Seagrass Meadows of the Laguna Madre of Texas

Seagrass Meadows of the Laguna Madre of Texas
		by *Christopher P. Onuf* National Biological Service

Fig. 1. Major bay systems along the Texas coast.

A series of lagoons forms an almost continuous fringe of water behind coastal barriers for 500 km (310 mi) from Galveston Bay, Texas, to the Mexican border (Fig. 1). At the northeast end, where river discharge and precipitation greatly exceed evaporation from the embayments, fringing marshes are the dominant wetland type. Toward the southwest, freshwater inputs decrease, fringing marshes are replaced by wind-tidal flats that support highly productive algal mats during periodic inundation, and seagrasses dominate the shallow waters of the embayments (Table).

Seagrasses are so prevalent in Laguna Madre that they define the structure of the physical environment, as well as being the source of biological production for the ecosystem. Consequently, seagrass meadows serve a critical nursery function in support of the region's rich fisheries, and one waterfowl species has established an exclusive dependence on Laguna Madre and its most common seagrass. More than 75% of the world population of redhead ducks (Aythya americana) winters in the greater Laguna Madre ecosystem (inclusive of the Laguna Madre de Tamaulipas, immediately south of the delta of the Rio Grande in Mexico; Weller [1964]) and feeds almost exclusively on one species of seagrass while in residence (shoal grass, Halodule wrightii). Because of the degree of dependence of the redhead population on the laguna and reports of major disruptions to the laguna's seagrass community, the National Biological Service began a research program in coastal Texas.

Table. Seagrass cover in bays of the Texas coast.

Bay system	Bottom vegetated (%)
Galveston Bay System*	0.3
Matagorda Bay System*	1.1
San Antonio Bay System**	5.0
Aransas-Copano Bay System**	5.2
Corpus Christi Bay System**	12.1
Upper Laguna Madre***	75.2
Lower Laguna Madre***	70.5


*Adair et al. 1994.
**Adair and Moore 1990.
***Quammen and Onuf 1993.

The Texas Parks and Wildlife Department inventoried aquatic vegetation in Laguna Madre in the 1960's (Texas Parks and Wildlife Department 1965-67). In 1988 the National Wetlands Research Center (now the Souhern Science Center) resurveyed the laguna (Quammen and Onuf 1993).
Distributional Patterns
Seagrass meadows are undergoing profound change in Laguna Madre. The area of vegetated bottom in upper Laguna Madre has increased 130 km² (50 mi²), from 120 km² (46 mi²) to 250 km² (97 mi²) between 1967 and 1988 (Quammen and Onuf 1993), an amount exceeding the total area of seagrass meadows in bays of the middle and upper Texas coast (Adair and Moore 1990; Adair et al. 1994). Concurrently, seagrass cover in lower Laguna Madre decreased by an even larger amount, 140 km² (54 mi²), from 620 km² (239 mi²) to 480 km² (185 mi²), confined to deeper areas (Quammen and Onuf 1993).

Changes in the species composition of seagrass meadows affected even larger areas of the lower laguna (Fig. 2). Shoal grass covered 82% of the bay bottom in 1965 compared to 33% in 1988. Over the same period, cover of bay bottom by manatee grass (Syringodium filiforme) increased from 9% to 27% and by turtle grass (Thalassia testudinum) from 1% to 7%.

Fig. 2. Dominant cover types in the continuously submerged portions of upper (a) and lower (b) Laguna Madre.

