webs, nutrient and mineral cycling, and coastal stabilization? 

 What processes in seagrass ecosystems are affected by environ- 

 mental changes or human induced perturbations? Are there 

 structural patterns in these ecosystems that allow them to per- 

 sist in changing environments? 



Research has now focused on understanding the development 

 of these systems in terms of both local and latitudinal gradients. 

 The central hypothesis of the project is based on the principals 

 of ecological succession — succession of species, structures, and 

 functions — and this unifying concept has led to considerable 

 understanding and progress. Work has generally been confined 

 to the Zostera (eelgrass) system, characteristic of north temper- 

 ate regions, and the Thalassia (turtle grass) system, characteristic 

 of the Tropics. 



Laboratory studies and field work in Alaska and St. Croix, 

 U.S.V.I., have shown that the development of the plant com- 

 ponent of the seagrass community, and subsequently the ani- 

 mal component, is keyed to chemical and microbial processes 

 in the sediments. This keying results in a gradient of develop- 

 ment that can be seen across any local seagrass bed and prob- 

 ably also in the latitudinal distribution patterns. These results 

 have led to the refined hypotheses that seagrass ecosystems 

 represent mature as well as colonizing and intermediate stages 

 of development, that the degree of development reached in any 

 specific location depends on the interaction of environmental 

 constraints and ecological processes, and that only under op- 

 timum conditions will a mature system develop. A corollary is 

 that less than optimum conditions will result in a less mature 

 seagrass system. 



To test these hypotheses for the tropical Thalassia system, 

 which in the American Tropics includes Thalassia testudimum, 

 Syringodium fili, Halodule wrightii, and Halophila spp., an ex- 

 pedition was made aboard the RV Alpha Helix to the Mis- 

 kito banks off the coast of Nicaragua. (See fig. 47.) The 

 Nicaraguan shelf is the largest in the Caribbean and is re- 

 nowned as a major feeding area for the seagrass-eating green 

 turtle {Chelonia mydas). Previously, work on the tropical sea- 

 grasses had been in Texas and Florida, near the northern limits 

 of the system, and in St. Croix on the eastern edge of the 

 Caribbean. The expedition to Nicaragua sought to examine the 

 Thalassia system, in what are presumably optimal conditions for 

 the Caribbean Sea, as a basis of comparison for the continuing 

 research at other sites. 



The Miskito banks were found to contain a huge offshore 

 seagrass meadow that extended out to depths of about 20 m, 

 but was excluded from a nearshore zone about 1 to 2 miles 

 wide by a belt of turbid, low-salinity water. A series of tran- 

 sects were used to quantitatively study the biota of the sea- 

 grasses and associated coral reef and mangrove habitats. The 

 results indicate a gradient of detritus in the sediment that de- 

 creases with distance from the mangrove cays. As in other 

 areas studied, the plant and animal community reflects the sedi- 

 ment gradient. Sea urchins are often major herbivores in sea- 

 grass beds, and on the Miskito banks the common species, 

 Lythechinus variegatus, was studied to determine its association 

 with the seagrasses. The conclusion of the studies was that in 

 contrast to other areas, Lythechinus in this area could hardly 

 be considered a herbivore, because it apparently derived most 

 of its nutrition from bryozoans that are epiphytic on seagrass 

 leaves. Related research examined the relationship between 

 coral reefs and seagrass beds; this relationship is most obvious 



where heavily grazed seagrasses around a reef form a sand 

 "halo." It appears that reef dwelling organisms on the Miskito 

 banks exert an influence on the surrounding seagrasses for as 

 much as 20 m from the reef. 



Green turtles concentrate on the Miskito banks during late 

 fall and winter. These animals are the basis of the culture and 

 a traditional fishery of the Miskito Indians. Green turtles are 

 primarily seagrass consumers, and hence the coastal peoples of 

 the region are closely tied to the productivity of the seagrass 

 beds. The results of the studies on the digestive physiology of 

 the turtles showed that these animals are indeed true herbivores 

 capable of symbiotic cellulose digestion; assimilation of total 

 carbohydrates and cellulose is estimated to exceed 90 percent. 

 The process is not unlike ruminant mammals, but this is the first 

 established example of symbiotic cellulose digestion in a rep- 

 tile. Analysis of the carbon isotope ratio of turtle flesh con- 

 firmed that the animal could not be isotopically distinguished 

 from the seagrasses it eats. In addition, T. Fenchel (coopera- 

 tive participant from Denmark) discovered a new cilliate spe- 

 cies (possibly genus) in the microbial fauna of the turtle gut. 

 In SES III (1978-80), the scientific approach developed in 

 the previous studies will be expanded and intensified. A range 

 of seagrass ecosystems will be examined over latitudinal and 

 local environmental gradients. Two major sites have been se- 

 lected for intensive study — a tropical one on St. Croix, 

 U.S.V.I., and a north temperate one on the Alaska Peninsula. 

 (See fig. 48.) There are also several ancillary sites on the 

 Pacific and Atlantic coasts of America that will be used to 

 interpret latitudinal patterns. In addition, two other major 

 expeditions are being planned as international cooperative 

 studies. First, in 1979 will be a cooperative expedition with 

 Australian, Danish, and Japanese scientists to the Torres Strait 

 region of Australia; this area is considered the biogeographical 

 center of seagrasses. The second expedition, still under con- 

 sideration, would be in cooperation with Mexican scientists to 

 study the seagrass beds of the Gulf of California; this area 

 represents the southern extent of the Zostera system on the 

 Pacific coast of America. Both expeditions are viewed as tests 

 of the general hypothesis from a latitudinal perspective. Table 

 16 identifies participants in SES. 



International cooperation and interest in seagrasses has in- 

 creased greatly in the past few years, and now at least 20 na- 

 tions have active groups of seagrass researchers. International 

 collaboration is generally maintained through the International 

 Seagrass Committee, whose members include: Tom Fenchel, 

 Denmark; J. M. Peres, France; Akhiko Hattori, Japan; D. Den 

 Hartog, the Netherlands; and Peter McRoy and Patrick L. 

 Parker, United States. A meeting of the Committee will be 

 held in conjunction with a seagrass symposium as a part of the 

 Second International Congress of Ecology in September 1978. 



SES Bibliography 



Fry. B., R. S. Scalan, and P. L. Parker. 



1977. Stable carbon isotope evidence for two sources of 

 organic matter in coastal sediments: seagrasses and plank- 

 ton. Geochim. et Cosmochim. Acta. 41:1875-1877. 



Phillips, R. C, and R. F. Shaw. 



1976. Zostera noltii Hornem. in Washington, USA. Syesis. 

 9:355-358. 



79 



