Floridas Seagrasses

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Floridas Seagrasses

• Maia McGuire, PhD
• FL Sea Grant Extension Agent

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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, Johnsons seagrass,
• Widgeon grass (freshwater grass with high
  salinity tolerance)

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Thalassia testudinumTurtle grass

• Largest and most robust of Floridas seagrasses
• Flat, wide (4-12 mm) leaf blades (10-35 cm in
  length)
• 2-5 leaves per shoot
• Forms extensive meadows

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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

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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

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Halophila engelmanni, H. decipiens, H. johnsonii

• Relatively sparsely distributed
• Paddle-shaped leaf blades
• Johnsons seagrass is listed as a threatened
  species

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Ruppia maritimaWidgeon grass

• Often found alongside Halodule in areas of lower
  salinity
• Important food for waterfowl
• Primarily a freshwater plant

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Seagrass distribution
Thalassia testudinum
Syringodium filiforme
Halodule wrightii
Halophila decipiens
Halophila johnsonii
Halophila engelmanii
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Ruppia maritima distribution
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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)

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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

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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

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Other seagrass community members

• Macroalgae
• Caulerpa spp.
• Several species of calcareous green algae
• Halimeda, Penicillus, Udotea

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

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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

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

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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

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

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Role of seagrass beds

• Primary producer
• Food for grazers produce detritus
• Habitat
• Nursery grounds
• Permanent home for many species
• Sediment stabilization

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Threats to seagrasses

• Physical damage
• Dredging, prop scars/blowouts
• Eutrophication
• Salinity stress
• Temperature stress