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 populations is discussed in terms of seasonal growth and flowering.

Keywords: Seagrasses; Oman; Arabian Sea upwelling; Grazing; Survival strategies

* Corresponding author.

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 contribute 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 development 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. 2). The shoreline transects were established to determine changes in macrophyte distribution as affected by water depth and distance from shore. Transects extended from the intertidal zone to a depth of == 3 m; four depth strata were sampled, intertidal (< 0.5 m), upper subtidal (< 1.3 m), lower subtidal (= 1.3 m) and deep subtidal (= 3.0 m). At each sample location the presence, cover, and abundance for each species was estimated visually using the Braun-Blanquet technique (Mueller-Dombois and Ellenberg, 1974) in four 625 cm2 quadrats placed approximately 2 m north, east, south and west of the sample point. For a particular sample quadrat, the observed seagrasses and macroalgae were first listed. A cover-abundance rating was then assigned using the following Braun-Blanquet (B-B) scale values: 5 cover of more than 75% of 204 B.P. Jupp et al./Aquatic Botany 53 (1996) 199-213 the quadrat; 4 50-75% cover; 3 25-50% cover; 2 5-25% cover; 1 numerous, but less than 5% cover or scattered with up to 5% cover; + few, with small cover (assigned a value of 0.5); r solitary, with small cover (assigned a value of 0.1). Frequency of occurrence, abundance, and density information for the seagrasses and macroalgae were calculated using the following formulae: Frequency = number of occupied quadrats/total number of quadrats Abundance = sum of B-B scale values/number of occupied quadrats Density = sum of B-B scale values/total number of quadrats Seagrass biomass and density were estimated from four replicate core samples (7.5 cm diameter) per transect station and three replicate core samples (15 cm diameter) per offshore station. In each core, the numbers of short-shoots of Halodule uninervis or leaf pairs of Halophila ovalis were recorded; the numbers of live rhizome apices of each species were also counted.

2.2. Other sites

At the NOAA/SQU site (Fig. 2), off the southwest point of Jazirat Shaghaf, two 0.25m2 quadrats were collected in June 1992 in patchy beds of both Halodule uninervis and Thalassodendron ciliaturn. At the site along the east side of Ghubbat Hashish (Fig. 11, one 0.25m2 quadrat was collected in June 1991 from a sparse bed of Halodule uninervis. In Sur Harbour (Fig. l), three 15 cm diameter cores were taken in May 1994 from mixed Halodule uninervis and Halophila ovalis beds; there was also a large mat of the filamentous green alga Rhizoclonium sp. in one of the cores. The site appeared to be polluted with sewage and is exposed at low tide. Plant material was separated into leaf, root and rhizome fractions, and short shoot and leaf pair densities were recorded. Material was cleared of epibiota by rinsing in 5% phosphoric acid, washed and then dried in an oven at 80Â° for at least 2 days.

2.3. Environmental data:

Water temperature and conductivity were measured with a WTW L191 meter. Salinity was calculated from conductivity after Dietrich et al. (1975). Oxygen was measured with a WTW OX1 191 meter. Photosynthetically active radiation (PAR = 400-700 nrn) was measured with a LI-COR LI-192 SB Quantum Sensor (2~).

3. Results

Spot measurements (n> 5) of environmental factors were made between 19 May and 7 June 1991 giving a mean water temperature of 33.8OC, salinity of 34.5 %O and oxygen content of 8.4 mg 1'(1 18%) in the Umm ar Rasas Bight and values of 30.ZÂ°C33.6 %O and 10.4 mg 0, I-' (146%) for offshore waters west of Shaghaf Island. PAR (400-700 nm) was measured just north of the Jetty site in Umm ar Rasas Bight at 09:50h on 19 B.P. Jupp er al./Aquatic Botany 53 (1996) 199-213

