长江口外海域微型和小型底栖生物群落结构和时空变化

Community structure and spatio-temporal variation of microbenthos and meiobenthos in sediments off the Yangtze Estuary

利用DAPI荧光计数法和Ludox-QPS方法并结合环境因子的分析,研究了2010年和2011年夏季(7月)及秋季(11月)长江口至济州岛沿线的CJ断面沉积物中的微型及小型底栖生物的组成、丰度和生物量及分布特点。结果表明,表层5 cm沉积物中,细菌(108个/cm3)、蓝细菌(106个/cm3)、自养(106个/cm3)和异养微型鞭毛虫(105—106个/cm3)的丰度较其他类群高3个数量级以上;细菌(19—24μg C/cm3)、自养(13—31μg C/cm3)和异养(5—44μg C/cm3)微型鞭毛虫的生物量明显较蓝细菌(1—2μg C/cm3)、小型底栖生物(0.9—1μg C/cm3)、纤毛虫(0.04—0.2μg C/cm3)、异养小鞭毛虫(0.02—0.08μg C/cm3)及硅藻(0.001—0.008μg C/cm3)高。除细菌和蓝细菌外,2011年微型底栖生物和小型底栖生物各类群的丰度和生物量大多高于2010年,且大多为秋季高于夏季。发现的166种纤毛虫中,肉食性纤毛虫所占现存量的比例最大,菌食性次之,藻食性和杂食性最小;肉食性纤毛虫在秋季所占比例高于夏季,而菌食性则相反。统计分析表明,微型底栖生物丰度和生物量年度间具有极显著差异,基于物种-丰度的纤毛虫群落结构年度间呈极显著差异,而小型底栖生物则未见显著差异。微型和小型底栖生物各类群夏季和秋季的分布受多因子而非单一环境因子的影响,长江冲淡水对该断面的微型和小型底栖生物的影响不显著。对微型底栖生物各类群及水体中相应类群的比较分析表明,该海域的微型底栖生物具有明显的数量和功能重要性。

Using epifluorescence microscopy and the Ludox-QPS method, we investigated the community composition, abundance, and biomass as well as distribution of microbenthos and meiobenthos in combination with the analysis of benthic environmental variables at four stations off the Yangtze Estuary in the summer （July） and fall （November） of 2010 and 2011, respectively. The abundances of bacteria （10^8 cells/cm^3）, cyanobacteria （10^6 cells/cm^3）, and phototrophic （PNF, 10^6 cells/cm^3） and heterotrophic nanoflagellates （HNF, 10^5-10^6 cells/cm^3） were much higher than those of other components in the upper 5 cm of sediments. The biomass partitioning indicates the primary importance of benthic bacteria （19-24 μg C/cm^3）, followed by PNF （13-31 μg C/cm^3）, HNF （5-44 μg C/cm^3）, cyanobacteria （1-2 μg C/cm^3）, meiobenthos （0.9-1 μg C/cm^3）, ciliates （0.04-0.2 μg C/cm^3）, heterotrophic microflagellates （HMF, 0.02-0.08 μg C/cm^3）, and diatoms （0.001-0.008 μg C/cm^3）. The standing crops of microbenthic and meiobenthic assemblages, except of bacteria and cyanobacteria, were generally higher in 2011 than in 2010 and usually higher in the fall than in the summer. About 84% of diatoms, 82% of ciliates, 65% of HMF, and 77% of meiobenthos were distributed in the upper 2 cm of the sediments, whereas no distinct vertical distribution was observed for bacteria, cyanobacteria, PNF, and HNF. The vertical distribution had little variation between the years and seasons. The estimated productivity of diatoms ranged from 0.05 to 0.4 mg C m^-2 d^-1 and contributed only 0.01%-0.1% of the primary productivity. Thus, most of the primary productivity was contributed by the pico- and nano-sized benthos. The estimated productivity of bacteria was 80-101 mg C m^-2 d^-1, about 17%-21% of the primary productivity. The biomass of bacteria was nine to 18 times higher than that of the meiobenthos, whereas the productivity was 30-61 times higher than that of the meiobenthos （2-3 mg C m^-2 d^-1）. The biomass of ciliates was much lower than that of the meiobenthos, while the ciliate productivity （3-23 mg C m^-2 d^-1） was one to 14 times higher than that of the meiobenthos. The relative metabolic rates indicate that benthic heterotrophic nanoflagellates constitute the most important group in sediment respiration, accounting for 92.9%-99.1% of the estimated combined metabolic rate of the microbenthic and meiobenthic consumers. Among the 166 identified ciliate species, carnivorous ciliates constituted the primary feeding type in standing crops, followed by bacterivores, algivores, and omnivores. Carnivorous ciliates contributed a relatively higher proportion of standing crops in the fall than in the summer, whereas the contribution of bacterivorous ciliates was inverse. The abundance and biomass of the microbenthos as well as the ciliate community structure based on species-abundance varied significantly between the years, while no significant difference was observed for meiobenthos. Statistical analyses show that the distribution of microbenthos and meiobenthos in the summer and fall was affected by multiple factors rather than a single environmental factor, and diluted water of the Yangtze River did not show an obvious impact on the microbenthos and meiobenthos. BIOENV analysis indicates that in the summer, the microbenthic abundance had the highest correlation with the chlorophyll a concentration and ciliate species-abundance had the highest correlation with the combination of the bottom water temperature, chlorophyll a concentration, and organic matter content, while no correlation was found for meiobenthos. In the fall, the meiobenthic abundance had the highest correlation with the median grain size of the sediments, while no correlation was found for the microbenthos and ciliates. Comparative analyses between the microbenthic assemblages and corresponding ones in the upper pelagic waters suggest that the microbenthos had numerical and functional importance in the study area.