Distribution, Abundance, and Species Composition of Seagrasses at Several Sites in Oman

Aquatic botany Aquatic Botany 53 (1996) 199-213 Distribution, abundance, and species composition of seagrasses at several sites in Oman B.P. Jupp * , M.J. Durako ", W.J. Kenworthy ', G.W. Thayer ', L. Schillak a Department of Biology, Sultan Qaboos University (SQU),P.O. Box 36. At-Khod 123, Muscat, Oman Florida Marine Research Institute, 100 Eighth Ave., S.E., St. Petersburg FL 33701. USA Beaufort Laboratory, National Marine Fisheries Service, National Oceanic and Atmospheric Administration fNOA.4). Beaufort NC 28516. USA " World Wide Fund for Nature (WWF-Germany),Hedderichstrasse 110,60591 Frankfurt. Germany Accepted 18 January 1996 Abstract Distribution, abundance, and biomass data for seagrass communities at several locations on the coast of Oman are presented. The main study site was on the western side of Masirah Island on the Arabian Sea coast of Oman. This area is an important feeding ground for the green turtle, Chelonia mydas L., and it is affected by upwelling of low temperature waters during the summer monsoon. The depth distributions of Halodule uninervis (Forssk.) Aschers. and Halophila ovalis (R. Brown) Hook. f., the two most abundant seagrasses at this site, overlapped but were inversely related. Halodule dominated the intertidal zone and Halophila was more predominant in the deep subtidal, although total biomass of the two seagrasses were similar in this depth zone. At all depths, biomass of Halophila was about equally distributed between leaves and roots and rhizomes. Leaf biomass of Halodule was only 7-20% of the total biomass and the highest below-ground biomass occurred in the intertidal zone. Biomass of these species here and at other sites and of Thalassodendron ciliatum (Forssk.) den Hartog at this site was generally lower than comparative data in the Gulf and the Red Sea. Small patches of Syringodium isoetifolium (~schers.)Dandy were also observed in Umm ar Rasas Bight making a total of four species recorded to occur in Oman. The reduced growth of seagrasses at Masirah Island seems to be due to stresses associated with the summer monsoon and grazing pressure. Survival of these popula- tions is discussed in terms of seasonal growth and flowering. Keywords: Seagrasses; Oman; Arabian Sea upwelling; Grazing; Survival strategies * Corresponding author. 0304-3770/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved Pit S0304-3770(96)01023-6 B.P. Jupp et al./Aquatic Botany 53 (1996) 199-213 1. Introduction The coastal regions of the Arabian peninsula have a diversity of geological and marine habitats. An arid climate, large seasonal temperature fluctuations and relatively high salinities all combine to make the Gulf (name used for Arabian, Persian Gulf, etc., see Sheppard et al., 1992) an extremely stressful marine environment (Sheppard et a]., 1992). In contrast, moderate temperature fluctuations and a much deeper basin con- tribute to less extreme environmental conditions in the Red Sea. Consequently, the Red Sea has higher animal and plant diversity than the Gulf. Within the Arabian region there have been several studies on seagrasses which are summarized in Price (1992). Most work has been done in the Red Sea (e.g. Price et a]., 1988) and in the Gulf (Basson et al., 1977; Price and Coles, 1992; Durako et al., 1993; Kenworthy et a]., 1993). Substantial differences are evident within marine plant communities of the Red Sea and Gulf, especially the seagrasses. Only four species are known for the Gulf, with most communities dominated by smaller-bodied species, Halodule uninervis (Forssk.) Aschers., Halophila ovalis (R. Brown) Hook. f., and Halophila stipulacea (Forssk.) Aschers. (Basson et a]., 1977; Coles and McCain, 1990; Price, 1992; Price and Coles, 1992; Kenworthy et al., 1993). A larger species, Syringodium isoetifolium (Aschers.) Dandy, occurs in the Gulf but is very rare. In contrast, at least ten and possibly 1 1 species are known for the Red Sea, including several of the larger-bodied, wide-leaved seagrasses like Thalassodendron ciliaturn (Forssk.) den Hartog, Thalassia hemprichii (Ehrenb.) Aschers., Cymodocea rotundata Ehrenb. et Hempr. ex Aschers. and Cymodocea serrulata (R. Brown) Aschers. et Magnus (Price, 1992). Although much less is known about seagrasses along the southern Arabian coastline, there is evidence that diversity and abundance are generally lower in Oman and Yemen than in the Red Sea. Price (1992) reported four species for S.E. Arabia and stated that seagrasses appear to be uncommon along much of the Oman and Yemen coastlines although isolated pockets of dense beds have been reported for eastern Oman near the Gulf of Masirah (Ross, 1985; Clarke et a]., 1986) and the Gulf of Aden (Hirth et al., 1973). Sparse beds of Halodule uninervis and Halophila ovalis are found along the north coast of Oman, being limited to sandy/silty areas such as Ras Suwadi, Bandar Jissah and Sur Harbour (Mardela Int., 1975; Salm and Dobbin, 1986; Salm et