Increased fitness of a key appendicularian zooplankton species under warmer, acidified seawater conditions

Ocean warming and acidification (OA) may alter the fitness of species in marine pelagic ecosystems through community effects or direct physiological impacts. We used the zooplanktonic appendicularian, Oikopleura dioica, to assess temperature and pH effects at mesocosm and microcosm scales. In mesocosms, both OA and warming positively impacted O. dioica abundance over successive generations. In microcosms, the positive impact of OA, was observed to result from increased fecundity. In contrast, increased pH, observed for example during phytoplankton blooms, reduced fecundity. Oocyte fertility and juvenile development were equivalent under all pH conditions, indicating that the positive effect of lower pH on O. dioica abundance was principally due to increased egg number. This effect was influenced by food quantity and quality, supporting possible improved digestion and assimilation at lowered pH. Higher temperature resulted in more rapid growth, faster maturation and earlier reproduction. Thus, increased temperature and reduced pH had significant positive impacts on O. dioica fitness through increased fecundity and shortened generation time, suggesting that predicted future ocean conditions may favour this zooplankton species.海洋变暖与酸化（Ocean Acidification, OA）可通过群落效应或直接生理影响，改变海洋浮游生态系统中物种的适合度。本研究以浮游尾海鞘类物种异体住囊虫（Oikopleura dioica）为实验材料，在中型实验生态系（mesocosm）与微型实验生态系（microcosm）尺度下评估温度与pH的调控效应。在中型实验生态系中，酸化与变暖均对连续多代的异体住囊虫种群丰度产生正向影响。在微型实验生态系中，观测到酸化的正向影响源自繁殖力提升；与之相反，例如在浮游植物水华期间出现的pH升高现象，会降低繁殖力。所有pH条件下的卵母细胞活力与幼体发育水平均无显著差异，这表明低pH对异体住囊虫种群丰度的正向影响主要源于产卵量的提升。该效应受食物数量与质量的调控，佐证了低pH环境下异体住囊虫的消化与吸收功能可能得到增强。更高的温度可促进生长速率、加快成熟速度并使繁殖提前。综上，升温与低pH通过提升繁殖力、缩短世代时间，对异体住囊虫的适合度产生了显著正向影响，这预示着未来预估的海洋环境或将有利于这类浮游动物的生存。