Home


Gulf of Mexico Coastal Wetlands: Case Studies of Loss Trends


by
James B. Johnston
National Biological Service
Mary C. Watzin
John A. Barras
Lawrence R. Handley
National Biological Service
The Gulf of Mexico's coastal wetlands are of special interest because the gulf is an exceptionally productive sea that yields more than 1.1 billion kg (2.5 billion lb) of fish and shellfish annually and contains four of the top five fishery ports in the nation by weight (U.S. Environmental Protection Agency 1988). The volume of commercial shrimp landings in the gulf has been statistically related to the areal coverage of gulf coastal wetlands (and seagrass beds) that provide crucial nursery habitat to the young (Turner 1977). Coastal wetlands (particularly salt marshes and mangroves) and associated shallow waters function similarly in support of many fish species of commercial interest (Seaman 1985). The gulf wetlands are also well known for their large populations of wildlife, including shorebirds, colonial nesting birds, and 75% of the migratory waterfowl traversing the United States (Duke and Kruczynski 1992). The extensive coastal wetlands that remain along the gulf make up about half of the nation's total wetland area (NOAA 1991).

General Trends
The National Oceanic and Atmospheric Administration (NOAA 1991) examined the areal extent and distribution of gulf coast coastal wetlands in the mid-1980's by using aerial photographs and maps from 1972 to 1984 (28% from 1979 and 42% from 1980 or later). Summaries of NOAA's data are shown in the Table for three wetland categories: marshes (fresh, brackish, and salt marshes), estuarine shrub-scrub (mangroves), and freshwater forested/shrub-scrub wetlands. Even though wetland area has diminished greatly along the gulf coast during the last 30 years, about 1.3 million ha (3.3 million acres) still remain in these three categories. Table. Total area (hectares) of selected vegetated wetlands by state for the Gulf of Mexico (from NOAA 1991).*
State Marsh Estuarine scrub-shrub Fresh forested and scrub-shrub Total % of total**
Texas 183,900 1,100 3,000 188,000 14
Louisiana 723,500 4,100 1,900 729,500 55
Mississippi 23,800 400 - 24,200 2
Alabama 10,400 1,100 800 12,300 1
Florida 108,100 248,400 7,400 363,900 28
Total 1,049,700 255,100 13,100 1,317,900 -
* Calculated based on Fish and Wildlife Service's National Wetlands Inventory maps. Area originally reported as acres x 100; hectare = 2.471 acres.
** Fractions of percent rounded to the next highest whole percent (1.6 = 2.0%).
 
Louisiana has the greatest area of coastal wetlands with 55% of the total, followed by Florida (18%), Texas (14%), and Mississippi (2%). Louisiana contains 69% of the marshes, while Florida has 97% of the estuarine scrub-shrub, most of which is mangrove. Of the three wetland types, 80% is marsh, 19% estuarine scrub-shrub, and 1% forested wetland.
Because of the age of the photographs used by NOAA and because national trends suggest that the area of most wetland types is still declining (Frayer et al. 1983), the wetland statistics presented by NOAA may be overestimates. No current studies summarize coastal wetland area or loss rates for the entire Gulf of Mexico region; therefore, four case studies conducted by the National Biological Service's Southern Science Center, the U.S. Fish and Wildlife Service's National Wetland Inventory, and their partners are presented to depict status and trends from the 1950's to 1970's and the 1970's to the late 1980's. The areas chosen (Fig. 1) represent a cross-section of current trends.

Fig. 1. Locations of wetland loss study sites along the Gulf of Mexico region.