al., 1988; Cordero, 1992). Some of the best developed beds in Oman reportedly occur (Salm, 1989) in the Masirah Island area of the east coast (Masirah Channel, the Ban- a1 Hickman peninsula, and Ghubbat Hashish Bay; see Fig. 1). Ghubbat Hashish is the centre of a significant penaeid prawn fishery (Mohan and Siddeek, 1996) including Penaeus semisulcatus de Haan, which depends on seagrass beds as nursery habitat (Coles et a]., 1987). Like the Gulf, the coastlines of Oman bordering the Arabian Sea are extremely stressed, but for an additional reason. During summer the coastal oceanography of this shoreline is dominated by a southwest monsoon ('khareef'). Strong winds blow parallel to the coastline from June to September, driving a highly significant upwelling, accompanied by maximum wave heights during July and August. Between June and October, warm oligotrophic coastal waters are replaced by cold, nutrient rich and turbid waters. The upwelling drops surface water temperatures to as low as 18OC and nutrient B.P. Jupp el al./Aquatic Botany 53 (1996)199-213 20 1 concentrations may rise to 20 pg-at. NO3-N 1'and 2.0 pg-at. PO4-P 1 ' (Savidge et al., 1990). The result is an upwelling zone ranked as one of the five most intense upwelling regions in the world (Currie, 1992; Ormond and Banaimoon, 1994). During the monsoon, prevailing conditions allow for the growth of the kelps Sargassopsis zanardinii (Schiff.) Nizam. et al, and Ecklonia radiata (C. Ag.) J. Ag., (Barratt et al., 1986) but are not well suited for the habitat requirements of larger-bodied tropical seagrasses. Along the eastern coast of Oman near Masirah Island is a region of unconsolidated sedimentary environments which may be partially shielded from the most direct effects of coastal upwelling. The Masirah channel has been identified as a major feeding ground for the green turtle (Ross, 1985). Two seagrasses, Halodule uninervis and Halophila ovalis, were recorded as important components of the green turtle diet, along with the algae Sargassum ilicifolium (Turn.) C. Ag. and Chaetomorpha aerea (Dillw.) Kiitz., by Ross (1985) who also reported biomass and cover data for mixed seagrass and algal samples on the west coast of Masirah Island. Seasonal physical oceanographic conditions here may prevent or minimize the establishment of larger-bodied seagrasses common in the Red Sea and, like the Gulf (Basson et al., 1977), species composition and abundance should be dominated by the smaller species capable of either tolerating extreme conditions or being able to recolonize substrates outside the seasonal period of upwelling. In this paper we report the results of a main field study undertaken in January 1993 to describe the species abundance and zonation of seagrasses in Umm ar Rasas Bight, a shallow channel on Masirah Island. The channel is centrally located on the western side of Masirah Island between land and the smaller island of Jazirat Shaghaf. This eastern coast of Oman is under the influence of the summer monsoon, but there may be a noticeable reduction in the monsoon's direct effects compared to regions further south (Barratt et al., 1984). Conditions within the channel may be ideally suited for develop- ment of the most mature seagrass beds in the region. Further limited samples obtained at different times from the same region and from a protected site in Sur Harbour on the northeast coast of Oman are also presented. 2. Materials and methods 2.1. Main study sites at Umm ar Rasas Bight and Jazirat Shaghaf The main study site at 2Oo28'N; 58'46'E is midway down the western side of Masirah Island facing the 20 km wide Masirah Channel off the eastern coastline of Oman in the Arabian Sea (Fig. 1). The mainland coastline of this region consists of long stretches of sandy beaches, littoral dunes and mud-flat with salt pans ('sabkhas') interrupted by rock outcrops. Masirah Island has more prominent rock formations but is generally low lying and surrounded by a shallow continental shelf with predominantly carbonate sediments. Coral patch reefs have been reported for this area. This is in contrast to the geology further south in Oman where rocky promontories, steep cliffs and hard substrates dominate the shoreline (Sheppard, 1992). B.P. Jupp et al./Aquatic Botany 53 (1996) 199-213 Fig. 1. Map of Sultanate of Oman with inset details of study sites. B.P. Jupp et a/./Aquatic Botany 53 (1996) 199-213 Fig. 2. Location of sampling sites at main study area, Jazirat Shaghaf and Umm ar Rasas Bight, Masirah Island, Oman. Sites shown are J, 'Jetty' transects; 'offshore' Sites 1, 2, 3, 4 and NOAA/SQU Site. Depth contours are in metres. The distribution, density and biomass of submerged macrophytes in January 1993 were determined firstly in Umm ar Rasas Bight by sampling along three onshore-to-offshore transects ('Jetty' sites T-1, T-2, T-3) and from four (three for biomass) randomly chosen sample sites from vegetated shoals (1 -2 m depths) surrounding Jazirat Shaghaf Island (Fig.

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