Flora and Fauna of Northwest Florida

Biology Department

University of West Florida

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Seagrasses

Seagrass meadows share many of the same ecological functions as saltmarshes, but these habitats are completely submerged. Like saltmarshes, seagrass meadows have very high productivity. Organisms in seagrass communities use the seagrasses directly, and also use the dead plant material (detritus) as a food source. Perhaps the greatest benefit to many organisms is the complex physical habitat provided by submerged grasses that provides a refuge from predation and a forage area for juvenile and adults of many species. Like marshes, seagrass meadows have a tremendous amount of surface area and reduced water velocity between grass blades that facilitates physical and biological filtration of suspended material in the overlying water.

Organisms like commercial brown shrimp (Penaeus aztecus; left) use the grass beds as both nursery and feeding areas. Some fish, like the Atlantic needlefish (Strongylura marina; above) and spotted sea trout (Cynoscion nebulosa) hover over and around the grass beds looking for smaller organisms to eat.

 

In Santa Rosa Sound, FL, extensive seagrass meadows are predominantly comprised of the flowering plants Halodule wrightii (Cuban Shoal Grass) and Thalassia testudinum (Turtle Grass). Both of these plants have rhizomes (under"ground" stems) and roots in anaerobic sediments. The blades or leaves of the plants extend out and above the sediment from nodes on the horizontal rhizomes. Thick intermeshed mats of rhizomes are typical in a mature seagrass bed, stabilizing the bottom.

Thalassia testudinum leaves washed up during the winter on Range Point, Santa Rosa Sound.

Thalassia (thal-ass-ee-a) testudinum has flat tape-like leaves. It is at the northern limit of its range in Santa Rosa Sound, and it is deciduous here, losing its leaves when water temperatures drop in winter. Thick piles of shed Thalassia blades are common on area beaches in the winter. Thalassia will not tolerate any air exposure in Santa Rosa Sound, and so it does not occur as close into shore as Halodule. Its shoreward distribution is mostly controlled by the lowest low tides that occur in winter. As a result, there is a band of pure Halodule close to shore, and mixed Halodule and Thalassia out further to a depth of 6-8 feet. Beyond that depth, light penetration to the bottom is reduced beyond the needs of both plants.

Halodule (hal-oo-do-lee) wrightii has thin rounded blades. This grass is relatively hardy, and is known as a pioneer species in bare sediment habitats. In Santa Rosa Sound the landward distribution of this species is limited by mean low water, but it does tolerate limited air exposure and low temperature associated with shallow water.

A third species commonly found is Ruppia maritima (Widgeon Grass). This grass has upright branched stems, and does not form the dense web of underground rhizomes, as do the other two species. Ruppia tolerates wide ranges of salinity and temperature, and is a very prolific grass, dispersing widely and spreading fast where it gains a foot hold.

Seagrasses are sensitive to water quality, and are endangered worldwide. Increased turbidity is the biggest problem, limiting photosynthetic ability of the plants by limiting light penetration. Increases in erosion increase suspended mineral particles. Increased nutrients result in increased phytoplankton growth that limits light penetration, and also increases the growth of epiphytes (aufwuchs) on grass blades and macroalage growth (Chondria, Gracilaria) that competes with seagrasses for light. Nutrients in the sediment are good for seagrass growth; nutrients in the water column are not. Nutrient/turbidity effects swamp most toxin effects (i.e. herbicides), but as with most stresses, effects are cumulative and often synergistic (sum of separate effects less than combined effect)

Seagrasses are also affected by physical damage. Dredging and filling activities have taken their toll on many grass beds. Prop scars from boat traffic in shallow areas leave 1 to 2 foot wide bare strips where the rhizomes have been removed. These take 3-5 years to heal, and so cumulative damage often occurs. Anchoring will also rip up rhizomes. Shading from piers will inhibit growth just as water turbidity does.

Seagrasses are also subject to bio-perturbation, or the digging activities of aquatic organisms. Skates dig pits looking for food, and crabs dig up rhizomes in burrowing for shelter.

 

Dominant Seagrass Organisms

Plants

Turtle Grass

Thalassia testudinum

Cuban Shoal Grass

Halodule wrightii

Widgeon Grass

Ruppia maritima

 

Crustaceans

Amphipods

Gammarus sp.

Green Striped Hermit Crab

Clibanarius vittatus

Blue Crab

Calinectes sapidus

Spider Crab

Lubinia emarginata

Mud Crab

spp

Grass Shrimp

Palaemonetes sp.

Broken Back Shrimp

Hippolyte sp.

White Shrimp

Penaeus setifers

Pink Shrimp

Penaeus duorarum

Brown Shrimp

Penaeus aztecus

Snapping Shrimp

Alepheus heterochaelis

 

Coelenterates

Hydroids

 

Molluscs

Crown Conch

Melongena corona

Oyster

Crassostrea virginica

Bay Scallop

Argopecten irradians

Lightning Whelk

Busycon contrarium

Mud Snail

Ilyanassa obseleta

Oyster Drill

Urosalpinx cinerea

 

Fishes

Goby

spp.

Pinfish

Lagodon rhomboides

(up to 90% of biomass)

Pipefish

Syngnathus scovelli

Toadfish

Opsanus beta

Bay Anchovy

Anchoa mitchelli

Silverside

Menidia berylina.

Killifish

spp

Spotted Seatrout

Cynoscion nebulosus

Red Fish (drum)

Sciaenops ocellatus

Croaker

Micropogonias undulatus

Spot

Leiostomous xanthurus

Needlefish

Strongylura marina

Ladyfish

Elops sarus

Striped Burrfish

Chilomycterus schoepfi