濒危海草贝克喜盐草在海南东寨港的分布及其群落基本特征

Distribution and community characteristics of threatened seagrass Halophila beccarii in Dongzhai Harbor, Hainan

关键词:  海草生物地理学, 东寨港红树林, 海草保护, IUCN, 土壤种子库

Key words:  seagrass biogeography, Dongzhaigang mangrove, seagrass conservation, IUCN, seed bank

摘要:

贝克喜盐草(Halophila beccarii)是当前全球面临灭绝风险的10种海草之一,被国际自然保护联盟(IUCN)列为易危(VL)种。由于贝克喜盐草生境的特殊性(受涨退潮影响的潮间带)及个体十分纤小且易被沉积物覆盖,加之种群更新速率极快,导致通常难以被发现。据报道,贝克喜盐草仅在我国的广西、广东、台湾、香港、福建和海南6个省(区)有小面积的分布。该研究首次报道了位于海南东寨港的贝克喜盐草海草的分布、群落基本特征以及生境概况。结果表明:与国内其它地区的贝克喜盐草相比,调查期间东寨港的贝克喜盐草面积较大,为15.4 hm²; 密度适中,为10 394 ± 576 茎·m^-2; 覆盖度较低,为～5%; 生物量较低,总生物量仅(9.224 ± 1.151)gDW·m^-2,地上生物量为(2.339 ± 0.245)gDW·m^-2,地下为(6.886 ± 1.055)gDW·m^-2,地上与地下生物量之比值较低(0.34); 叶片较小,叶长为(4.83 ± 0.87)mm,叶宽为(1.44 ± 0.21)mm,调查时在植物体上未发现有花或果,但在沉积物中发现有一定数量的贝克喜盐草种子(2 105 ± 664)粒·m^-2。该研究区的海水受到一定程度的水体污染。建议海南东寨港国家级自然保护区在今后的管理工作中,应把海草也作为重点保护对象。

Abstract:

Recent mangrove surveys have discovered a seagrass bed of Halophila beccarii within Dongzhaigang National Natural Reserve, Hainan that have not previously been reported. The distribution, basic biological traits of the seagrass bed and associated habitat(sediment and sea water)were studied in the paper. H. beccarii is among one of the ten seagrass species which are at elevated risk of extinction in the world, and it was listed as ‘Vulnerable' by the International Union for Conservation of Nature(IUCN)under criterion B2. It is generally hard to be found in the field due to its very tiny leaves and its shoots are easily buried by sediment. In addition, this species has a very narrow, restricted depth range right at the intertidal zone(usually mud flat or seaward mangrove areas), where is submersed tidally every day. Moreover, rapid population turnover and low awareness of seagrass do not facilitate the discovery of this species in China. Until today, very limited H. beccarii resources have been reported along the coast of Fujian, Taiwan, Guangdong, Hong Kong, Guangxi and Hainan, with an estimation of total area of less than 200 hm² in China. In this study, we used a hand-hold GPS to map the extent of seagrass distribution in the study area. Besides, we sampled seagrass plants, associated sediment and seawater for analysis in the lab. Compared with other H. beccarii sites in China, the area in Dongzhaigang was relative big, with a total area of 15.4 hm² and intermediate density of(10 394 ± 576)shoots·m^-2, ranking the second largest H. beccarii bed in China according the published data. However, the biomass(9.225 ± 1.151 gDW·m^-2)and percent coverage(5.0% ± 1.1%)at this site was very low partly due to its pretty small leaf [leaf length:(4.83 ± 0.87)mm; leaf width:(1.44 ± 0.21)mm]. The ratio of above- to below-ground biomass was also very low(0.34), with(2.339 ± 0.245)gDW·m^-2 of above-ground biomass and(6.886 ± 1.055)gDW·m^-2 of below-ground biomass. No flower and fruit of the plant was found during our survey, but an intermediate density(2 105 ± 664 seeds·m^-2)of seeds in the sediment was found. The organic carbon content and bulk density of sediment in the study area was(2.08 ± 0.13)% [0-10 cm:(2.42 ± 0.03)%; 10-20 cm:(2.17 ± 0.03)%; 20-30 cm:(1.65 ± 0.20)% and(0.98 ± 0.01)g·cm^-3. 0-10 cm:(0.93 ± 0.01)g·cm^-3; 10-20 cm:(0.97 ± 0.01)g·cm^-3; 20-30 cm:(1.02 ± 0.02)g·cm^-3] respectively. In addition,(6.67 ± 0.24)‰ of salinity,(7.33 ± 0.03)of pH,(-1.53 ± 1.85)mV of oxidation reduction potential(ORP)and(6.65 ± 0.31)mg·L^-1 of dissolved oxygen(DO)of overlying water in the study area was measured in situ. On the other hand,(1.90 ± 0.03)mg·L^-1 of total nitrogen content(TN),(0.13 ± 0.01)mg·L^-1 of total phosphorus content(TP)and(5.89 ± 0.06)mg·L^-1 of organic carbon content(C_org)of sampled-seawater during high tide in the study area was detected in the lab. A certain level of pollution in sea water of the site was demonstrated, which was consistent with the previous research in the same study area. We suggest that the seagrass plants and their associated habitats should also be listed in the key protection objects by the Management Bureau of Dongzhaigang National Natural Reserve besides mangrove ecosystem.