. Field surveys found large rays to be abundant in turtlegrass habitats only during July-August. These data indicate that stingrays a re not responsible for the widespread occurrence of unvegetated patche s within St. Joseph Bay. Additional exclosure experiments found that s and dollars (Mellita quinquiesperforata Leske) did not affect turtlegr ass colonization of unvegetated sand flats. Comparisons of sand flat p erimeters enclosing sand dollars at ambient densities (5-15 individual s/m2) and those from which sand dollars were removed showed no inhibit ion of turtlegrass colonization of unvegetated sand patches after a 2- yr period. However, stone crab (Menippe spp.) burrow construction at t he seaward edge of turtlegrass habitats was found to cause significant , previously unreported, losses of turtlegrass habitat (average seagra ss recession > 1 m/7 months). We conclude that bioturbation has less i mpact on turtlegrass than eelgrass habitats due to thicker, deeper tur tlegrass rhizomes, and coarser sediment grain size.