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

A survey of mudflat gastropods in Deep Bay, Hong Kong

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. A SURVEY OF MUDFLAT GASTROPODS IN DEEP BAY, HONG KONG M.W. Cha* The Swire Institute of Marine Science, The University of Hong Kong, Cape d'Aguilar, Shek 0, Hong Kong ABSTRACT Seventeen species of gastropods were recorded from a survey conducted on the Mai Po Marshes Nature Reserve mudflat. Sermyla tornatella was the dominant species and contributed 73.1 % and 61.3% to the total gastropod density and biomass, respectively. Density of this species averaged at 816 individuals·m-2 and was significantly correlated with total gastropod density on the mudflat. Although the lower mudflat hosted higher gastropod densities and biomasses, the high shore and areas along one of the three transects had higher species diversity. The importance of S. tornatella to feeding waterfowl is discussed. INTRODUCTION The intertidal mudflat of Deep Bay is an important feeding habitat for migratory waterfowl passing through Hong Kong along the East Asian / Australasian flyway and for wintering birds (Young & Melville 1993). More than 68,000 individuals of waterfowl have been recorded utilising different parts of Deep Bay in a census conducted in January 1996 (Hong Kong Bird Watching Society 1997). With the rapid development of the northwest New Territories in Hong Kong and Shenzhen Special Economic Zone in China, pollution is now, however, considered a serious threat to Deep Bay. The biochemical oxygen demand (BODs) loadings entering Deep Bay is estimated at 35,000 t·yr1 (Peking University 1994). Escherichia coli counts approaching 104 per 100 ml have been detected repeatedly in these areas (EPD 1994). This is mainly due to inputs of largely untreated domestic sewage as well as commercial and livestock wastes. Organic * Present address: Environmental Protection Department, Southom Centre, Hennessy Road, Wan Chai, Hong Kong 33 34 M.W.CHA pollution can cause significant adverse effects on macrobenthic populations (Pearson & Rosenberg 1978) and, thus, on the abundance and distribution of prey for feeding shorebirds which depend on such communities (Goss-Custard 1977a; Green et al. 1992; Yates et al. 1993). Information on mudflat invertebrates in Deep Bay can, therefore, help us understand more about the effects of pollution on that habitat and prey availability for migrating birds. The Gastropoda are the second most abundant group of invertebrates recorded on the Deep Bay mudflat, only exceeded in abundance by the tiny bivalve Pseudopythina maipoensis (McChesney 1997). The gastropods are mainly concentrated in the top 30 mm of the mud surface. It is not, however, clear how the distribution and abundance of this group of invertebrates respond to pollution stress and the timing and distribution of migrant birds on the mudflat. This study, therefore, aims at gathering baseline data on the distribution and diversity of mudflat gastropods at Mai Po. The distribution pattern is compared and related to the pollution status and distribution of migratant birds on the Mai Po mudflat. MATERIALS AND METHODS The mudflat (4.06 km2 ) adjacent to the Mai Po Marshes Nature Reserve is bounded by the Shenzhen River to the north and the Shan Pui River to the south (Fig. 1). These two rivers are the main sources of largely untreated sewage in Deep Bay. In September 1993, 0.25 m2 quadrat samples were collected along three transects (A, B and C) which, respectively, were located in the northern, central and southern part of the mudflat, to estimate the general pattern of gastropod density and biomass (Fig. 1). At least six replicated random samples were collected from each of the upper, mid and lower regions of the three transects. The top 30 mm of sediment within a quadrat was collected and sieved through a 1 mm mesh. Gastropods retained on the sieve were counted and identified to species. Total with-shell dry weights of the few numerically dominant species, i.e., Sermyla tornatella, Stenophyra spp. and Salinator spp., in each sample were recorded. Uncommon species were grouped together before recording. The relative percentage importance of these species in terms of density and biomass at each station were calculated and compared by the Kruskal-Wallis and Wilcoxon tests. Shannon-Wiener diversity index (8') (Krebs 1978) was also calculated for each station and transect. Regression relationships between the density of S. tornatella and total density and biomass of all gastropods were calculated...