Coastal Wetland Loss: Gulf of Mexico Case Studies
Galveston Bay
White et al. (1993) reported both gains and losses in Galveston Bay wetlands from the 1950's to 1989, but the net trend was one of wetland loss, going from 69,800 ha (171,000 acres) in the 1950's to 56,100 ha (138,600 acres) in 1989. The rate of loss decreased over time from about 405 ha (1,000 acres) per year between 1953 and 1979 to about 283 ha (700 acres) per year between 1979 and 1989. The rate of loss from 1979 to 1989 would probably be lower if inaccuracies in wetland interpretation of the 1979 photographs could be taken into account. In general, freshwater scrub-shrub habitats decreased in area from the 1950's to 1979 and 1989, while forested wetlands increased. Marshes (fresh and non-fresh) decreased from about 67,000 ha (165,500 acres) in the 1950's to about 52,800 ha (130,400 acres) in 1989, producing a total net marsh loss of about 21% of that resource.
The five key factors contributing most to wetlands decline in the Galveston Bay since the 1950's are (1) industrial development; (2) urbanization; (3) navigation channels; (4) flood control and multipurpose water projects to meet Houston's future water demand, especially upstream impoundments on the Trinity and San Jacinto rivers; and (5) pollution due to agricultural runoff despite the diminished acreage lost to agricultural expansion. It should be noted that human-induced subsidence due to industrial development (oil and gas activities) and urbanization (groundwater withdrawals) are considered in this analysis (D. Whitehead, U.S. Fish and Wildlife Service, personal communication).
Coastal Louisiana
Coastal wetland loss for Louisiana represents 67% of the nation's total loss. For the time period 1978-90, the loss was 177,625 ha (290,432 acres), representing an annual loss rate of 9,802 ha/yr (24,203 acres/yr) for this 12-year period; that is equal to 97.9 km2 or 37.8 m2/yr. For the time period 1956-78, net wetland loss was even greater, 267,800 ha (661,700 acres), representing a loss rate of 12,170 ha/yr (30,000 acres/yr); that is equal to 121.7 km2 or 47 mi2/yr.
Although much of this loss is only indirectly linked to human activities, most of the net current, catastrophic wetland loss is primarily the result of altered hydrology stemming from navigation, flood control, and mineral extraction and transport projects (Sasser et al. 1986; Louisiana Wetland Protection Panel 1987; Turner and Cahoon 1988). These operations do not always destroy wetlands directly, but they do amplify tidal forces in historically low-energy systems, which upsets the balance of subsidence and accretion, reduces nutrient and sediment influx, decreases freshwater retention, and increases the levels of salt, sulfate, and other substances potentially toxic to indigenous plant species (Good 1993).
Current wetland losses are concentrated in the southern Deltaic Plain (78%; Fig. 1). In this region, losses are especially severe in the fringing marshes of the Terrebonne and Barataria basins (Figs. 2 and 3). Previous losses in the Deltaic Plain occurred primarily in large areas of interior lands. In the Chenier Plain (Fig. 1), loss rates were more constant (22%); many of the larger areas of loss there seem related to impounded areas with managed water levels.
Fig. 2. Coastal Louisiana basins as defined in the Coastal Wetlands Planning, Protection, and Restoration Act Plan.
The Barataria and Terrebonne basins suffer the highest land loss rates (all land but mostly wetlands) in Louisiana (2,880 ha/yr [7,120 acres/yr] and 2,630 ha/yr [6,500 acres/yr], respectively), accounting for 64% of all land loss in the 1978-90 period. In contrast, this area accounted for only 43% of all loss in the 1956-78 period. The Mermentau and Sabine basins (Fig. 2) have the next highest loss rates (1,080 ha/yr [2,670 acres/yr] and 660 ha/yr [1,630 acres/yr]), with losses largely confined to the northern and central portions, except for shoreline erosion along the Mermentau Basin's coastline. Loss rates within the Teche-Vermilion, Mississippi, Breton Sound, and Pontchartrain basins (Fig. 2) are all less than 930 ha/yr (2,300 acres/yr), which seems to indicate more stable environments. The Atchafalaya and Pearl River basins (Fig. 2) experienced losses of less than 130 ha/yr (321 acres/yr). In summary, land loss rates in coastal Louisiana, although decreasing, remain high for the 1978-90 period.
Fig. 3. Coastal landloss in Louisiana and elsewhere is analyzed by using computerized geographic information systems that produce graphics such as this map.
The National Biological Service is providing future land loss updates for coastal Louisiana by using Landsat Thematic Mapper satellite imagery on a 3-year basis.
Mobile Bay
Non-freshwater marshes surrounding Mobile Bay declined by more than 4,047 ha (10,000 acres) from 1955 to 1979, representing a loss of 35% (Roach et al. 1987). Freshwater marshes in all of coastal Alabama declined by about 69% from 1955 to 1979. More than 2,500 ha (6,200 acres) were lost during that time (Roach et al. 1987).
When comparing these data to 1988 wetland habitat maps prepared for upper Mobile Bay, it appears that in this portion of the bay no additional net loss of non-freshwater marsh has occurred since 1979. Some marsh has obviously continued to be lost in certain areas, primarily because of dredge disposal associated with navigation and industry. These losses, though, seem to have been offset by the growth of emergent marsh in existing spoil sites (Watzin et al. 1994).
The Southern Science Center's 1988 areal estimates show a substantial increase of 189 ha (467 acres) in freshwater marsh from 1979 to 1988 in upper Mobile Bay. Further investigation revealed that some of this gain was the result of the growth of emergent vegetation in existing disposal areas and in ditches along railroads and highways. Because of disparities in photointerpretation between dates, it is also quite likely that some of these differences are simply due to mapping errors and differences in mapping technique (Watzin et al. 1994).
As a result of mapping errors associated with interpreting forested and scrub-shrub wetlands in the 1956 photographs, Roach et al. (1987) had little faith in the quantitative estimate of change between 1956 and 1979 for these wetland types. The Southern Science Center's 1988 wetland area figures for forested wetlands appear relatively accurate; they indicate that about 486 ha (1,201 acres) of forested wetlands (2.7%) were lost in upper Mobile Bay between 1979 and 1988. These losses can be attributed to conversion of forested habitats to scrub-shrub areas (e.g., clearcutting associated with timber harvest), small impoundments, and commercial and residential development (Watzin et al. 1994).
Tampa Bay
Haddad (1989) reported emergent wetlands decreased from 29,000 ha (71,700 acres) in the 1950's to 23,900 ha (59,100 acres) in 1982, about an 18% loss. Mangroves decreased from 8,629 ha (21,320 acres) to 8,032 ha (19,847 acres), a decline of about 7%. Salt marshes declined from 2,063 ha (5,097 acres) to 1,423 ha (3,538 acres), or a loss of 30%. Freshwater wetlands decreased 21% from 18,335 ha (45,305 acres) to 14,440 ha (35,681 acres).
Lewis et al. (1985) estimate that 44% of the salt marsh and mangrove has been lost in Tampa Bay since the late 1800's. Although their numbers and those of Haddad (1989) are not readily comparable because of differences in time frame, methodology, vegetation classification, and area mapped, the results taken together confirm that significant losses of wetland habitat have occurred. Marsh and mangrove losses are the product of dredge and fill activities that are now under strict regulatory control; although permitted dredging continues, protective measures exist to minimize loss that is not for public benefit.

