Temporal variations in four species of intertidal foraminifera, Bahrain, Arabian Gulf

This study deals with the temporal variations in four infaunal foraminiferal taxa sampled monthly over a 25-month period (February 1991 February 1993 inclusive) from the lower intertidal zone on the east coast of Bahrain, Arabian Gulf. Standing crop, relative abundance and changes in species diversity and annual production of tests were measured for Ammonia beccarii, Elphidium advenum, Brizalina pacifica and Nonion sp. Additionally, the greatest diameter of each individual of A. beccarii was measured to determine monthly size changes and periods of reproduction. An attempt was made, through the use of replicate sampling, to determine homogeneity or patchiness (similarity) in species distributions and abundance patterns. Major changes are noted in species diversity and abundance between July and August 1992. The standing crop of the whole assemblage and of A. beccarii doubled, that of E. advenum increased three to four times and, for B. pacifica and Nonion sp. , the increase varied between 30 and 100 times. Overall, standing crop varied from 1-118 per 10cm3 and annual production of tests ranged from 2 to 2550 per 10cm3. A. beccarii is thought to reproduce throughout the year. Species diversity is characteristically low in this hypersaline habitat. Whereas the living assemblage is dominated by infaunal taxa, the dead assemblage is composed largely of transported epiphytic taxa, so the two are very different. INTRODUCTION The objective of the study was to monitor temporal changes in selected infaunal foraminiferal species from a subtropical lower intertidal zone in an arid climatic region. Replicate samples were taken to investigate homogeneity or patchiness of species distributions and their abundance patterns. Sampling was carried out monthly over a 25-month period in order to determine periods of reproduction, annual production of tests and changes in species diversity. The collection site is located in a bay on the east coast of Bahrain (text-fig. 1); the area is microtidal (<lm) with a diurnal rhythm. The intertidal zone is broad and gently sloping, characterized by a silty, sandy carbonate-sediment veneer discontinuously overlying lithifled hardground. There is sparse development of seagrass on the sediment with isolated patches of benthic algae, including the small shrub-like calcareous alga Jania, concentrated on other algae and on the exposed hardground. The salinity is 45%o throughout the year and temperatures range from around 17? to 31? C (Basson et al. 1989). There are no previous publications on recent foraminifera of Bahrain but 14 publications deal specifically with the taxonomy and ecology of foraminifera of the Arabian/Persian Gulf: Murray 1965a,b, 1966a,b,c, 1970a,b; Al Abdul-Razzaq and Bhalla 1987; Ahmed 1991; Haake 1970, 1975; Lutze 1974; Lutze et al. 1971; Lutze and Wolf 1976. There is a synthesis of Gulf foraminiferal ecology in Murray (1991) and a further publication by Coles and McCain (1990) in which foraminifera are noted as a small part of the infaunal community in relation to the environmental factors in the western portion of the Gulf. A general account of the marine biology and ecology of the Gulf is given by Basson et al. (1977). MATERIALS AND METHODS Replicate samples were collected monthly over a 25-month period (February 1991 to February 1993 inclusive) from the lower part of the intertidal zone of the bay between Askar village and Ras Hayyan on the east coast of Bahrain (text-fig. 1). A plastic ring 1cm high, 10cm in diameter and enclosing a volume of 78.5cm was pressed into the silty, sandy carbonate sediment, an aluminum plate slid underneath and the contents scooped out and placed in a collecting jar. Samples were preserved in methanol. In the laboratory, samples were wet sieved (63gm mesh), stained in rose Bengal TEXT-FIGURE 1 Location of the study area (head of arrow). micropaleontology, vol. 41, no. 1, pp. 69-76, text-figures 1-14, table 1, appendix 1, 1995 0 25 kms I I I I I t N