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

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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.