Table 1 Total biomass of three seagrass species from several sites in Oman Species and site Date N Total biomass Range Depth (g DW m-2 Â s.e.) (g DW m- *) (m) Halodule uninervis I. Masirah Island (Main site) Urnm ar Rasas Bight (Jetty T- 1, T-2, T-3) Intertidal January 1993 Upper subtidal January 1993 Lower subtidal January 1993 Deep subtidal January 1993 Jazirat Shaghaf ('offshore') Site 1 January 1993 Site 2 January 1993 Site 3 January 1993 NOAA/SQU site June 1992 2. Ghubbat Hashish June 1991 3. Sur Harbour May 1994 Halophila ovalis 1. Masirah Island Umm ar Rasas Bight Intertidal January 1993 Upper subtidal January 1993 Lower subtidal January 1993 Deep subtidal January 1993 Jazirat Shaghaf Site 1 January 1993 Site 2 January 1993 Site 3 January 1993 2. Sur Harbour May 1994 Thalassodendron ciliaturn Jaziraf Shaghaf NOAA/SQU site June 1992

N, number of samples; g DW, gram dry weight; only one sample.

May 1991 when surface irradiance was 1600 pmol m2sf' and irradiance at 2 m depth (just off the bottom) was 395 pmol m2s-I. Total biomass data for three seagrass species from several sites in Oman are shown in Table 1. Variation of biomass with depth is seen at the main study site transects in Umm ar Rasas Bight. Along all three shoreline transects at Umm ar Rasas Bight, Halodule uninervis and Halophila ovalis depth distributions were inversely related (Fig. 3, Fig. 4, Fig. 5). However, the depth distributions for both Halodule and Halophila generally overlapped; both genera were found in all four depth zones (Fig. 3, Fig. 4). Halodule dominated the intertidal zone, in terms of biomass (Fig. 3, Fig. 4), density, abundance and frequency (Fig. 5). Halophila was more predominant (higher frequency of occurrence, overall bottom coverage, leaf pair density and apical meristem density; Fig. 5, Fig. 6) in the deep subtidal, although total biomass of the two species was very similar B.P. Jupp et al./Aquatic Botany 53 (1996) 199-213

20 , JETTY T-1

100 J JETTY T-2

0 20 40 IH. uninervis leaves ? 60 B H. ovalis leaves ' 80 IH. uninervis rhizomes and roots UID H. ova/& rhkoms and rwts g 100 _ 2o _ JETTY T-3

INTERTIDAL UPPER LOWER DEEP F- SUBTIDAL SUBTIDAL suBnox

20 -i OFFSHORE -

:: j 1 SITE-1 SITE-2 SITE-3 100 Fig. 3. Seagrass biomass (meanÂ±s.ein g DW m2)along three onshore-to-offshore transects in Urnm ar Rasas Bight and from three offshore shoal sites at Jazirat Shaghaf, Masirah Island. in this depth zone (Table 1, Fig. 4). At all depths, biomass of Halophila was about equally distributed between leaves and roots and rhizomes, and highest biomass occurred in the deeper sites (Table 1, Fig. 3, Fig. 4). Leaf biomass of Halodule was only 7-20% of the total biomass and the highest below-ground biomass occurred in the intertidal zone (Fig. 3, Fig. 4). A small patch of Syringodium isoetifolium less than 2.0 m in diameter was observed near one of the deep subtidal transect stations in the channel, but was not actually detected in the sample quadrats or cores. In the shoals surrounding Jazirat Shaghaf Island, Halodule and Halophila exhibited patchy distribution patterns (Fig. 3). Both species were observed in about half of the quadrats. Where present, they exhibited < 25% cover, but the mean biomass of Halodule was over three times greater than that of Halophila (Table 1, Fig. 3, Fig. 4). Macroalgal distribution was even more patchy, macroalgae being present in only 25-30% of the quadrats. However, where present, red-algal cover averaged about 75%. B.P. Jupp er aI./Aquatic Botany 53 (1996) 199-213

INTERTIDAL UPPER LOWR DEEP OFFSHORE SUBTIDAL SUBTIDAL SUBTIDAL

10 A 2 20 Â¥ 30 w> IH. uninewis leaves 3 40 0 rn H. ovalis leaves m so IH. uninewk rhizomes and roots

60 IKOI H. ovalis rhuoms and rmb

80 Fig. 4. Summary of depth-related changes in seagrass biomass (mean in g DW m2)in Umm ar Rasas Bight and offshore sites, Jazirat Shaghaf, Masirah Island,

