274 



Coashil & Miirmt' Ecosyslcins— Our Living Resources 



1990), which reduced light penetration and pro- 

 duced 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 Hats 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 mead- 

 ows, with only minor changes in density. In the 

 lower bay. some shifting of locations and 

 changes in density occuired. 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 develop- 

 ment: and the high wave energy, overwash. sed- 

 imentation, 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 nonheni 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 sea- 

 grasses 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 sub- 

 merged aquatic vegetation but only a small por- 

 tion is seagrasses (5.657 ha [13,974 acres] in 

 1988). Since the mid-l950"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 

 hackbarrier lagoon. These seagrass beds are vir- 

 tually 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, ero- 

 sion, sedimentation, changes in water depth, 

 and turbidity. For example. Hurricane Camille 

 in August 1969, with a storm surge of nearly 1 1 

 m (36 ft) on the Mississippi mainland, caused a 

 loss of 530 ha ( 1 .310 acres), or 22% of the sea- 

 grasses, 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 huiTicanes, two tropical storms, and count- 

 less 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 Hunicane Carla in 1970. In 

 West Bay nearly 890 ha (2,200 acres) of sea- 

 grasses were completely lost, primarily through 

 human activities including industrial, residen- 

 tial, and commercial development: wastewater 

 discharges: chemical spills: and increased tur- 

 bidity from boat traffic and dredging (Pulich 

 and White 1991 ). In Christmas Bay, which has 

 the largest concentration of seagrass beds in the 



