Seagrasses

*General Importance:

Seagrasses are flowering plants (Angiosperms) that grow entirely underwater. They may tolerate or require various concentrations of salt.

Primary productivity in seagrass beds is among the highest measured (500 - 4000 g C/m2/year)

Seagrasses provide important habitat for marine species (shrimp, scallops, fishes)

Seagrasses stabilize sediment and filter water

*Seagrass Diversity:
 
*Major polyhaline (18-30 ppt) species of seagrasses
Scientific name Common name Salinity range (ppt) Occurrence 
Zostera marina eelgrass
7 - 35 
 Found in cold waters. Reported for Florida but not Louisiana
Halodule wrightii shoal grass
12-35
 Plaquemines and Terrebonne Parishes; Chandeleur Islands
Thalassia testudinum turtle grass
Tolerance: 3.5 - 60

Optimum: 24-35

 Chandeleur Islands; Cameron Parish?
Syringodium filiforme

(Cymodocea filiformis)

 manatee grass
24-35
 Chandeleur Islands
Halophila engelmannii "Halophila"
35
 Chandeleur Islands
Ruppia maritima widgeon grass
2 - 70
 Mostly coastal LA parishes

 
 
*Major mesohaline (5-18 ppt) species of seagrasses
Scientific name Common name Salinity range (ppt) Occurrence 
Vallisneria americanum wildcelery or freshwater eelgrass
0 - 9 
 Mostly coastal parishes
Potamogeton pectinatus Sago pondweed
0 - 9
 Several parishes in Louisiana
Potamogeton perfoliatus redhead grass
0 - 9
 Lake Pontchartrain
Zanichellia palustris horned pondweed
0 -25
 Mostly in extreme southeastern LA
Myriophyllum spicatum Eurasian watermilfoil
0 -10
 Mostly in the southern one-third of LA
Najas guadalupensis bushy pondweed
1- 15
 Throughout LA

Studies comparing productivity of seagrass beds and other ecosystems:
 
Seagrass productivity
Species
 Location  Productivity (gC/m2/day)  
*Thalassia testudinum Florida*

Puerto Rico*
0.9 -16*

2.5 - 4.5*

  
*Syringodium filiforme Florida*

Texas*
0.8 - 3.0*

0.6 - 9.0*

  
*Halodule wrightii North Carolina*
0.5-2.0*
  
*Zostera marina North Carolina*

Rhode Island*
0.2-1.7*

0.4-2.9

  
Wheat1 World average
.94
  
Corn1 World average
1.1
  
Sugar1 World average
4.7
  
Temperate forest1 World average
4.4
  
Tropical rainforest1 World average
5.5
  
       

1Source: Bertness, M. D. The Ecology of Atlantic Shorelines. Sinauer Associates, Inc., Sunderland, Massachusetts.
  *Habitat Complexity:

Habitat complexity is high in seagrass beds/SAV (Submerged Aquatic Vegetation)

Seagrass provides a refuge from predation. Predation is inhibited because the prey have many places to hide

Densities of many invertebrates (infaunal and epifaunal) and small fishes are greater in SAV than in nearby unvegetated areas
 
 

*Types of animals in seagrass beds:

Infauna: bury in sediment. The roots and rhizmome mat protects various Polychaete worms, amphipods, clams from predators.

Epifauna: Live on surface of blades and sediment and receive refuge among the shoots and leaves. Epifaunal organisms include the meiofauna (harpacticoid copepods), macrofauna (amphipods. isopods), and tube-dwelling sessile polychaetes, gastropods, decapods.

Mobile fauna: Live in water over canopy and can swim into the canopy for protection from predators.

*Source: Information marked with "*" is from or modified from "Estuarine Ecology, BIOL 6010, Dr. Joe Luczkovich, East Carolina University, Institute for Coastal and Marine Resources (252) 328-1759, Department of Biology (252) 328-2847, http://drjoe.biology.ecu.edu/estuary/estuary.htm"

More on the importance of seagrasses:

The following quote emphasizes the significance of seagrasses. Source: Godfrey, R. K. and J. Wooten. 1979. Aquatic and Wetland Plants of Southeastern United States. University of Georgia Press. Athens. Pg. 31.

"Zostera marina is considered to be the food at the base of a pyramid of living things 25 million tons of which are required to produce eventually 5 million tons of birds, 5 thousand tones of halibut, flounder and plaice, and 6 thousand tons of cod. In the early 1930s about 90 percent of the eelgrass along the eastern seaboard of North America was devastated by an epidemic fungal disease. This in turn greatly reduced animal populations dependent upon it for food either directly or indirectly. It is held that in the web of life where eelgrass is abundant, it serves as a friction filter for silts and pollutants. In its absence, where waters are silt-laden and polluted, the silts, sewage and other wastes seep out of the river mouths unimpeded wiping out marine life. How bountifully the eelgrass has come back to date we do not know."