This cover was dominated by at least two species of Gracilaria. Green algae were encountered more frequently than red algae, but they were less abundant. Halimeda tuna (Ell. and Soland.) Lamour. and Udotea indica Gepp and Gepp were the dominant, green

, , INTERTIDAL UPPER LOWER DEEP OFFSHORE SUBTIDAL SUBTIDAL SUBTIDAL

0Habdu/e unimwis Halophila ovalis U Red Macroalgae IIlIj Green Macroalgae Fig. 5. Macrophyte density, abundance and frequency from Braun-Blanquet sampling in Umm ar Rasas Bight and offshore sites, Jazirat Shaghaf, Masirah Island. B.P. Jupp er al./Aquatic Botany 53 (1996) 199-213

INTERTIDAL UPPER LOWER DEEP SUBTIDAL SUBTIDAL SUBTIDAL Fig. 6. Rhizome apical meristems (top panel) and and short-shoot or leaf pair densities (bottom panel) as a function of tidal zone for Halodule unineruis and Halophila ovalis in Umm ar Rasas Bight, Masirah Island (mean Â s.e.). macroalgae. Other green algae found in this area include an Acetabularia sp., Caulerpa mexicana (Sond.) J. Ag., Caulerpa sertularioides (Gmel.) Howe and Chaetomorpha aerea (Dillw.) Kiitz. Brown macroalgae collected from this region include Cystoseira myrica (S.G. Gmel.) C. Ag., Padina sp., Sargassum spp. and Stokeyia indica Thivy and Joshi. The red alga Laurencia papillosa (C. Agardh) Grev. has also been found here. The total biomass of Halodule uninervis from other sites (Ghubbat Hashish, Sur Harbour (Table 1)) was of the same order as those found at the main study sites. A small total biomass of Halophila ovalis was found in Sur Harbour (Table 1). That this site is eutrophic was indicated by a nearby sewage outfall and a high maximum biomass of 632 g DW m2for a Rhizoclonium sp. mat here. A mean biomass of 146 g DW m2was found for Thalassodendron ciliaturn in patchy communities on the shallow rock ledges southwest of Shaghaf Island (NOAA/SQU site (Table 1, Fig. 2)).

