ELSEVIER J. Exp. Mar. Biol. Ecol. 178 (1994) 181-192 JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY Effects of bioturbation in controlling turtlegrass (Thalassia testudinum Banks ex Kiinig) abundance: evidence from field enclosures and observations in the Northern Gulf of Mexico* John F. Valentine +**, Kenneth L. Heck, Jr. a, Patric Harper a, Michael Beck b a Marine Environmental Sciences zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG Consortium, P.O. Box 367-70, Dauphin Island Sea Lab, Dauphin Island, AL 36528. USA; Department of Marine Science, University of South Alabama. Mobile, AL 36688, USA; hDepartment @ “Biological Science, Florida State Universiry, Tallahassee, FL 32306, USA (Received 16 February 1993; revision received 14 December 1994; accepted 20 December 1993) Abstract Bioturbation by stingrays and sand dollars can limit the distribution of temperate zone seagrass (eelgrass, Zosteru marina Lamarck) meadows through the disruption of the root-rhizome matrix. The effects of bioturbation by these organisms on tropical or subtropical seagrass (turtlegrass, Thalussiu testudinum Banks ex Kdnig) meadows are less well known. Stingray enclosure stud- ies 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 20.9 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 occur- rence of unvegetated patches within St. Joseph Bay. Additional exclosure experiments found that sand dollars (Me/k quinquiesperforuta Leske) did not a.tTect turtlegrass colonization of unveg- etated sand flats. Comparisons of sand flat perimeters enclosing sand dollars at ambient den- sities (5-15 individuals/m*) and those from which sand dollars were removed showed no inhi- bition of turtlegrass colonization of unvegetated sand patches after a 2-yr period. However, stone crab (M enippe spp.) burrow construction at the seaward edge of turtlegrass habitats was found to cause significant, previously unreported, losses of turtlegrass habitat (average seagrass reces- sion > 1 m/7 months). We conclude that bioturbation has less impact on turtlegrass than eel- grass habitats due to thicker, deeper turtlegrass rhizomes, and coarser sediment grain size. Kell words: Bioturbation; Sand dollar; Stingray; Stone Crab; Turtlegrass * MESC Contribution No. 217. ** Corresponding author. 0022-0981/94/$7.00 0 1994 Elsevier Science B.V. All rights reserved SSDl 0022-0981(94)00005-X