Florida’s Seagrasses

1. Florida’s Seagrasses Maia McGuire, PhD FL Sea Grant Extension Agent

2. Seagrasses • Fully submerged marine plants; true angiosperms • True roots, vascular system, flowers • Reproduce asexually using rhizomes • 6-7 species; 3 common • Turtle grass, Shoal grass, Manatee grass • Star grass, Paddle grass, Johnson’s seagrass, • Widgeon grass (freshwater grass with high salinity tolerance)

3. Thalassia testudinumTurtle grass • Largest and most robust of Florida’s seagrasses • Flat, wide (4-12 mm) leaf blades (10-35 cm in length) • 2-5 leaves per shoot • Forms extensive meadows

4. Halodule wrightiiShoal grass • Early colonizer of disturbed areas or areas too deep or shallow for other seagrasses • Tolerates wide range of T, S • Leaves flat (1-3 mm wide), 10-20 cm long • Tips of leaves have 2-3 points

5. Syringodium filiformeManatee grass • Leaves are round (like spaghetti); 1-1.5 mm in diameter; length varies but can reach 50 cm • Commonly found mixed with other seagrasses or in small monospecific patches

6. Halophila engelmanni, H. decipiens, H. johnsonii • Relatively sparsely distributed • Paddle-shaped leaf blades • Johnson’s seagrass is listed as a threatened species

7. Ruppia maritimaWidgeon grass • Often found alongside Halodule in areas of lower salinity • Important food for waterfowl • Primarily a freshwater plant

8. Seagrass distribution Thalassia testudinum Halodule wrightii Syringodium filiforme Halophila decipiens Halophila johnsonii Halophila engelmanii

9. Ruppia maritima distribution

10. Seagrass biology • Growth • Thalassia blades can grow as much as 1 cm/day • Growth is slowed by cooler temperatures • Extremes in temperatures (hot or cold) can kill leaf blades • Optimal temperature range 20-30° C • Optimal salinity range 24-35 ppt • Extensive seagrass beds not found deeper than 10-15 m (light and pressure are both factors)

11. Zonation • Halodule grows in shallowest water and has highest tolerance to exposure • Thalassia is most dominant; forms large meadows in waters up to 10-12 m deep • Syringodium forms meadows in deep water (up to 15 m) • Halodule and Halophila can grow in even deeper water, but sparsely

12. Productivity • For Thalassia, range of 0.9 – 16 grams C/m²/day • 10 g C/m²/day = 3.65 kg C/m²/year • Measurements usually include associated plants (macroalgae, epiphytes)… • Highly productive ecosystems • Important food source, for grazers and as detritus

13. Other seagrass community members • Macroalgae • Caulerpa spp. • Several species of calcareous green algae • Halimeda, Penicillus, Udotea

14. Epiphytic algae • 113 species identified on Thalassia (Humm, 1964) • Include coralline red algae • Includes N-fixing blue-greens • Leaf tips usually more heavily epiphytized

15. Invertebrates • Mollusks • Conch (Strombus gigas), tulip shell (Fasciolaria tulipa), nudibranchs, emerald nerites (Smaragdia viridis), pen shell (Atrina spp.), scallops • Echinoderms • Sea urchins, sea stars • Sea cucumbers

16. Crustaceans • Amphipods, shrimp, crabs • Corals • Sponges • Polychaetes • E.g. Medusa worm

17. Vertebrates • Fish • Permanent Residents • Pipefish, seahorses, gobies, lizardfish, parrotfishes, eels… • Seasonal Residents • Pinfish, spot, spotted seatrout, silver perch, pigfish & other juvenile grunts, snappers, sheepshead, red drum, gag grouper, sharks…

18. Reptiles • Green sea turtle (Chelonia mydas) • Birds • Great blue heron, great egret, osprey, brown pelican… • Mammals • Manatee, bottlenose dolphin

19. Role of seagrass beds • Primary producer • Food for grazers; produce detritus • Habitat • Nursery grounds • Permanent home for many species • Sediment stabilization

20. Threats to seagrasses • Physical damage • Dredging, prop scars/blowouts • Eutrophication • Salinity stress • Temperature stress