Factors Responsible
Processes responsible for the loss of seagrass from deep areas are different from those for the other changes. The loss of seagrass has resulted from reduced light reaching the bottom in deep areas near navigation channels because of increased turbidity caused by maintenance dredging. In 1988-89, waves generated by frequent episodes of high winds resuspended fine materials from dredge deposits and increased light attenuation for more than a year after a dredging project was completed (Onuf 1994). Since the interval between dredging projects is 2 years, the reduction in available light is essentially permanent.
Hydrological modifications of the laguna are most likely the primary cause of the expansion of seagrass cover in upper Laguna Madre and the shift in the composition of surviving seagrass meadows in lower Laguna Madre. Historically, a 20-km (12.4-mi) expanse of usually emergent flats separated the two sections of the laguna. Salinities greater than 60 ppt in the lower laguna and greater than 100 ppt in the southern part of the upper laguna were not unusual.
In 1949 the Gulf Intracoastal Waterway was completed, providing a continuous water connection between the two parts of the laguna, improving exchange with the Gulf of Mexico and moderating the salinity regime of the laguna. Since completion of the waterway, salinities have seldom reached 50 ppt in the lower laguna and 60 ppt in the upper laguna, even during extreme drought (Quammen and Onuf 1993).
Isolation from source populations of seagrass probably accounts for the slower colonization of the upper laguna than the lower laguna, after the environment became tolerable. The displacement of shoal grass by manatee grass and turtle grass after salinity moderation is consistent with the relative intolerance of those species to hypersalinity (high salinity) and their superior competitive capabilities under benign conditions. The current distributions of the three species are consistent with their relative colonizing abilities since salinity moderation: shoal grass is most widespread, manatee grass is intermediate, and turtle grass is most closely confined to its point of origin at the south end of the laguna (Quammen and Onuf 1993).
Management Implications
The dramatic decrease of shoal grass in the lower laguna is a particular concern to natural resource managers because redheads feed almost exclusively on shoal grass while in winter residence. Historically, there were several other important wintering areas for these ducks, such as Chesapeake Bay, Pamlico Sound, and Galveston Bay. The possibility existed that other areas could absorb additional birds if habitat quality in Laguna Madre deteriorated. Now, none of the alternative areas support significant winter populations of redheads, and few others do either, making the condition of Laguna Madre all the more critical for redheads.
Changes in the upper laguna since 1988 are almost certain to worsen the problem of redhead habitat deterioration. Whereas increases in the upper laguna compensated for about 40% of the losses of shoal grass in the lower laguna over the period of this analysis, a persistent phytoplankton bloom known as the brown tide has been resident in the upper laguna since 1990. The bloom is so dense in some locations that it reduces light penetrating 1 m (3.3 ft) by more than 50% (Dunton 1994). This light reduction is leading to loss of shoal grass in the deep areas most influenced by the brown tide.
Displacement of shoal grass by manatee grass was not evident in the upper laguna in 1988 but is now. In all likelihood, the same processes responsible for the profound changes in the composition of seagrass meadows in the lower laguna will now take hold in the upper laguna. The greater isolation of the upper laguna from a source population of the invader probably accounts for the much later initiation of the replacement process than in the lower laguna.
A final factor further magnifies the importance to redheads of these changes in seagrasses of the Laguna Madre of Texas. The Laguna Madre de Tamaulipas, just south of the delta of the Rio Grande, is an integral part of the winter life-support system of redheads. In most years, more redheads overwinter in Texas than Mexico; however, in years of drought in Texas, more ducks continue south into Mexico. The large geographic extent of available habitat apparently buffers the population by increasing the probability that suitable conditions prevail somewhere in the system every year. The governor of the State of Tamaulipas, however, is now promoting the extension of the Gulf Intracoastal Waterway through the Laguna Madre in Mexico. In all likelihood, this development will reduce the support capacity of the laguna in Mexico for redheads, further increasing the reliance of the ducks on the laguna in Texas.
Modification of dredging practices in Texas and planning of waterway construction in Mexico hold the most promise for sustaining seagrasses and habitat for redheads to the maximum extent possible. At present, most dredge disposal is to submerged receiving areas along the channel, where bay resources are directly affected and wave-caused resuspension sometimes impairs water clarity for long periods after dredging. Land-based or deep-sea disposal would alleviate these problems. In Mexico, conducting an inventory of key resources, prominently including seagrasses and redheads, routing the waterway to avoid concentration areas, and implementing environmentally sound construction and disposal practices will ensure the greatest security for the wintering habitat of redheads and other resources linked to seagrass meadows.
		For further information: Christopher P. Onuf National Biological Service Southern Science Center Campus Box 339 6300 Ocean Dr. Corpus Christi, TX 78412

References
Adair, S.E., and J.L. Moore. 1990. A survey of seagrass distribution of the middle Texas coast. Chapter 11 in S.E. Adair, J.L. Moore, W.H. Kiel, Jr., and M.W. Weller, eds. Winter ecology of redhead ducks in the gulf coast region. Final Rep., Cooperative Agreement 14-16-0009-87-909 between Texas A & M University and the U.S. Fish and Wildlife Service, National Wetlands Research Center. Adair, S.E., J.L. Moore, and C.P. Onuf. 1994. Distributional ecology of submerged aquatic vegetation in estuaries of the upper Texas coast. Wetlands 14(2):110-121. Dunton, K.H. 1994. Seasonal growth and biomass of the subtropical seagrass Halodule wrightii in relation to continuous measurements of underwater irradiance. Marine Biology 120:479-489.	Onuf, C.P. 1994. Seagrasses, dredging and light in Laguna Madre, Texas, USA. Estuarine, Coastal and Shelf Science 39:75-91. Quammen, M.L., and C.P. Onuf. 1993. Laguna Madre: seagrass changes continue decades after salinity reduction. Estuaries 16:302-310. Texas Parks and Wildlife Department. 1965-67. Coastal waterfowl project. Federal Aid Projects W-29-R-18 to 24. Austin, TX. Weller, M.W. 1964. Distribution and migration of the redhead. Journal of Wildlife Management 28:64-103.

References

Adair, S.E., and J.L. Moore. 1990. A survey of seagrass distribution of the middle Texas coast. Chapter 11 in S.E. Adair, J.L. Moore, W.H. Kiel, Jr., and M.W. Weller, eds. Winter ecology of redhead ducks in the gulf coast region. Final Rep., Cooperative Agreement 14-16-0009-87-909 between Texas A & M University and the U.S. Fish and Wildlife Service, National Wetlands Research Center.

Adair, S.E., J.L. Moore, and C.P. Onuf. 1994. Distributional ecology of submerged aquatic vegetation in estuaries of the upper Texas coast. Wetlands 14(2):110-121.

Dunton, K.H. 1994. Seasonal growth and biomass of the subtropical seagrass Halodule wrightii in relation to continuous measurements of underwater irradiance. Marine Biology 120:479-489.

Onuf, C.P. 1994. Seagrasses, dredging and light in Laguna Madre, Texas, USA. Estuarine, Coastal and Shelf Science 39:75-91.

Quammen, M.L., and C.P. Onuf. 1993. Laguna Madre: seagrass changes continue decades after salinity reduction. Estuaries 16:302-310.

Texas Parks and Wildlife Department. 1965-67. Coastal waterfowl project. Federal Aid Projects W-29-R-18 to 24. Austin, TX.

Weller, M.W. 1964. Distribution and migration of the redhead. Journal of Wildlife Management 28:64-103.

Seagrass Meadows of the Laguna Madre of Texas

Distributional Patterns

Factors Responsible

Management Implications