latitudes, and turtle grass ( Thalassia 

 testudinum ) from the tropics. The sa- 

 linity optimum of these species is in 

 the range of 15 to 35 ppt. Within the 

 MDPR, eelgrass is absent and turtlegrass 

 is limited to higher salinity areas, 

 such as the waters to the west of the 

 Chandeleur Islands in the Pontchartrain 

 hydrologic unit. 



The limited areal distribution of 

 EAB in the MDPR is undoubtedly a result 

 of low light rather than nutrient limi- 

 tation. Most coastal areas where EAB 

 habitat occurs are less turbid than in 

 the MDPR, and submergent macrophytes are 

 reported in water as deep as 10 m (33 

 ft) (Phillips 1960). Nutrient levels in 

 very clear waters are typically low, 

 however, and in many areas submergent 

 macrophytes may be limited by low nu- 

 trients. Open water areas in the MDPR 

 are, by contrast, nutrient rich and 

 turbid, and submergent macrophytes are 

 therefore rarely found in waters of more 

 than 1 m (3.3 ft) deep. This suggests 

 that light penetration (turbidity) is 

 the major limiting factor. 



The distribution of EAB within 

 individual hydrologic units supports 

 this hypothesis. The Mississippi Delta, 

 Atchafalaya, and Vermilion units have 

 the lowest area of EAB. These zones are 

 riverine influenced and very turbid. The 

 Terrebonne and Mississippi Sound units, 

 with generally less turbid waters, have 

 the largest areas of EAB habitat. The 

 Pontchartrain unit probably once had the 

 largest area of EAB, but erosion of the 

 Chandeleur Islands and increase in tur- 

 bidity of Lake Pontchartrain have 

 resulted in the reduction in area suit- 

 able for this habitat. 



Although there are few studies on 

 EAB in the MDPR, there is an extensive 

 literature in other areas that gives 

 insight into the role of EAB in the 

 MDPR. Sea grasses in coastal marine and 

 estuarine ecosystems constitute a unique 

 shallow water community. They are 

 widespread throughout the world, and 

 they make a substantial contribution to 

 overall coastal productivity (McRoy and 

 McMillan 1979). Work has focused on the 

 two most common species, eelgrass and 



turtlegrass. Annual production of sub- 

 mergent vegetation is extremely high, 

 ranging from about 500 g C/m 2 /yr in the 

 temperate zones to 1,000 g C/m 2 /yr in 

 the tropics. In addition, Gentner 

 (1977) descibed the important role sea- 

 grass plays in the uptake and transport 

 of iron and phosphate from the sub- 

 strate. Primary production in seagrass 

 communities is divided among several 

 components: seagrass, benthic macro and 

 microalgae, epiphytic algae, and phyto- 

 plankton. Jones (1968) and Penhale 

 (1977) determined that 18% to 20% of 

 total productivity was contributed by 

 epiphytes attached to seagrass. 



In spite of the high rate of pro- 

 duction in grass beds, only a few 

 heterotrophs are known to utilize 

 macrophyte tissue directly (Fenchel 

 1977; Zieman et al. 1979). Grazers in 

 the water column eat phytoplankton, 

 epiphytes, and benthic microalgae, 

 rather than live rooted plants. The 

 largest percentage of carbon produced by 

 submerged vascular plants is consumed as 

 detritus after it dies and begins de- 

 composing (Thayer et al. 1975). The 

 utilization of the dead particulate 

 matter from seagrass involves a wide 

 variety of organisms with complicated 

 food interrelationships. 



Although faunistic studies in sub- 

 mergent grassbeds abound, few studies 

 have dealt quantitatively with the spe- 

 cific fate of seagrass-produced carbon 

 as it passes through estuarine food webs 

 (Kikuchi and Peres 1977). Sea grass 

 habitats characteristically have a high 

 density of animals residing in them and 

 a concommitantly high rate of secondary 

 production. Such high rates of second- 

 ary production are attributed to a high 

 rate of detritus production, and to the 

 seagrass itself, which serves as a 

 refugium, for stabilizing sediments, and 

 for creating micro-habitats (Thayer et 

 al. 1975; Kikuchi and Peres 1977). The 

 beds are important nursery and feeding 

 areas for many marine nekton species 

 (Yanez-Arancibia et al. 1979). Recently 

 there have been indications of a con- 

 siderable transport of seagrass off- 

 shore, where it may serve as food for 

 both surface and benthic feeding organ- 

 isms (Wolff 1976). 



68 



