Oxygen constraints at the primary production - consumer interface: the aquatic keystone species Daphnia and its stoichiometric regulation

据预测，人类增加的营养物质和升高的水温(全球变化)将促进缺氧和蓝藻水华的发生，对水生生态系统和水质造成众所周知的有害影响。蓝藻缺乏必需的脂肪酸和甾醇，对捕食藻类的浮游动物，如优势食草动物和关键物种大型水蚤来说，食物质量很差。当同时发生缺氧和低质量水华的负面影响时，因为通常控制水华的达夫类动物面临着两难境地，即在低氧浓度下，它们无法通过更高的呼吸速率来弥补糟糕的食物质量。然而，它们可能会增加对限制性饮食化合物的吸收，或减少对其他饮食成分的过量吸收，或增加其排泄。我们对低氧对这种化学计量比调节的潜在限制知之甚少，无法判断其在田间条件下的相关性。因此，在不同的氧气条件下，应通过生活史实验和~(14)C放射性标记饲料追踪碳的流动来研究不同食物质量和数量下的同化、呼吸和排泄的化学计量规律。这将提高我们对水蚤对环境氧气、温度、食物浓度和质量变化的反应的理解，并使预测初级生产和消费者之间由水蚤介导的界面的未来反应成为可能。

Anthropogenically increased nutrients and enhanced water temperatures (global change) are predicted to promote the occurrence of oxygen deficiencies and cyanobacterial blooms, both with the well-known detrimental effects on aquatic ecosystems and water quality. Cyanobacteria lack essential fatty acids and sterols, and are of poor food quality for algae-grazing zooplankton such as the dominant grazer and keystone species Daphnia. When occurring simultaneously, the negative effects of oxygen deficiency and low-quality blooms enhance each other because daphnids that usually control the blooms face the dilemma that under low oxygen concentrations they cannot compensate for poor food quality by higher respiration rates. However, they might enhance the assimilation of limiting dietary compounds or lessen the assimilation of other dietary ingredients in excess or increasing their excretion. We know too little about potential constraints by low oxygen of such stoichiometric regulation to judge its relevance under field conditions. Therefore, under different oxygen conditions the stoichiometric regulation of assimilation, respiration and excretion at different food qualities and quantities shall be investigated by carrying out life history experiments and using 14C radio-labelled diets to track the flow of carbon. This will improve our understanding of Daphnia’s response to changing environmental oxygen, temperature, food concentration and quality, and makes it possible to predict future reactions of the Daphnia-mediated interface between primary production and consumers.