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Florida’s Seagrasses Maia McGuire, PhD FL Sea Grant Extension Agent 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) Thalassia testudinum Turtle 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 Halodule wrightii Shoal 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 Syringodium filiforme Manatee 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 Halophila engelmanni, H. decipiens, H. johnsonii • Relatively sparsely distributed • Paddle-shaped leaf blades • Johnson’s seagrass is listed as a threatened species Ruppia maritima Widgeon grass • Often found alongside Halodule in areas of lower salinity • Important food for waterfowl • Primarily a freshwater plant Seagrass distribution Thalassia testudinum Halodule wrightii Syringodium filiforme Halophila engelmanii Halophila decipiens Halophila johnsonii Ruppia maritima distribution 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) 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 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 Other seagrass community members • Macroalgae – Caulerpa spp. – Several species of calcareous green algae • Halimeda, Penicillus, Udotea • Epiphytic algae – 113 species identified on Thalassia (Humm, 1964) – Include coralline red algae – Includes N-fixing blue-greens – Leaf tips usually more heavily epiphytized 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 • Crustaceans – Amphipods, shrimp, crabs • Corals • Sponges • Polychaetes – E.g. Medusa worm 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… • Reptiles – Green sea turtle (Chelonia mydas) • Birds – Great blue heron, great egret, osprey, brown pelican… • Mammals – Manatee, bottlenose dolphin Role of seagrass beds • Primary producer – Food for grazers; produce detritus • Habitat – Nursery grounds – Permanent home for many species • Sediment stabilization Threats to seagrasses • Physical damage – Dredging, prop scars/blowouts • Eutrophication • Salinity stress • Temperature stress