长江口海域桡足类对低氧的响应及其生存策略

In recent years, summertime bottom hypoxia off the Changjiang estuary has increased in frequency and extent, threatening the safety of marine ecosystem. As one of the key groups off Changjiang Estuary, population fluctuations of copepods caused by hypoxia would affect marine food web and biological pump through top-down and bottom-up control, which is one of the key links to understand ecological effects of hypoxia. Our preliminary work discovered that some copepods could reside higher in the water column, while another species could assemble inside the hypoxic water. Thus, we speculated that copepods had two different survival strategies: hypoxia-sensitive species could confine diel vertical migrations and inhabit higher in the water column to avoid hypoxia, while hypoxia-resistant ones would adapt hypoxia by changing metabolic type or elevating antioxidant level. To confirm this hypothesis, in this project, we plan to reveal copepods’ response to hypoixa in carcass proportion, species conposition, abundance and vertical distribution through neutral red staining and species identification; to detect mortality rate, metabolic rate and type, antioxidant level of the dominant copepods to uncover their hypoxia tolerance and adaption in lab; to discuss the survival strategies of both hypoxia sensitive and resistant species with the results above. This project would not only enrich the study of biological tolerance and adaption to hypoxia, but also provide evidence to assess the ecological effects caused by hypoxia.

近年来，长江口夏季底层低氧现象日趋严重，甚至出现无氧区，严重威胁海洋生态安全。桡足类是该海域的关键类群，由低氧引起的种群变动将通过上下行作用影响海洋食物网和生物泵的运转，是了解低氧生态效应的关键环节之一。前期工作发现，桡足类在低氧期偏向分布于表层，而部分种类却聚集在低氧层。据此推测：桡足类有两种生存策略，即低氧敏感种通过减小垂直移动范围、栖息于表层水体来规避低氧胁迫，而耐低氧种通过转变代谢类型或提高抗氧化水平等生理调控来适应低氧环境。为此，本项目拟通过中性红染色法和镜检法，从死亡比例、种类组成、丰度和垂直分布等方面阐明桡足类对低氧的响应；模拟低氧环境，从死亡率、代谢率、代谢类型和抗氧化水平等生理状态指标，揭示桡足类对低氧的耐受性和适应性；在此基础上，探讨低氧敏感种和耐低氧种的生存策略。本项目不仅可丰富生物的低氧耐受性和适应性研究，也为评估近海低氧生态效应提供直接观测依据。

近年来，夏季长江口海域底层低氧现象日趋严重，严重威胁海洋生态安全。桡足类是该海域的关键类群，由低氧引起的种群变动将通过上下行作用影响海洋食物网和生物泵的运转，是了解低氧生态效应的关键环节之一。本项目获取了长江口低氧区内外浮游动物和桡足类死亡比例的样品，通过中性红染色法甄别桡足类死体，从死亡比例、种类组成、丰度和空间分布等方面阐明桡足类对低氧的响应。在水平空间分布上，桡足类的死体在长江口海域普遍存在，优势种的平均死体比例为4.8—69.5%。桡足类雄体对环境变化更为敏感，其死体比例高于雌体。桡足类受温度、盐度、溶解氧、叶绿素浓度等环境因子影响，其死体比例的分布情况与其生态类型相关。在垂直空间分布上，低氧主要出现在温跃层以下水层，是影响浮游动物群落结构的主要环境因子。与其他水层相比，缺氧层（DO<1mg/L）中浮游动物丰度和生物量较低，其中桡足类等甲壳动物占比降低而胶质动物占比提高，群落结构明显不同。在非低氧站位，桡足类在表层、中层和底层均匀分布。而在低氧站位，桡足类主要分布在表、中层的非低氧区域，活体丰度占比高达83.53%—98.48%。桡足类总死亡比例为3.87%—61.64%，不同物种死亡比例不同。研究筛选出耐低氧种1种（背针胸刺水蚤）和低氧敏感种6种（微刺哲水蚤、伯氏平头水蚤、精致真次水蚤、异尾宽水蚤、狭额次真哲水蚤和近缘大眼水蚤）。耐低氧种（如背针胸刺水蚤）在低氧层的死亡比例低于非低氧层，在低氧站位主要分布在低氧层，以规避鱼类等的捕食。低氧敏感种主要分布在非低氧水层，当低氧发生时可通过改变垂直移动习性，白日栖息在上层水域以规避底层低氧的胁迫。低氧敏感种和耐低氧种对低氧的不同响应方式和生存策略均是种群保存与延续的重要途径，有利于季节性低氧现象衰退后的种群恢复。本项目的实施不仅揭示长江口桡足类对低氧的响应及其生存策略，也丰富了生物对低氧的耐受性和适应性研究，为评估低氧对海洋生态系统的影响效应提供直接观测依据。