1. Global change assessments have typically ignored synthetic chemical pollution, despite the rapid increase of pharmaceuticals, pesticides, and industrial chemicals in the environment. The paucity of research on the ecological effects of these micropollutants undermines our efforts to address the freshwater biodiversity crisis. Understanding the responses of individual organisms to chemical pollution should help address this knowledge gap because individual‐level effects can cascade across populations, communities, and ecosystems with devastating consequences.
2. Inputs of treated municipal wastewater are a major source of micropollutants in receiving environments. Here, we assessed population and individual‐level influences of treated wastewater on freshwater gammarid amphipods (Gammarus spp.) in Swiss lowland streams in situ and tested effects of a micropollutant mixture on individual Gammarus fossarum using a common garden laboratory experiment. We hypothesised that population‐level effects of wastewater are mediated through asymmetric sensitivities of juvenile and adult gammarids to chemical pollutants. We expected that life‐stage specific sensitivities would reflect allometric theory relating body size to a wide range of organismal characteristics, including metabolism, growth, and mortality.
3. At the population level (i.e. field survey), we observed greatly reduced abundances of juvenile gammarids downstream of the wastewater discharge in three of the six sites surveyed, indicating the potential for demographic effects of pollution. At the individual level in a field transplant experiment, we found that the presence of wastewater led to a steeper positive relationship between gammarid body size and leaf consumption. In the laboratory experiment, we found that micropollutants had negative effects on consumption and growth rates, but a positive effect on survival. Differences in the relationship between gammarid body size and performance were subtle across treatments, although flatter slopes in growth appear to be a consistent response to chemical pollution. Faster growth rates appeared to be connected with reduced survival, placing individuals in control treatments and/or with smaller body size at greatest risk of mortality. Notably, juvenile gammarids had faster growth rates and lower survival than adults when exposed to micropollutants.
4. Our results demonstrate the potential for negative impacts of micropollutants in freshwater ecosystems. However, the results also show that organismal responses to chemical pollutants can be complex, whereby impacts at the individual level may act counter‐intuitively to population‐level dynamics. This highlights the need for more realistic experiments to better assess how organismal responses depend on life stage and body size, and how individual‐level effects propagate to higher levels of biological organisation. Our study shows how allometric theory can be used to examine the effects of stressors on underlying organismal biology, population demographics, and link with broader macroecological patterns.

中文翻译：

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溪流两栖动物的大小依赖性敏感性表明人口对化学污染的反应

1. 尽管环境中的药品，农药和工业化学品迅速增加，但全球变化评估通常忽略了合成化学物质的污染。对这些微量污染物的生态影响的研究很少，破坏了我们应对淡水生物多样性危机的努力。理解个体生物对化学污染的反应应该有助于解决这一知识鸿沟，因为个体层面的影响可能会在人口，社区和生态系统之间形成级联，带来毁灭性后果。
2. 经过处理的市政废水的输入是接收环境中微量污染物的主要来源。在这里，我们评估了处理过的废水对瑞士低地溪流中淡水伽马立足类双足纲动物（Gammarus spp。）的种群和个体水平的影响，并使用普通的花园实验室实验测试了微污染物混合物对单个Gammarus 镰刀菌的影响。我们假设废水的人口水平影响是通过对化学污染物的不对称幼稚和成年γ-线虫介导的。我们预期生命阶段的特定敏感性将反映出将身体大小与各种机体特征（包括代谢，生长和死亡率）联系起来的异速测量理论。
3. 在人口水平（即实地调查），我们观察到在六个被调查地点中的三个地点，废水排放下游的幼小γ闪藻的数量大大减少，这表明了潜在的人口统计学影响。在田间移植实验的个体水平上，我们发现废水的存在导致了γ体大小与叶片消耗之间更陡的正相关。在实验室实验中，我们发现微污染物对消耗和生长速率有负面影响，但对生存有正面影响。尽管治疗中的平坦斜率似乎是对化学污染的一致反应，但是在各个处理中，γ射线体大小与性能之间关系的差异微妙。更快的生长速度似乎与存活率降低有关，将个体置于对照治疗中和/或体型较小，则有最大的死亡风险。值得注意的是，当暴露于微污染物时，幼稚的γ-内酰胺类具有比成人更快的生长速率和更低的存活率。
4. 我们的结果证明了微污染物对淡水生态系统的潜在负面影响。但是，结果还表明，生物体对化学污染物的反应可能很复杂，因此在个人层面上的影响可能与人口层面的动态有悖常理。这凸显了需要进行更现实的实验，以更好地评估生物反应如何取决于生命阶段和体型以及个体水平效应如何传播到更高水平的生物组织。我们的研究表明，变长理论可以用来检查压力源对潜在生物生物学，人口统计学的影响，以及与更广泛的宏观生态学模式的联系。