Florida’s SeagrassesFlorida’s Seagrasses

Maia McGuire, PhDFL Sea Grant Extension Agent

SeagrassesSeagrasses

• 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 testudinumThalassia testudinumTurtle grassTurtle 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 wrightiiHalodule wrightiiShoal grassShoal 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 filiformeSyringodium filiformeManatee grassManatee 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. Halophila engelmanni, H. decipiens, H. johnsoniidecipiens, H. johnsonii

• Relatively sparsely distributed • Paddle-shaped leaf blades• Johnson’s seagrass is listed as a

threatened species

Ruppia maritimaRuppia maritimaWidgeon grassWidgeon grass

• Often found alongside Halodule in areas of lower salinity

• Important food for waterfowl

• Primarily a freshwater plant

Seagrass distributionSeagrass distribution

Thalassia testudinum

Syringodium filiforme

Halodule wrightii

Halophila engelmanii

Halophila decipiens

Halophila johnsonii

Ruppia maritimaRuppia maritima distributiondistribution

Seagrass biologySeagrass 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)

ZonationZonation

• 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

ProductivityProductivity

• 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 Other seagrass community membersmembers

• 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

InvertebratesInvertebrates

• 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

VertebratesVertebrates

• 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 bedsRole of seagrass beds

• Primary producer– Food for grazers; produce detritus

• Habitat– Nursery grounds– Permanent home for many species

• Sediment stabilization

Threats to seagrassesThreats to seagrasses

• Physical damage– Dredging, prop scars/blowouts

• Eutrophication• Salinity stress• Temperature stress