Constraints by oxygen and food quality on carbon pathway regulation: a co‐limitation study with an aquatic key herbivore

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.