4. Discussion and conclusions

The seagrass community inside the channel at Umm ar Rasas Bight was dominated by the same two species most abundant in the Gulf (Basson et al., 1977; Price, 1992; Kenworthy et al., 1993). Halodule unineruis was relatively more abundant than Halophila ovalis in the intertidal zone where the maximum total biomass was observed (Table 1, Fig. 3, Fig. 4, Fig. 5). The abundance of Halodule uninervis in the intertidal zone of Masirah Island may reflect the relatively high desiccation tolerance of this species, due largely to its flaccid leaves which lie on the sediment surface when the tide is low (Phillips, 1960; Lewis et al., 1985). This, however, contrasts with observations B.P. Jupp el at. /Aquatic Botany 53 (1996) 199-2 13 209 from the northwestern Gulf in Saudi Arabia where seagrasses were almost completely absent from the intertidal zone, presumably due to the stress induced by wide temperature fluctuations, hypersalinity, and possible damage from the Gulf War oil spill (Kenworthy et al., 1993; Durako et al., 1993). In the channel at Masirah Island the intertidal mean biomass of Halodule uninervis (77 g DW m2)was similar to the total biomass of Halodule unineruis in the shallow subtidal (< 1.0 m depth) inner bay stations sampled in February 1992 in northwestern Saudia Arabia (Kenworthy et al., 1993; Fig. 3). In these same two Gulf embayments total biomass of Halodule uninervis at deeper stations (2-10 m) ranged between 150 and 300 g DW m2,whereas the total biomass at the subtidal stations on Masirah Island was only 8-16 g DW m2.Since the samples were taken at the same point in time in the seasonal cycle it appears that Halodule uninervis beds in the northwestern Gulf develop a higher biomass. This also appears to be the case in the Red Sea since the maximum biomass of Halodule uninervis (177 g DW mm2)found in this study contrasts with the mean biomass of 230 g DW m2and a maximum biomass of 400 g DW m-2 for Halodule uninervis dominated beds in the northern Red Sea (Lipkin, 1979; Wahbeh, 1980). The Sur Harbour site is not affected by the S.W. monsoon but, despite the shelter and nutrient-rich site there, the maximum biomass of Halodule uninervis found in May (Table 1) was only 109 g DW m2.The mean density of short shoots of Halodule uninervis at the transect sites in January was generally less than 3500 m2(Fig. 61, contrasting with data in the Gulf where densities were generally greater than 3500 shoots m2in late March (Kenworthy et al., 1993). Halophila ovalis leaf pair densities were also lower (< 1000 m2)here than in the Gulf. Halophila ovalis exhibited little change in abundance at the two deeper depth zones (Fig. 5), but biomass, leaf-pair densities and apical meristem densities all increased with depth (Fig. 4, Fig. 6). Because of the relatively small rhizomes and, hence, low respiratory demand of this genus (Duarte, 1991), Halophila is characteristically found to dominate the deeper edges of seagrass beds. Reproduction by seed may also enhance survival in deeper water by allowing Halophila to persist through periods of un- favourable environmental conditions (poor monsoonal water quality) as temporary seed banks in the sediment (Kenworthy et al., 1989). Of all the seagrasses, this genus is one of the most prolific seed producers and some species rely almost completely on seed reproduction to maintain populations in seasonally fluctuating environments (Kenworthy, 1992). Although flowering was not observed, probably because sampling was done too early in the season, it would be an ideal mechanism for both Halophila ovalis and Halodule uninervis to survive in deeper water under monsoon conditions. Future studies in this region should examine the reproductive mechanisms of these two species in order to better understand how they survive in stressful periods. These observations suggest that full development of seagrasses on Masirah Island is limited by some environmental factor, or that sampling was done too early in the growing season to detect maximum abundance of these species. Full maturation of seagrass beds in this region of Oman may not occur until April or May, just prior to the onset of the monsoon. Given the known seasonal fluctuation in growth and biomass of Halodule spp. (Basson et al., 1977; Coles et a]., 1989; Hillman et a].. 1989; Kenworthy, 19921, it is possible that the biomass of these beds could double between February and 210 B.P. Jupp er al./Aquatic Botany 53 (1996) 199-213

