|Place of Conferral||中国科学院海洋研究所|
|Keyword||异体住囊虫 小型化 提前成熟 种群大小 环境变化|
在雨季胶州湾连续调查中，异体住囊虫种群丰度与微型浮游植物（<20 µm）生物量显著正相关，高丰度集中在靠近河口的富营养化海区，最高丰度为28107 ind. m-3。本研究结果表明，由降雨和陆源输入引起的海水富营养化，致使微型浮游植物生物量迅速增加，通过食物链的传递进一步导致异体住囊虫种群丰度在短时间内实现指数式增长，种群丰度随微型浮游植物的暴发达到峰值，并在小型浮游植物盛行时降到低值，是用于近海环境指示的良好生物指标。受摄食偏好影响，异体住囊虫对微型浮游植物造成巨大的摄食压力，有利于小型浮游植物恢复成为优势种，进而促进浮游植物群落大型化。此外，在雨季胶州湾富营养化的环境中，异体住囊虫成熟个体的体长范围在115到619 µm之间，远小于同一温度条件下实验室种群的成熟个体大小。与实验室种群相比，在同一体长下，胶州湾异体住囊虫的性腺更大；其中，成熟个体性腺长的差值较小，在 -1到6 µm之间，未成熟个体性腺长的差值较大，在25到31 µm之间。本研究结果表明，富营养化海域异体住囊虫个体小型化是以降低繁殖力为代价，通过缩短世代周期快速发育成熟实现的，这种提前成熟的生殖适应性有利于提高种群内禀增长率，促进种群快速补充，以便及时响应快速变化的环境资源。本研究解释了自然环境中异体住囊虫个体小型化的原因，并揭示了异体住囊虫对浮游植物群落大小和结构的影响。
在室内培养实验中，异体住囊虫的生长和繁殖不仅受饵料浓度影响，还受种群密度制约，与PFS（个体可获食物量）显著相关。每个异体住囊虫一个世代内可获食物量达到8 µg C 的阈值时，才能正常生长和繁殖；个体一个世代内可获食物量低于4.5 µg C 的阈值时，不能按时成熟；而个体一个世代内可获食物量在4.5到8 µg C 之间时，成熟个体小且少。本研究结果表明，异体住囊虫的r值受种内竞争调节，通过改变成熟时间、成熟个体的大小以及成熟个体的比例来实现。在PFS饱和的条件下，种内不存在竞争，异体住囊虫通过增加成熟个体的大小和数量提高r值，以便及时利用充足的饵料资源；相反，在PFS受限制的条件下，种内竞争激烈，异体住囊虫通过降低繁殖力、减少成熟个体的数量以及延长世代周期降低r值，以便避免种群过度繁殖后崩溃。本研究阐明了种内竞争调节种群规模的机制，解释了野外异体住囊虫种群是如何在食物短缺时维持生存，如何对浮游植物水华做出快速响应以及如何在水华结束时避免过度繁殖。
The appendicularian Oikopleura dioica is a filter-feeding gelatinous zooplankton with global distribution. This species holds a series of distinctive characters, such as simplified body plan, short generation time, high reproductive rates, small genome and efficient feeding ability, which make it attractive to both biologists and ecologists. Influenced by climate change and human activities, coastal eutrophication is becoming more and more serious, the population size of O. dioica shows a trend of explosive growth. The questions, why does individual miniaturization appear in eutrophic coastal waters, and whether the population increase can change from an exponential to a logistic model to avoid a final sudden collapse are still unanswered. Thus, it is necessary to evaluate the rapid response of O. dioica to the fluctuating environmental conditions. For one hand, it is of great practical and theoretical significance to understand the population dynamics and recruitment mechanism in eutrophic coastal waters, and for another hand, it can provide data support for exploring the impact of O. dioica on the eutrophic marine ecosystem.
First, we studied the adaptability of O. dioica to nutrient fluctuations by a continuous survey (twice a week) in the Jiaozhou Bay from August 5 to September 27, 2011. By analyzing the spatio-temporal variation of nutrients, chlorophyll a concentrations, and O. dioica abundance, the response time and pattern of population size to phytoplankton were revealed. Furthermore, by comparing the body sizes of individuals under natural and laboratory conditions, the reproductive mechanism and significance of early maturation under natural conditions were discussed. Second, we studied the regulation mechanism of intraspecific competition on population size via laboratory incubations in July 2017. To replace competitive pressure with the per capita food supply (PFS), by analyzing the PFS effects on somatic growth, reproduction performance, and the intrinsic rate of natural increase (r), the mechanism of population size regulation was revealed, and the population dynamics in natural conditions were discussed. Finally, we studied the effect of O. dioica on nutrient consumption via field experiment in the Jiaozhou Bay from October 22 to October 29, 2015. By comparing the pathways of nutrient consumption and the scale of phytoplankton blooms between the field experiment and the field investigations, it will provide a basis for further research on the control of eutrophication and phytoplankton blooms by O. dioica in the future.
