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Seagrass in Galveston Bay

In some areas of the bay where water is shallow and clear, seagrasses can establish dense meadows on the bay bottom. The submerged aquatic vegetation (SAV) and associated animals compose the seagrass meadow community. The fauna associated with patches of SAV is quite diverse. Juvenile shrimp, crab and fish use this habitat as a nursery area and migrate to other habitats when they mature.

Trends in Seagrass Distribution

The majority of Texas seagrass meadows occur along the middle to lower Texas coast where waters are warm, clear, and have higher salinities (such as the Laguna madre). Estuarine seagrass species include shoalgrass (Halodule wrightii), clover grass (Halophila englemanni), and turtlegrass (Thalassia testudinum). Some species of seagrass, like widgeongrass (Ruppia maritima), can be found in fresh water. Turbidity and salinity, rainfall are the controlling factors for seagrass growth in Galveston Bay.

In the Galveston Bay system, seagrasses historically flourished in four locations:

  1. Trinity River Delta (widgeongrass and tape grass);
  2. Western shoreline of Galveston Bay from Seabrook to San Leon (widgeongrass);
  3. Southern shoreline of West Bay (shoalgrass mixed with widgeongrass); and
  4. Christmas Bay (shoalgrass mixed withturtlegrass and clovergrass)

Most of the seagrass beds once present in the Galveston Bay system have been lost since the late 1950s. Between the 1950s and late 1980s, approximately 1,700 acres of seagrass meadows disappeared. By 1987, seagrass meadows were gone from West Bay and the western shore of Upper Galveston Bay. By that time, seagrass meadows were limited only to Christmas Bay (nearly 400 acres) while upper Trinity Bay (less than 300 acres). In 1996, seagrasses in Christmas Bay were estimated to toal 424 acres. In 2005, the Texas Parks and Wildlife Department estimated that seagrasses in Christmas Bay covered 436.6 acres of bay bottom.

Widgeongrass is ephemeral and more tolerant of lower salinities and can still be found scattered in upper Trinity Bay near the mouth of the Trinity River, in Galveston Bay tributaries and in isolated ponds. The widgeongrass beds along the western shore of upper Galveston Bay have disappeared without remnant populations.

Causes of Seagrass Loss

The causes of seagrass loss can be categorized into natural and anthropogenic disturbances. Natural disturbances are produced by storms (such as hurricanes), floods and droughts. These directly impact seagrass growth and survival through changes in turbidity and sedimentation. Anthropogenic disturbances are of three basic types: dredging, boating and pollution. Dredging increases the suspended solids in the water and may impact seagrass through light attenuation (the reduction in light penetration) or direct burial of plants. Powerboats, while not a significant threat in Galveston Bay, can directly excavate plants and roots, leaving propellor scars that take years to revegetate. Nutrient enrichment from agricultural runoff, aquaculture effluents, improperly functioning sewage treatment systems or groundwater discharged from septic system drainage fields can lead to excessive algal growth that shades the grass or leads to low oxygen conditions.

The exact reasons for the decline of submerged aquatic vege­tation in Galveston Bay are not known. Plausible reasons include:

  1. Light attenuation
  2. Subsidence
  3. Effects of hurricanes on western Galveston Bay
  4. Human activities including development, wastewater discharges, chemical spills, and dredging activities in West Bay.

As in other estuaries, light attenuation is presumably the major limiting factor to seagrass growth in Galveston Bay. In addition, submerged aquatic vegetation requires a low‑energy environment with limited water current and turbulence. High wave energy and turbidity in locales where sub­merged aquatic vegetation formerly existed may reduce the potential for re‑establishment. Placement of geotubes for protection of shorelines at Galveston Island State Park has certainly encouraged the growth of seagrass meadows in that location.

Seagrass Bed Restoration and Creation

 In recognition of the value being lost with declining seagrass resources, state agencies approved a Seagrass Conservation Plan for Texas. The plan recommends management actions to reverse the decline in this resource.

Seagrass restoration is more challenging than marsh cordgrass restoration. Historically, restoration and creation of seagrass meadows has been quite difficult and not very successful. This seems to be changing in the Galveston Bay system, and more seagrass restoration activity is occurring.

 Biologists and managers working on Galveston Bay are optimistic about the prospects for restoring seagrass habitat in the bay system because the water quality has improved. Open bay disposal of dredged material is no longer a threat to seagrass populations, since this disposal method is has been replaced by beneficial use of dredge material for marsh creation.
 
Studies have shown that planting shoalgrass can lead to restoration of a seagrass ecosystem . Recently a seagrass ecosystem colonized the terraces created for the Galveston Island State Park marsh restoration project. Three seagrass planting techniques were utilized:
  • Broadcasting of live plant material
  • Planting of seagrass in peat pots
  • Bare-root planting via pontoon/tractor boat.

Of the three, the broadcast method was found to be the most successful technique. More restoration projects are proposed in support of the objective of the Galveston Bay Plan which calls for creation of 1,400 acres of seagrass beds.

Additional Information

 

Literature Cited:

Pulich, W. M. and W. A. White. 1991. "Decline of submerged vegetation in the Galveston Bay system: chronology and relationship to physical processes." Journal of Coastal Research 7: 1125-1138.

Pulich, W. M., Jr. 1996. Map of Galveston Bay submerged aquatic vegetation. Compilation of a digital data layer composed of wetland habitats and coastal land cover data: final report to Natural Resources Inventory Program. Austin, Texas, Texas Parks and Wildlife Department.

Pullen, E. J. 1960. A checklist of marsh and marine flora in area M-2. Texas Game and Fish Commission, Marine Fisheries Division Project Reports, 1959-1960, Project No. M-2 R-2, Job No. C-2. Austin, Texas.

Renfro, W. C. 1959. Checklist of the flora of area M-2. Texas Game and Fish Commission, Marine Laboratory Reports, 1959, Project No. M-2 R-1, Job No. C-2. Austin, Texas

 
 

 

 

Related Pages:

Habitat
Water and Sediment Quality

Wetlands

 

 

Seagrass meadows were once extensive in Upper Galveston Bay and West Bay. Image courtesy Texas Parks and Wildlife Department. Seagrass meadows were once extensive in Upper Galveston Bay and West Bay. Image courtesy Texas Parks and Wildlife Department.

 

 

Seagrass areas in the Galveston Bay System Map depicting status of seagrass habitat in Galveston Bay in the 1956, 1987, and 1996. Data sources: Renfro 1959; Pullen 1960; Pulich and White 1991; Pulich 1996.

 

 

 

Image of dredging a shipping channel. Image © 2009 iStockphoto.com/Jeffrey Diamond.Dredging a shipping channel. Image © 2009 iStockphoto.com/Jeffrey Diamond.

 
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