Transcript Florida's Seagrasses - St. Johns County Extension Office

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