The Ecology, Biometry and Biomass of the Seagrass Halophila stipulacea Along the Jordanian Coast of the Gulf of Aqaba

Aspects of the ecology, biometry and biomass of the seagrass Halophila stipulacea along the Jordanian coast of the Gulf of Aqaba have been investigated. The vertical distribution covers a depth range of 1 to at least 50 m. There is a decrease in density and biomass with increasing depth to 45 m and an increase in leaf length with increasing depth. Vertical and habitat patterns of distribution are discussed in terms of ecological factors and inherent characters of H. stipulacea. Two populations based on differences in mean leaf length and width are recognized and these are related to major habitats rather than depth. Introduction Most of the species of marine angiosperms or seagrasses have a distribution restricted to the tropics; other species occur in temperate and warm-temperate to subtropical areas (Dawson 1966). The vertical distribution is, however, generally restricted to shallow water primarily because of the dependence on light for photosynthesis. The seagrass community is one of the most productive of the primary producing marine communities (Bakus 1969). Additionally, seagrass beds serve as breeding and nursery grounds for many commercially important fishes and crustaceans. Seagrass beds are also feeding grounds for many coral reef-associated and other species (Randall 1965); important in the biogeochemical cycle of nutrients (McRoy and Barsdate 1970); and in the formation of and as a reservoir for detritus (Harrison and Mann 1976). Seagrass beds are indicative of a number of environmental conditions including light penetration, fine-grained sediment with high organic content, reduced current velocity and stabilization of the bottom (den Hartog 1971). The primary producing communities in the Jordan Gulf of Aqaba include phytoplankton, macroalgae, components of the coral reef complex (those Cnidaria containing Zooxanthellae) and the seagrasses. In view of the fundamental importance of seagrasses in productivity, a program of investigation has been initiated and includes mapping the horizontal and vertical distribution, using a combination of photographs taken from a helicopter and SCUBA diving, productivity, biomass, biometry and nutrientseagrass interaction. Three species of seagrasses have been identified from the Jordanian coast of the Gulf of Aqaba (Boulos and Lahham 1977). The species include Halophila stipulacea, Halophila ovalis and Halodule uninervis. Lipkin (1975) reported two additional species from Aqaba based on previous collections, including Syringodium isoetifolium and Thalassodendron ciliatum. To date these species have not been found along the Jordanian coast. Mergner and Schuhmacher (1974) determined the zonation of H. stipulacea and H. uninervis within a reef lagoon and to a depth of 25 m at one locality on the coast of Jordan. Methods and Materials Halophila stipulacea was selected for detailed study because of its wide horizontal and vertical distribution along the coast of Jordan. The selection of the six localities for study shown in Figure 1 was based on uniformity of the seagrass beds and to provide maximum habitat variation in the occurrence of//, stipulacea within the capabilities of sampling by using SCUBA. Locality 1, on the north coast, is characterized by a gentle gradient and fine-grained sediment. Corals are, for the most part, absent but there is a high concentration of mobile macrofauna and infauna based on the observation of a large number of trails and mounds. One effect of the presence of the fauna is the continual vertical disturbance of the sediment. Localities 2 and 0006-8055/79/0022O425S02.00 © by Walter de Gruyter & Co. · Berlin · New York 426 Hulings: Ecology, Biometry, Biomass ofHahphila stipulacea Jordan Coast, Aqaba