Lake water chemistry and population of origin interact to shape fecundity and growth in Daphnia ambigua

Freshwater environments vary widely in ion availability, owing to both natural and anthropogenic drivers. Field and laboratory work point to the importance of overall salinity as well as cation depletion, in shaping the physiology, behavior, and ecology of freshwater taxa. Yet, we currently have a poor understanding of the degree to which populations may vary in response to ion availability. Using Daphnia collected from three lakes that differ greatly in salinity and calcium availability, we conducted a laboratory reciprocal transplant experiment to assess how animals representing these populations vary in fecundity, body size, and survival when reared in lake water from each environment. The lake water environment and population of origin strongly interacted to shape Daphnia growth and reproduction. Surprisingly, we found only modest evidence that lake water with abundant calcium (5.5 mg/L vs. 1.2-2.3 mg/L) increased Daphnia growth or reproduction. In contrast, water from a relatively ion-rich lake (400 µS/cm specific conductance) strongly boosted Daphnia fecundity over lower-ion lake water (20-50 µS/cm), especially for the population originating from the high-ion environment. Our results suggest that ion-poor conditions common in regions around the world may exert stress on freshwater organisms, even for populations inhabiting these environments. Meanwhile, moderate salt enrichment may not prove harmful but could even benefit freshwater taxa in these ion-poor regions. The context dependence of how and when lake water chemistry affects Daphnia and other freshwater taxa deserves greater attention, in both ion-depleted and ion-rich conditions. Daphnia are key members of lake food webs and serve as an important model for ecology, evolution, and toxicology research. Consideration of how lake water chemistry may influence how Daphnia populations respond to abiotic and biotic stress may improve the ability to evaluate and predict ecological and evolutionary dynamics in lakes of varying chemical composition.

淡水环境的离子可利用性存在显著差异，这一现象由自然与人为双重驱动因素共同促成。野外与实验室研究均证实，总盐度与阳离子耗竭在塑造淡水类群（taxa）的生理、行为与生态特征中发挥关键作用。然而，当前我们对不同种群响应离子可利用性变化的程度仍知之甚少。本研究采集自三个盐度与钙可利用性差异显著的湖泊中的水蚤（Daphnia），开展了实验室互惠移植实验，以评估来自这些种群的个体在各湖泊来源的湖水环境中饲养时，其繁殖力、体型与存活率的差异。湖水环境与种群来源二者存在显著交互作用，共同塑造水蚤的生长与繁殖状况。令人意外的是，本研究仅获得有限证据表明，高钙湖水（钙浓度5.5 mg/L，对比1.2~2.3 mg/L）可提升水蚤的生长与繁殖能力。与之形成对比的是，相对富离子湖泊的湖水（比电导率400 µS/cm）相较于低离子湖水（20~50 µS/cm）可显著提升水蚤的繁殖力，这一效应在源自高离子环境的种群中尤为突出。研究结果表明，全球多地普遍存在的低离子环境可能对淡水生物造成胁迫，即便对于栖息于此类环境中的种群亦是如此。与此同时，适度的盐富集对于这些低离子区域的淡水类群而言，或许非但无害，反而可能带来益处。湖水化学特征影响水蚤与其他淡水类群的方式与时机存在情境依赖性，这一现象在离子耗竭与富离子环境中均需得到更多关注。水蚤是湖泊食物网的关键类群，同时也是生态学、进化生物学与毒理学研究的重要模式生物。探究湖水化学特征如何影响水蚤种群对非生物与生物胁迫的响应，有助于提升我们评估与预测不同化学组成湖泊中的生态与进化动态的能力。