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Seagrass Distribution in the Northern Gulf of Mexico


by
Lawrence R. Handley
National Biological Service
Seagrass ecosystems are widely recognized as some of the most productive benthic habitats in estuarine and nearshore waters of the gulf coast. Seagrass meadows provide food for wintering waterfowl and important spawning and foraging habitat for several species of commercially important finfish and shellfish. Physical structure provided by seagrasses affords juveniles refuge from predation and allows for attachment of epiphytes and benthic organisms. Seagrass communities also support several endangered and threatened species, including some sea turtles and manatees. Changes in seagrass distribution can reflect the health of a water body, and losses of seagrasses may signal water-quality problems in coastal waters. Losses of seagrasses in the northern Gulf of Mexico over the last five decades have been extensive--from 20% to 100% for most estuaries, with only a few areas experiencing increases in seagrasses.
Although often considered continuous around the entire periphery of the gulf, seagrasses exist only in isolated patches and narrow bands from Mobile Bay, Alabama, to Aransas Bay, Texas (Figure). This pattern of occurrence results from a combination of low salinities, high turbidity, and high wave energy in shallow waters. Seagrasses are more extensively developed from Mobile Bay to Florida Bay (Figure). Although freshwater submerged aquatic vegetation also occurs throughout gulf coast estuaries and river deltas, its distribution is not considered in this article.

Figure. Study sites along the Gulf of Mexico region.
Seagrass habitats in the Gulf of Mexico have declined dramatically during the past 50 years, mostly because of coastal population growth and accompanying municipal, industrial, and agricultural development. Although proximate causes of local declines can sometimes be identified, most habitat loss has resulted from widespread deterioration of water quality (Neckles 1993).
The total seagrass coverage in the shallow, clear waters in protected estuaries and nearshore waters of the Gulf of Mexico coastal states is estimated to be 1.02 million ha (2.52 million acres; Duke and Kruczynski 1992). About 693,000 ha (1.71 million acres) of seagrasses occur in waters of the Florida Big Bend and Florida Bay (Figure). The remaining 324,000 ha (800,000 acres) are within gulf estuaries, with about 95% in the estuarine areas of Florida and Texas. Florida Bay seagrass meadows occupy about 550,000 ha (1.36 million acres), while the seagrass meadows of the Florida Big Bend area cover about 300,000 ha (740,000 acres; Zieman and Zieman 1989).
Six species of seagrasses occur in the Gulf of Mexico: turtle grass (Thalassia testudinum), shoal grass (Halodule wrightii), manatee grass (Syringodium filiforme), star grass (Halophila engelmanni), Halophila decipiens, and widgeon grass (Ruppia maritima). The latter has a distribution in water with lower salinity, but is commonly reported in association with the seagrasses throughout the gulf coast.

