Ecology, 95(11), 2014, pp. 3068–3079 Ó 2014 by the Ecological Society of America Constraints by oxygen and food quality on carbon pathway regulation: a co-limitation study with an aquatic key herbivore MARCUS LUKAS 1 AND ALEXANDER WACKER Department of Ecology and Ecosystem Modeling, Institute of Biochemistry and Biology, University of Potsdam, Am Neuen Palais 10, D 14469 Potsdam, Germany Abstract. In food webs, herbivores are often constrained by low food quality in terms of mineral and biochemical limitations, which in aquatic ecosystems can co-occur with limited oxygen conditions. As low food quality implies that carbon (C) is available in excess, and therefore a regulation to get rid of excess C is crucial for the performance of consumers, we examined the C pathways (ingestion, feces release, excretion, and respiration) of a planktonic key herbivore (Daphnia magna). We tested whether consumer C pathways increase due to mineral (phosphorus, P) or biochemical (cholesterol and fatty acid) limitations and how these regulations vary when in addition oxygen is low. Under such conditions, at least the capability of the upregulation of respiration may be restricted. Furthermore, we discussed the potential role of the oxygen-transporting protein hemoglobin (Hb) in the regulation of C budgets. Different food quality constraints led to certain C regulation patterns to increase the removal of excess dietary C: P-limited D. magna increased excretion and respiration, while cholesterol- limited Daphnia in addition upregulated the release of feces. In contrast, the regulative effort was low and only feces release increased when D. magna was limited by a long-chain polyunsaturated fatty acid (eicosapentaenoic acid, EPA). Co-limiting oxygen did not always impact the discharge of excess C. We found the food-quality-induced upregulation of respiration was still present at low oxygen. In contrast, higher excretion of excess C was diminished at low oxygen supply. Besides the effect that the Hb concentration increased under low oxygen, our results indicate a low food-quality-induced increase in the Hb content of the animals. Overall, C budgeting is phenotypically plastic towards different (co-)limiting scenarios. These trigger specific regulation responses that could be the result of evolutionary adaptations. Key words: carbon budget; carbon stoichiometry; cholesterol; co-limitation; Daphnia; EPA; hemoglobin; oxygen; phosphorus; polyunsaturated fatty acid; zooplankton. INTRODUCTION An optimal consumer performance requires all essential dietary components to be present in balanced ratios, i.e., in terms of the concentrations of essential elements, essential biochemical molecules, and all major classes of biochemical compounds. Accordingly, much attention has been given to aspects of food quality limitations in terrestrial and aquatic food webs (e.g., Elser et al. 2000). Such limitations may coincide with abiotic factors such as low oxygen availability, partic- ularly in aquatic systems. At the beginning of the food quality debate, studies focused on the effects of mineral nutrients (such as phosphorus, P) on the plant– consumer interface and used laboratory experiments with herbivorous zooplankton feeding on P-limited cultured algae (Urabe and Watanabe 1992, Sterner 1993, Sterner and Hessen 1994). Limited P concentra- tions cause reduced production of aquatic herbivores (Sterner and Hessen 1994, Sterner and Elser 2002) due to decreased tissue P contents at low dietary P availability (Lukas et al. 2011). Evidence of P limitation of aquatic herbivores in nature was previously indirect and based on carbon : phosphorus ratios of lake seston (Hessen 1992, Elser and Hassett 1994) and was directly shown recently in a field study (Hartwich et al. 2012). In recent years, aspects of food quality have become broadened. Researchers have paid additional attention to essential biochemical food components that cannot be synthesized by a consumer, e.g., polyunsaturated fatty acids (PUFAs), amino acids, vitamins and sterols (Mu¨ller-Navarra 1995, Anderson et al. 2004, Wacker and Martin-Creuzburg 2012). Sterols and PUFAs are important cell membrane components and essential food components for herbivorous arthropods (e.g., Behmer and Nes 2003). In aquatic ecosystems, growth limita- tions by sterols and PUFAs are assumed to occur when cyanobacteria dominate the seston (Sperfeld et al. 2012). These phytoplankton species usually lack sterols (Volk- man 2003) and PUFAs (Wacker and Martin-Creuzburg 2012). However, limitations by these biochemical essentials might not be restricted to cyanobacteria blooms, as eukaryotic algae can also be poor in long- Manuscript received 9 January 2014; revised 24 April 2014; accepted 29 April 2014. Corresponding Editor: E. Van Donk. 1 E-mail: marcus.lukas@gmail.com 3068