May. Since there was a moderate density of rhizome apical meristems that could also contribute to vegetative reproduction (Fig. 6), favourable growing conditions in spring could then result in additional above ground and below ground biomass. It should be noted that Ross (1985) found similar values of biomass (70 g DW m2)for mixed Halodule unineruis and Halophila oualis samples at Safaiq (near the 'Jetty' transects; see Fig. 2), and 110 g DW m2at Dawwah (just north of these sites) in March 1979, indicating no large seasonal increase by then from data taken for this study in late January. Environmental data in late May/ early June indicated the onset of full monsoon conditions had not yet occurred; underwater irradiance at this time of 395 pmol m2s' at 2 m depth is an order of magnitude greater than the light-saturation levels of 30-35 pmol m2s' reported by Durako et al. (1993) for leaf tissue of Halodule unineruis and Halophila oualis. Another explanation for the relatively low biomass of both Halodule uninervis and Halophila oualis in the Masirah Channel area is the potential grazing activity of green turtles, Chelonia mydas. Ross (1985) identified the area around the Masirah Channel as an important nesting and feeding habitat for Chelonia mydas. In that same study the stomach contents of a number of Chelonia mydas indicated they were feeding on both Halodule unineruis and Halophila oualis with no clear indication of a preference for either species. Some preference feeding tests were carried out by Al-Ajzoon (1993) with young adults of Chelonia mydas, which were offered clumps of macroalgae (preference: Sargassopsis > Sargassum > Ulua) and seagrasses (preference: Halophila > Syringodium > Halodule). Evidently, with the relatively low biomass and abundance of larger-bodied seagrasses, turtles in this area rely heavily on Halodule unineruis, Halophila oualis, and macroalgae as their principal sources of nutrition (Ross, 1985). The relatively low abundance of larger-bodied seagrasses is another indication of stressful conditions in the vicinity of Masirah Channel. The patchy, isolated occurrences of Thalassodendron ciliatum and Syringodoium isoetifolium indicate that seagrass bed development beyond the smaller species is severely constrained. The mean biomass of Thalassodendron ciliatum of 146 g DW m2found off the southwest point of Shaghaf Island is insignificant compared to the very high biomass of this species where it can form a thick rhizome layer (Lipkin, 1979; Brouns, 1985). Whether this is typical for the general region, or only for Masirah Island, is unclear. Reports of fairly well developed seagrass beds in the Gulf of Masirah and in Ghubbat Hashish Bay suggest that environmental conditions are suitable for seagrasses, possibly even the larger species (Salm, 1989). Further south in the Gulf of Aden, well developed beds of the large-bodied seagrasses Cymodocea serrulata and Syringodium isoetifolium occur at Khor Umaira, in Yemen (Hirth et al., 1973). These beds also provide an important source of nutrition for green turtles in the coastal area of Yemen (FAO, 1968), and indicate that development of beds with larger species outside the high diversity region of the Red Sea is possible. The physical features of the Arabian Sea coastlines of Yemen and Oman, along with the water quality conditions during the upwelling season, create a highly dynamic and fluctuating environment. Additionally, there are substantial populations of herbivorous echinoderms and fish which graze the vegetation intensively (Barratt et al., 1986). Only seagrasses uniquely equipped for survival in these conditions should be dominant, yet B.P. Jupp et al./Aquatic Botany 53 (1996) 199-213 21 1 there may be regions of the Oman coastline where seagrasses are more abundant than at Masirah Island and where beds of larger-bodied species occur. In summary, a number of factors may be contributing to low species diversity, and to limited distribution and abundance of seagrasses on Masirah Island. Many factors are contributing to the stressful nature of this coastal region including environmental stress, physical isolation from larger seagrass populations, and grazing pressure. Future ecological research in the area could benefit our understanding of the life history and population dynamics strategies utilized by seagrasses to survive in stressful environments. The establishment of seagrass beds with larger-bodied species and/or high biomass may be indicative of unusual conditions for this coastline. These areas would provide a unique opportunity to examine seagrass bed development under stressful conditions. Moreover, these local pockets of well developed seagrass beds may be a resource oasis among a wider regional distribution of smaller species and lower biomass, providing valuable habitat for fish, crustaceans, herbivores and other important wildlife species.

Acknowledgements

Support for this work was provided by Sultan Qaboos University, Muscat; Regional Organization for the Protection of the Marine Environment (ROPME), Kuwait; National Oceanic and Atmospheric Administration (NOAA), USA; National Marine Fisheries Service, NOAA, US Department of Commerce; Florida Marine Research Institute, St Petersburg, FL, USA., and the Marine Spill Response Corporation, Washington, DC, USA. Part of this study has been supported by Wiedleplan Consult GmbH, Stuttgart, Germany. We thank Professor Dr. C. den Hartog, Catholic University of Nijmegen, The Netherlands and Dr. R.C. Phillips, Washington, USA for taxonomic advice. We are grateful to Ali A. Al-Kiyumi, Ministry of Regional Municipalities and Environment, Muscat for much assistance and support. We also thank Robert Baldwin, Stephen Coles and Captain Richard Permenter, Ltjg Scott Stolz (Dive Master) and crew of NOAA ship 'Mt. Mitchell' (Leg VII, 1992) for help in the field. Special thanks also to Peter Penlerick and staff at Arabian Mapping Co., Muscat for producing the figures. Discus- sions with Rodney Salm and J. Perran Ross were most helpful.

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