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), cm long Tips of leaves have 2-3 points

Syringodium filiforme Manatee grass Leaves are round (like spaghetti); 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 Syringodium filiforme Halodule wrightii 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 ppt –Extensive seagrass beds not found deeper than 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 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