| Seagrass Beds and Intertidal Mudflats |
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| 4.2.1.38 |
Seagrass beds occur in shallow, sheltered intertidal or subtidal areas and are recognised as areas of high biological productivity. As such, they provide high value habitat as feeding and nursery grounds for a range of marine species. Local seagrass beds have a very limited distribution and occupy less than 0.1% of the total land area. |
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| 4.2.1.39 |
Seagrass beds are dominated by one or more species of specialised marine grasses. Four species of seagrasses, namely Halophila ovata, H. beccarii, Ruppia maritima and Zostera japonica have been recorded in Hong Kong (Hodgkiss and Morton 1978a, b), and all co-occur with mangroves. Halophilia ovata and H. beccarii are the most abundant amongst the four local seagrasses (Fong, 1998a), but still are restricted to only a few remote and less disturbed localities in Hong Kong (Lee 1994). H. ovata is a short lived annual plant which can colonise some areas in a short time but disappears quickly under unfavourable conditions (such as typhoons and strong water currents). It has been recorded at San Tau (Lantau), Sheung Sze Wan in Sai Kung and Lai Chi Wo in Crooked Harbour (Fong 1998b). H. beccarii appears to be extending its range on the intertidal mudflat along the southern fringe of Deep Bay, and the largest seagrass bed in Hong Kong comprises this species, occupying an area of around 4 ha at Ha Pak Nai where it provides an important nursery and feeding ground for horseshoe crabs (Fong, 1998a). The two most common species show seasonal highs in biomass during the wet season (AFCD pers comm, CW Fong pers comm) whereas Z. japonica grows most favourably during the dry season (Fong et al 1998). |
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| 4.2.1.40 |
Seagrasses have been documented to be threatened by development (Lee 1997) and are protected under the Marine Parks Ordinance (Cap 476) under the category of 'marine grasses' in designated Marine Parks and Marine Reserves only. Rapid development of the inshore environment and the associated terrestrial catchments, together with impacts to offshore water quality threaten the survival of the Hong Kong seagrasses, and hence the extent of the seagrass bed habitat. |
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| 4.2.1.41 |
The seagrass bed at Sham Sha Po (Northwest New Territories) was found during surveys to be patchy and composed of Halophila beccarii only. Zostera japonica was the dominant seagrass species recorded on the mudflats at So Lo Pun and Sham A Chung in Northeast New Territories. These seagrass beds, though small in size, were found to be growing in good condition. These habitats are remote and free from human disturbance and assigned high ecological value in the survey. |
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| 4.2.1.42 |
Intertidal mudflats often lie at sheltered heads of harbours or in bays land-locked by the surrounding coasts. They are classified as areas of fine-grained sediment (ie silt or fines) which lie between the high and low tide marks and which are not covered by seagrasses, mangroves or typical wetland vegetation. The habitat is generally fed with freshwater streams. Wave action is weak but currents agitate fine sediments from the substrate and create permanent turbidity. The habitat is susceptible to pollution because of its poor flushing conditions. Mudflats are known to support a variety of soft-bottom invertebrates and thus provide valuable feeding habitat for migratory bird populations (Turner and Wong 1991) as well as other organisms (eg horseshoe crabs). |
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| 4.2.1.43 |
The largest intertidal mudflat occurs in Deep Bay where the ample sediment load provided by the Pearl River and shelter from wave action allow the accumulation of silt. Deep Bay's mudflat benthos forms the trophic basis for the large number of resident and wintering birds in the area. However, the benthic population is affected by pollutants discharged into the two local river courses, the Shenzhen River and Yuen Long Creek. The polychaete-dominated nature of the benthic community (eg Capitella sp) observed in the river mouths of the two watercourses is indicative of gross organic pollution present (Morton et al 1996). |
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| 4.2.1.44 |
Some of the surveyed intertidal mudflats (eg Ho Chung) appear to have been affected by pollution as odorous and black-coloured waters were observed during the field visits undertaken for the SUSDEV 21 habitat survey. In such disturbed habitats, a comparatively lower species diversity and richness were generally recorded and ecological value was thus downgraded. Urban development appears to have adversely impacted some intertidal mudflats, particularly at Tung Chung where the habitat area has decreased and habitat quality in nearby areas has been affected. Despite this, extensive mudflats were recorded in Deep Bay and Tung Chung. |
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