Case Histories
Sarasota Bay
Between 1948 and 1974, South Sarasota and Roberts bays lost 193 ha (477 acres) or 25%; Dryman, Blackburn, Dona, and Roberts (a different Roberts Bay) bays lost 31 ha (77 acres) or 29%; and Lemon Bay lost 55 ha (136 acres) or 21% of seagrasses (Evans and Brungardt 1978). Losses have been attributed mainly to dredge-and-fill activities and decline in water quality (Wolfe and Drew 1990). Improved water quality in Little Sarasota Bay caused seagrasses to increase between 1948 and 1974 by 14 ha (34 acres) or 9%.
Tampa Bay
In Tampa Bay (Figure), turtle grass and shoalgrass are dominant, and widgeon grass, manatee grass, and star grass are also found. A historical estimate places 30,970 ha (76,527 acres) occurring within the shallow- water margins of Tampa Bay before human influence (ca. 1876; Lewis et al. 1985). Based on 1981 estimates of seagrass coverage, a reduction of 81% of seagrasses has occurred in Tampa Bay; 5,750 ha (14,208 acres) were present in 1981. The most striking decrease occurred between 1940 and 1963, when about 50% of the grass beds were lost (Lewis et al. 1985). During this period, Hillsborough Bay alone lost 94% of its grass beds, Old Tampa Bay lost 45%, and Tampa Bay proper lost 35%. These losses have been attributed primarily to direct dredging of grassbeds and major shoreline modifications through filling and siltation (Wolfe and Drew 1990), which reduced light penetration and produced bottom sediments that are not conducive to seagrass growth and development.
Since 1963, grass beds have continued to decline in the upper bays of Tampa Bay to a point where Hillsborough Bay has lost the remaining 139 ha (343 acres) and Old Tampa Bay has lost nearly 60% (Figure). In lower Tampa Bay, grass beds have regained some area, increasing about 14% or 435 ha (1,075 acres). Tampa Bay as a whole has lost 5,984 ha (14,786 acres), or 51% of seagrasses between 1940 and 1983.
Perdido Bay
In Perdido Bay (Figure), widgeon grass and shoal grass are the predominant species. They grow in large and small beds, in numerous patches along shallow sandy reaches of the shoreline, and in large shallow flats in the lower bay and outlet. From 1940 to 1987, changes in the upper and middle parts of the bay consisted mainly of shifts in the locations of small meadows, with only minor changes in density. In the lower bay, some shifting of locations and changes in density occurred, and the coverage of seagrasses declined from 486 ha (1,201 acres) in 1940-41 to 251 ha (619 acres) in 1987. While the loss of seagrasses for the whole area was nearly 48%, some areas in U.S. Geological Survey quadrangles lost as much as 82% of the seagrasses delineated between 1940-41 and 1987. The changes in the extent of seagrasses are due to increased turbidity caused primarily by channel dredging and boat traffic; shoreline modifications; decreasing water quality and sedimentation from increasing farmlands and residential, commercial, and industrial development; and the high wave energy, overwash, sedimentation, erosion, and runoff from Hurricane Frederick in 1979.
Mississippi Gulf Coast
Along the Mississippi gulf coast, the Gulf Islands National Seashore includes most of the state's barrier islands (Figure). Manatee grass and shoal grass are the dominant seagrasses found in the shallow water on the northern side of the barrier islands, where they are protected from the high wave energy of the open gulf. Between 1956 and 1987, 416 ha (1,029 acres) of seagrasses declined to 140 ha (345 acres), a loss of 66%. The largest concentration of seagrasses was found on the north side of Horn Island, where 169 ha (417 acres) in 1956 declined to 56 ha (138 acres) by 1987, and to 6 ha (14 acres) by 1992.
Coastal Louisiana
Coastal Louisiana has a large amount of submerged aquatic vegetation but only a small portion is seagrasses (5,657 ha [13,974 acres] in 1988). Since the mid-1950's Louisiana has lost all of its seagrass in Lake Pontchartrain, in the Mississippi River Delta, behind the south coast barrier islands and Marsh Island, and in the coastal lakes (White, Calcasieu, and Sabine). The only remaining seagrass beds in coastal Louisiana exist in Chandeleur Sound behind the Chandeleur Islands. Turtle grass, shoal grass, manatee grass, widgeon grass, and star grass are present in the sandy sediments of the shallow backbarrier lagoon. These seagrass beds are virtually unaffected by human impacts because of their distance from the mainland, and they are controlled by high waves from chronic frontal passages and hurricanes causing overwash, erosion, sedimentation, changes in water depth, and turbidity. For example, Hurricane Camille in August 1969, with a storm surge of nearly 11 m (36 ft) on the Mississippi mainland, caused a loss of 530 ha (1,310 acres), or 22% of the seagrasses, on the North Islands (USGS 1:24,000 quadrangle), and a loss of 303 ha (749 acres) or 54% of the seagrasses, on Chandeleur Light (USGS 1:24,000 quadrangle).
The Chandeleur Islands (Figure) have been intensively mapped for wetland and seagrass habitats for 1978, 1982, 1987, and 1989. The areal extent of seagrasses for the Chandeleur Islands has remained relatively constant over the 11-year period, from 6,409 ha (15,831 acres) in 1978 to 5,657 ha (13,974 acres) in 1989. This constitutes a loss of only 12% of the seagrasses from 1978 to 1989, a period that had two hurricanes, two tropical storms, and countless cold fronts that influenced these islands.
Galveston Bay
In the Galveston Bay estuary (Figure), the distribution of seagrasses, predominantly shoal grass and widgeon grass, decreased in areal extent from more than 2,024 ha (5,000 acres) in the mid-1950's to about 283 ha (700 acres) in 1989, a loss of 1,471 ha (3,635 acres) or about 85% (White et al. 1993). The most significant losses were along the margins of western Galveston Bay and were related to the effects of subsidence and Hurricane Carla in 1970. In West Bay nearly 890 ha (2,200 acres) of seagrasses were completely lost, primarily through human activities including industrial, residential, and commercial development; wastewater discharges; chemical spills; and increased turbidity from boat traffic and dredging (Pulich and White 1991). In Christmas Bay, which has the largest concentration of seagrass beds in the Galveston Bay estuarine system, seagrass areal extent declined from 121 ha (300 acres) in 1975 to 81 ha (200 acres) in 1987, but increased to 156 ha (385 acres) by 1989.