In the investigations of the Jiaozhou Bay during flood seasons, the abundance of O. dioica was significantly positively correlated with the biomass of nano- and pico-phytoplankton (<20 µm). They swarmed in the north part of the bay near the estuary, with the highest station-specific abundance of 28107 ind. m-3. Our results showed that the eutrophication of seawater caused by rainfall and land-based input had led to a rapid increase in the biomass of nano- and pico-phytoplankton, which further led to an exponential growth of O. dioica population in a short time through the transmission of the food chain. The population size peaked with the blooms of nano-phytoplankton, and fell to a low value when micro-phytoplankton became prevalent, this can be used to indicate environmental changes in ecological study. Affected by feeding preference, O. dioica caused huge feeding pressure on nano-phytoplankton, which was conducive to the restoration of micro-phytoplankton as dominant species, and thus promoted the large-scale phytoplankton community. In addition, in the eutrophic environment of the Jiaozhou Bay during the flood seasons, mature individuals of O. dioica attained body lengths ranging from 115 to 619 mm, which were much shorter than the laboratory population at the same temperature. Compared with the body/gonad length relationship established in laboratory studies, the natural population in the Jiaozhou Bay had greater gonad length at the same body length. The difference of gonad length between natural and laboratory specimens was -1-6 mm for matures and 25-31 mm for immatures at various food regimes. Our results showed that the miniaturization of O. dioica in eutrophic coastal waters was achieved by shortening generation time and reducing fecundity. Considering that maturation at smaller body size and shorter generation time arising therefrom can result in higher intrinsic rates of natural population increase, it suggests that early maturation favors a rapid response of O. dioica to fleeting phytoplankton blooms. Our results explained why smaller body size is usually achieved in natural environments comparing to those in laboratory, and revealed the response and impacts on size structure of phytoplankton communities.
For O. dioica cohorts that were cultivated in various food and density regimes, the growth and reproduction performance were affected by the food concentrations and population densities, which were significantly related to PFS. Somatic growth, represented by body length, was saturated above the PFS of 8 µg C ind-1, and below this threshold, individuals reached small body and gonad lengths, and maturation was rarely observed during the incubation period. Mature individuals were present at all food concentrations, but only above another PFS threshold, 4.5 μg C ind-1. Our results showed the r values were regulated by competition pressure via variabilities in maturation duration and the proportion of mature individuals in the cohorts. When the minimum PFS was satisfied in the designated generation time, no competition within the species, the r value tended to be regulated by the spawning proportion in the population. Otherwise, prolonged development duration and decreased r values were expected. Our results explained how natural populations maintain themselves when food is scarce, respond rapidly to phytoplankton blooms, and avoid overproliferation at the end of bloom periods. More importantly, for the first time, a mechanism was proposed in this study for efficient responses to fluctuations in food availability.
In the field experiment of nutrient enrichment, no significant zooplankton grazing on the phytoplankton was observed, and the rapid consumption of nutrients was caused by the phytoplankton's explosive proliferation. However, in the eutrophic environment of the Jiaozhou Bay during flood seasons, a significant O. dioica grazing on the phytoplankton was observed, and the rapid consumption of nutrients was caused by the trophic cascade interactions, thus the occurrence of phytoplankton blooms were avoided. Our results showed O. dioica population can make a greater contribution to the consumption of eutrophication and fleeting phytoplankton blooms. This provides a good research direction for the in-depth understanding of the ecological significance of O. dioica.
|MOST Discipline Catalogue||理学::海洋科学|
|Funding Project||National Basic Research Program of China[2015CB452903] ; Basic Work of Science and Technology Project of China[2013FY111100-03] ; Basic Work of Science and Technology Project of China[2013FY111100-03] ; National Basic Research Program of China[2015CB452903]|
|Table of Contents|
|李帅. 胶州湾异体住囊虫对环境快速变化响应的研究[D]. 中国科学院海洋研究所. 中国科学院大学,2020.|
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