The Mangrove Ecosystem of Deep Bay and the Mai Po Marshes, Hong Kong

Composition and zonation of benthic macrofauna in the Mai Po Marshes mangrove forest

The Mangrove Ecosystem ofDeep Bay and the Mai Po Marshes, Hong Kong (ed. S.Y. Lee). Proceedings of the International Workshop on the Mangrove Ecosystem of Deep Bay and the Mai Po Marshes, Hong Kong, 3-20 September 1993. Hong Kong: Hong Kong University Press, 1999. COMPOSITION AND ZONATION OF BENTHIC MACROFAUNA IN THE MAl PO MARSHES MANGROVE FOREST C. Anderson and S. McChesney Department of Ecology & Biodiversity and The Swire Institute of Marine Science, The University of Hong Kong, Cape d'Aguilar, Hong Kong ABSTRACT The composition and horizontal zonation of the benthic faunal assemblage of a mangrove forest in the Mai Po Marshes Nature Reserve, Hong Kong was examined in September 1993, in relation to edaphic properties of substrate water content, pH, Eh, and macrodetritus content. Thirty-two morphospecies were recorded from 7 phyla, with gastropods and annelids being numerically dominant. Of the 5 numerically dominant species, 3 showed distinct zonation patterns; lravadia (Fairbankia) bombayana was most abundant in the landward station and Assiminea brevicula and Assiminea sp.2 most abundant in the seaward station. A significant proportion of he variation in these species an total faunal abundance was explained by one or more of pH, detritus content and water content. INTRODUCTION The marginal, 'open-system' character of mangrove forests is reflected in their faunal assemblage, which typically contains pre-adapted, intertidal species, as well as species with characteristics more representative of marine or terrestrial systems (Macnae 1968, Odum et al. 1982). Tidal inundation, which allows flux of species and matter between the mangrove forest and surrounding systems, restricts utilisation of the forest by marine and terrestrial species, both temporally (e.g. fish are only able to utilise mangrove resources for food or shelter at high tide; Morton 1990) and spatially (e.g. mangrove folivorous caterpillars do not occur on leaves below the high water mark; Anderson and Lee 1995). The pre-adapted, intertidal species (with arboreal, benthic or epibenthic habits) endure no such restrictions, but exhibit responses to different conditions on the shore by occurring in distinct zonation patterns. These may be in a vertical (ascending mangrove stems or descending into the substrate; Dye 1983; Perry 1988) or horizontal (landward limit to seaward limit; Sasekumar 1974) direction. 45 46 C. ANDERSON & S. MCCHESNEY Horizontal zonation in coastal systems is thought to be, ultimately, a product of the competitive ability of a species and its degree of adaptation to biotic and abiotic shore conditions (Barnes and Hughes 1988; but see Underwood and Denley (1984) for a discussion of the paradigms resulting from rocky shore studies, where much of the research on intertidal assemblage has been carried out). The mangrove shore is somewhat benign in comparison to other shores, lacking large fluctuations in temperature and humidity over the tidal cycle by virtue of a canopy and moisture-retaining substrate. Changes in tidal inundation frequencies down the shore do, however, influence the faunal assemblage at different shore heights, either directly (e.g. species with specific adaptations for dry conditions are found in the high shore, Sasekumar 1974), or indirectly (e.g. by affecting soil waterlogging and pore water salinity which may affect species distributions; Smith 1992). In addition, gradients in other abiotic conditions (e.g. edaphic properties such as sediment grain size and sediment chemistry; Boto and Wellington 1984) may occur down the shore as a result of vegetational changes, which also typically exhibit a zonation pattern (Alongi 1987; Smith 199:2). The relationships between abiotic conditions and faunal distributions are complex, however, with reciprocal effects also observed (e.g. benthic invertebrate burrows alter sediment properties such as porosity, rigidity, nutrient status and redox potential; Smith 1991, 1992; Jones and Jago 1993). Biotic interactions in mangrove fauna such as predation (Morton 1990) and competition (Branch and Branch 1980, Dye and Lasiak 1986) are poorly studied, and effects on species distribution patterns are unknown. Previous studies of the mangrove-associated invertebrate fauna of the Mai Po Marshes are general accounts (Melville and Morton 1983; Morton and Morton 1983), focused on particular taxa (Anderson 1989; Tong 1990; Choi 1992, Leung 1993), concerned with particular ecological roles (Lee 1988; Anderson and Lee 1995), or related to pollution (Chiu 1992). Lee (1988, 1993) provides extensive data on the invertebrates found in the Mai Po Marshes including 13 new species. An ongoing study is examining the species composition and seasonal changes in benthic fauna of the mudflat which are the food source of migrating shorebirds and waterfowl. To date, no such study has been carried out...