Conclusions
Losses of seagrasses in the northern Gulf of Mexico have been extensive over the last five decades, with losses varying 20%-100% for most estuaries of the northern Gulf of Mexico. Only a few locales have experienced increases in seagrasses. The high productivity of the Gulf of Mexico seagrass beds as spawning, nursery, food, and shelter areas increases the importance of the loss of this valuable habitat far beyond the areal extent of the resource. Regionwide, the loss of seagrasses is attributable to natural causes (hurricanes, cold-front storms, and increased or decreased salinities) and human-induced effects (increased turbidity and decreases in water quality resulting from dredging, boating activities, and other development pressures), which work in concert to deterioriate the environmental quality of the habitat.
For further information:
Lawrence R. Handley
National Biological Service
Southern Science Center
700 Cajundome Blvd.
Lafayette, LA 70506
References
Duke, T.W., and W.L. Kruczynski, eds. 1992. Status and trends of emergent and submerged vegetated habitats of the Gulf of Mexico. Gulf of Mexico Program, U.S. Environmental Protection Agency, Stennis Space Center, MS. 161 pp.

Evans, M., and T. Brungardt. 1978. Shoreline analysis of Sarasota County Bay systems with regard to revegetation activities. Pages 193-206 in D.P. Cole, ed. Proceedings of the Fifth Annual Conference on Restoration of Coastal Vegetation in Florida. Environmental Studies Center, Hillsborough Community College, Tampa.

Lewis, R.R., M.J. Durako, M.D. Moffler, and R.C. Phillips. 1985. Seagrass meadows of Tampa Bay--a review. Pages 210-246 in S.F. Treat, J.L. Simon, R.R. Lewis III, and R.L. Whitman, Jr., eds. Proceedings of the Tampa Bay Area Scientific Information Symposium, May 1982. University of South Florida, Tampa.

Neckles, H.A., ed. 1993. Seagrass monitoring and research in the Gulf of Mexico: draft report of a workshop held at Mote Marine Laboratory in Sarasota, Florida, January 28-29, 1992. National Biological Survey, National Wetlands Research Center, Lafayette, LA. 75 pp.

 Pulich, W.M., Jr., and W.A. White. 1991. Decline of submerged vegetation in the Galveston Bay system: chronology and relationship to physical processes. Journal of Coastal Res. (4):1125-1138.

White, W.A., T.A. Tremblay, E.G. Wermund, Jr., and L.R. Handley. 1993. Trends and status of wetland and aquatic habitats in the Galveston Bay system, Texas. The Galveston Bay National Estuary Program, Publ. GBNEP-31. 225 pp.

Wolfe, S.H., and R.D. Drew, eds. 1990. An ecological characterization of the Tampa Bay watershed. U.S. Fish and Wildlife Service Biological Rep. 90(20). 334 pp.

Zieman, J.C., and R.T. Zieman. 1989. The ecology of the seagrass meadows of the west coast of Florida: a community profile. U.S. Fish and Wildlife Service Biological Rep. 85(7.25). 155 pp.


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