276 



Coastal & Marine Enisysreins — Our Liviii); Resources 



k 



1967 





I 

 N 



1965^ 



I I bare 



1 I shoal grass 



^1 manatee grass 



^■1 clover grass 



I I turtle grass 



ns not sampled 



Fig. 2. Dominant cover types in 

 the continuously submerged por- 

 tions of upper (a) and lower lb) 

 Laguna Madre. 



Laguna Madre de Tamaulipas. immediately 

 south of tlie delta of the Rio Grande in Mexico; 

 Weller [1464]) and feeds alinost exclusively on 

 one species of seagrass while in residence 

 (shoal grass. Halodide wiightii). 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. 



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 Onul' 199.^). 



Distributional Patterns 



Seagrass meadows are undergoing profound 

 change in Laguna Madre. The area of vegetated 

 bottom in upper Laguna Madre has increased 

 130 km- (50 mi-), from 120 km^ (46 mi-) to 

 250 km- (97 mi-) between 1967 and 1988 

 (Quammen and Onuf 1993). an amount exceed- 

 ing 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 km- 

 (54 mi-), from 620 km- (^239 mi-) to 480 km- 

 ( 185 mi-), confined to deeper areas (Quammen 

 and Onuf 1993). 



Changes in the species composition of sea- 

 grass 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 bot- 

 tom by manatee grass (Syrini>()diiim filiforme) 

 increased from 9% to 27% and by turtle grass 

 (Thalassia testiiJiiuiiin from 1% to 7%. 



Factors Responsible 



Processes responsible for the loss of sea- 

 grass 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 cau.sed by maintenance 

 dredging. In 1988-89, waves generated by fre- 

 quent 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 essen- 

 tially 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 sea- 

 grass meadows in lower Laguna Madre. 

 Historically, a 20-km ( 12.4-mi) expanse of usu- 

 ally emergent fiats separated the two sections of 

 the laguna. Salinities greater than 60 ppt in the 

 lower laguna and greater than 100 ppt in the 

 southern pail of the upper laguna were not 

 unusual. 



In 1949 the Gulf Intracoastal Waterway was 

 completed, providing a continuous water con- 

 nection between the two pails of the laguna. 

 improving exchange with the Gulf of Mexico 

 and moderating the salinity regime of the lagu- 

 na. 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 sea- 

 grass probably accounts for the slower colo- 

 nization 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 rela- 

 tive colonizing abilities since salinity modera- 

 tion: 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 win- 

 ter 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 sig- 

 nificant winter populations of redheads, and few 

 others do either, making the condition of 

 Laguna Madre all the more critical for red- 

 heads. 



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 phyto- 

 plankton bloom known as the brown tide has 

 been resident in the upper laguna since 1990. 