Future Concerns
To protect the future of gulf coastal wetlands, status and trends over time must be continually recorded and noted in the scientific and public literature. Preliminary data from selected coastal areas studied in the 1980's show a reduced rate of wetland loss compared with earlier decades. While this is good news, the pressures of a continuously expanding human population make it unclear whether this trend will continue into the 21st century. Only additional monitoring data can answer this question.
For further information:
James B. Johnston
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, USA. Gulf of Mexico Program, U.S. Environmental Protection Agency, John C. Stennis Space Center, MS. 161 pp.

Frayer, W.E., T.J. Monahan, D.C. Bowden, and F.A. Graybill. 1983. Status and trends of wetlands and deepwater habitats in the coterminous United States, 1950's to 1970's. Colorado State University, Department of Forest and Wood Sciences, Fort Collins. 32 pp.

Good, B. 1993. Louisiana's wetlands: combatting erosion and revitalizing native ecosystems. Restoration and Management Notes. 11:125-133.

Haddad, K.D. 1989. Habitat trends and fisheries in Tampa and Sarasota bays. Pages 113-128 in Tampa and Sarasota bays: issues, resources, status, and management. National Oceanic and Atmospheric Administration, Estuary-of-the-Month Seminar Series 11.

Lewis, R.R., III, M.J. Durako, M.D. Moffler, and R.C. Phillips. 1985. Seagrass meadows of Tampa Bay. Pages 210-246 in S. Treat, J. Simon, R. Lewis III, and R. Whitman, Jr., eds. Proceedings Tampa Bay Area Scientific Information Symposium. Florida Sea Grant Rep. 65.

Louisiana Wetland Protection Panel. 1987. Saving Louisiana's coastal wetlands: The need for a long-term plan of action. U.S. Environmental Protection Agency, EPA-230-02-87-026. 102 pp.

NOAA. 1991. Coastal wetlands of the United States: an accounting of a national resource base. National Oceanic and Atmospheric Administration Rep. 91-3. 59 pp.

 Roach, E.R., M.C. Watzin, and J.D. Scurry. 1987. Wetland changes in coastal Alabama. Pages 92-101 in T.A. Lowery, ed. Symposium on the Natural Resources of the Mobile Bay Estuary. Alabama Sea Grant Extension Service, Mobile, AL. MASGP-87-007.

Sasser, C.E., M.D. Dozier, J.G. Gosselink, and J.M. Hill. 1986. Spatial and temporal changes in Louisiana's Barataria Basin marshes, 1945-1980. Environmental Management 10:671-680.

Seaman, W., Jr. 1985. Florida aquatic habitat and fishery resources. Florida Chapter of the American Fisheries Society, Kissimmee. 543 pp.

Turner, R.E. 1977. Intertidal vegetation and commercial yields of penaeid shrimp. Transactions of the American Fisheries Society, Kissimmee, FL. 543 pp.

Turner, R.E., and D. Cahoon, eds. 1988. Causes of wetlands loss in the coastal Gulf of Mexico. Vol 1. Executive summary. Minerals Management Service OCS Study/MMS 87-0119.

U.S. Environmental Protection Agency. 1988. The gulf initiative: protecting the Gulf of Mexico. John C. Stennis Space Center, MS.

Watzin, M.C., S. Tucker, and C. South. 1994. Environmental problems in the Mobile Bay ecosystem: the cumulative effects of human activities. U.S. Environmental Protection Agency Tech. Rep. In press.

White, W.A., T.A. Tremblay, E.G. Wermund, 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. GBNEP-31. 225 pp.


Home