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

Observations on the mangrove community at the Mai Po Marshes, Hong Kong: Implications for conservation

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. OBSERVATIONS ON THE MANGROVE COMMUNITY AT THE MAl PO MARSHES, HONG KONG: IMPLICATIONS FOR CONSERVATION K. Kathiresan Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai 608502, India ABSTRACT Field observations of the mangrove forest at the Mai Po Marshes, Deep Bay, were made on: (a) rate of seaward extension of the mangrove fringe; (b) sedimentation on mangrove leaves; (c) foliovory; (d) canopy structure; and (e) the condition of the seedlings of Avicennia marina and Kandelia candel. Based on these observations, some considerations relevant to the conservation of the Mai Po Marshes are proposed. These include: (a) diking and channelisation to improve propagule dispersal; (b) dredging of water channels to reduce sedimentation; (c) collection and plantation of mangrove propagules to enhance mangrove expansion; and (d) application of natural insecticides to control large-scale defoliation. INTRODUCTION Mangrove ecosystems in Hong Kong are threatened by rapid urban growth, placing an urgent need on developing strategies for their conservation. There is, however, little information on the local mangroves directly relevant to their conservation (Irving and Morton 1988, Lee 1988). In the present study, observations were made on aspects of the mangroves at the Mai Po Marshes, with a view of generating a conservation strategy for the mangroves in this nature reserve. A map of Mai Po Marshes shows 'finger-like' projections extending from the mangrove fringe, notably associated with drainage water channels (Fig. 1). This prompted an investigation to ascertain the link between growth of these mangrove fingers and the channels. Changes to the mangrove forest fringe depend on how mangrove seedlings are recruited and get established. This further depends on: (a) canopy structural characteristics; (b) the production, dispersal and distribution of propagules; and (c) the conditions under which the propagules germinate and establish. 179 K KATHIRESAN o f mangroves Leaves 0 . 180 0 in the study area0 0 hotosynthettc done prevIOusly interfere wIth p k h s not been 0 0 s that may Such wor a hObit two COndll1on cially those were observed t~:~e:lth of the trees: the foliage of mangroves espe P roductivIty and t. f fine particles on d' ntatlOn 0 d (a) the se Ime h channels an , growing alon~ t.e d by herbivores. s mfhcte (b) the damage Mudflat Fig. 1 CONSERVATION OF THE MAl PO MANGROVES 181 These two aspects of the mangrove's ecology were studied in an effort to provide further information to the future conservation of the Mai Po mangroves. MATERIALS AND METHODS Study area The Mai Po Marshes (22°30'N; 114°02'E) are located on the south-eastern shores of Deep Bay, northwest Hong Kong. Deep Bay has a total area of 115 km2 with an average depth of 3 m. A vast expanse of mudflat is exposed at low tide (Irving and Morton 1988). Dense stands of mangroves exist along the shoreline (Lee 1988). Observations and measurements were made at mangrove areas around the birdwatching hide (Fig. 1) during September 1993. Mangrove forest expansion To study recent changes of the mangrove forest fringe, aerial photographs of the study area were traced on translucent graph paper. 'Finger-like' projections of the first were demarcated from the core mangrove fringe and are shown as shaded areas in Fig. 1. The length and area of the projections were measured on the graph paper on wither sides of the water channels. The length of the water channels from the gei wai slice gate was also measured. Sedimentation on mangrove leaves The percentage of the canopy of trees affected by sedimentation was calculated as the ratio of the height of the canopy covered with sedimentation to the total tree height multiplied by 100. To quantify the sedimentation amount, leaves were collected from the fourth, fifth and sixth positions from shoot apices from the lower parts of the canopy and placed in polythene bags. The leaves were then immersed in clean distilled water for 24 hours with gentle shaking. The leaves were then removed and their leaf area measured. The water with the sediment washed off from the leaves was filtered through preweighed Whatman glass micro-fiber filters (4.7 cm) which were...