莱州湾水母种类多样性及群集结构的季节变化

Annual survey of the species diversity and assemblage dynamics of medusae in Laizhou Bay,Bohai Sea

基于2011年5月至2012年4月(除冰期12月和翌年1—2月外)在渤海莱州湾逐月采集的数据资料,系统地开展水母种类组成、数量分布的周年季节变动,弥补自20世纪90年代以来该区域相关资料的不足,有利于掌握该水域水母(主要是小型水母)主要种类数量时空分布特点,促进莱州湾生物类群对生境改变响应的相关研究。结果显示:所有调查中,共出现水母30种,无全年出现的种类。各月中,莱州湾水母以8月种数最多,5月数量居多。各月水母总数量的80%多由1—2种优势种贡献所致;水母优势种类组成季节更替明显。3—5月优势种为八斑唇腕水母;6月为嵊山秀氏水母和贝氏真囊水母;7月为贝氏真囊水母、9月有细颈和平水母;10月为细颈和平水母和大西洋五角水母,8月和11月无明显的优势种,但8月细颈和平水母和曲膝薮枝螅水母数量较多。莱州湾小型水母聚集结构主要呈现季节性时间格局上的变动。春季有八斑唇腕水母为代表、在湾内分布的聚集组;夏季有以嵊山秀氏水母和贝氏真囊水母为代表、在湾口西侧和湾中部分布的聚集组,以及细颈和平水母为代表、在湾中底部分布的聚集组;秋季有四枝管水母和大西洋五角水母为代表、在湾口和中部分布的聚集组。水温对小型水母聚集结构分布具有较为重要的影响作用。大型水母在本调查出现较少。海蜇和沙海蜇出现于6—8月个别站位;海月水母7—10月均有出现,但高密集区出现在10月紧邻莱州湾的湾底水域。

Laizhou Bay, an estuary of the Yellow River, lies to the north of the Shandong Peninsula and is one of the three largest and most important bays of the Bohai Sea. It is characterized by a high level of primary productivity and is an important spawning, nursery, and feeding ground for many fish species in the Yellow Sea and Bohai Sea. Medusae play crucial roles, not only ecologically in the marine ecosystem but also economically, as commercial target species for some large scyphomedusae in Laizhou Bay. In the past few decades, the interacting impacts of climate change and human activities have changed the ecosystem of Laizhou Bay significantly, leading to regime shifts in biological organisms, including medusae. Recently, increasing concern has prompted studies on the ecological impacts of changes in the gelatinous species community of the estuary and coastal area. To explore the temporal and spatial distribution of the composition and abundance of medusa species in the bay, annual surveys were designed and carried out from May 2011 to April 2012 （excluding December 2011 and January and February 2012, because of the presence of sea ice）. Based on the data collected from 147 samples collected over the course of 9 cruises, nearly thirty cnidarians including twenty-four hydromedusae, one siphonophore, and three large scyphomedusan jellyfish were identified. The species of hydromedusae and siphonophore were small, and were collected by being vertically hauled from the seafloor to the surface using a mesoplankton net （mesh size 160 μm）. Of the smaller medusae, no common species were collected throughout the year. Species richness peaked in August, while the total abundance peaked in May. One or two dominant species contributed over 80% of the total monthly abundance. The dominant species varied among months, with Rathkea octopunctata dominating from March to May, Sugiura chengshanense and Euphysora bigelowi dominating in June, E. bigelowi dominating in July, Eirene menoni dominating in September, and E. menoni and Muggiaea atlantica dominating in October. In August, E. menoni and Obelia geniculata were abundant, but were distributed regionally in the central parts of the bay. Very few medusa species were collected in November. Multiple statistical analyses, including cluster analysis and BVSTEP modules conducted with Primer 6.0 software, were used to identify the best matches for environmental factors that affect the composition of the medusa assemblages. Based on the abundance of small medusae, cluster analysis recognized four seasonal and spatial assemblage groups. The first group, with R. octopunctata as its representative species, was collected at the runoff inlet of the bay in spring. The second group was collected in the western section of the river mouth or the central parts of the bay in summer. The second group＇s representative species were S. chengshanense and E. bigelowi. The third group also occurred in summer, but its location was very similar to the spring group, and it included E. menoni as its representative species. The fourth group appeared in autumn and was collected in the river mouth or the central part of the bay, and included two indicator species：M. atlantica and Proboscidactyla flavicirrata. According to the results of the BVSTEP modules analysis, the water temperature had a more significant effect on the seasonal and spatial distribution of the hydromedusae than salinity, pH, or dissolved oxygen. The three large scyphomedusan jellyfish were only caught by fishing methods that utilized a net with a 20 mm mesh size. They occurred only at a few inlet sites after June. Aurelia spp. peaked later in October. Rhopilema esculentum and Nemopilema nomurai were collected from June to August at only a few sites.