The combined effects of hypoxia and fish kairomones on several physiological and life history traits of Daphnia

Abstract

Abstract Numerous studies have tested the combined effect of the threat of predation by fish and low oxygen concentrations on the phenotypic plasticity of Daphnia. These studies assessed the trade‐off between minimising predation risk and the negative effects of oxygen deficiencies in the context of depth selection behaviour. We tested whether this trade‐off also affects physiological and life history traits. We expected an interactive effect between the threat of fish predation and low oxygen concentrations, such, that the net effect of both stressors would be antagonistic (lower than the sum of each of the stressors acting separately), rather than additive (or synergistic) on the majority of traits investigated, but we predicted synergistic effects on heat shock proteins (HSPs). To test this, we performed life table experiments in different oxygen concentrations (normoxia and hypoxia) and levels of predation threat (the presence and absence of fish kairomones) on HSP70 and putative HSP110, haemoglobin concentration and life history traits with small‐bodied Daphnia galeata and large‐bodied Daphnia pulex originating from waterbodies where there were different risks of fish predation. As predicted, the net effect of both stressors was antagonistic for most of the physiological and ecological variables studied. The presence of kairomones resulted in decreased body size of adults, egg size, egg size in relation to brood chamber volume, and in increased clutch size in relation to body size. These effects were weaker in hypoxia than in normoxia, which may suggest an existence of adaptive responses caused by a lower perceived risk in hypoxia than in normoxia, as the foraging abilities of fish are limited by oxygen deficiencies. The presence of kairomones hampered the production of haemoglobin in hypoxia for the clones of larger‐bodied species, which suggests the existence of a trade‐off between reduced visibility under positive‐size selective predation risk and increased efficiency of oxygen transport to body tissues. The presence of kairomones and hypoxia resulted in an increased level of putative HSP110, and the effect of kairomones was stronger in hypoxia than in normoxia. More complex results were obtained for the effect of both stressors on the level of HSP70. The findings of our study provide a new insight on the interactions between planktivorous fish and zooplankton in aquatic food webs. More specifically, these findings suggest the existence of unexplored size‐dependent eco‐physiological trade‐offs between minimising predation risk and mitigating the negative effects of oxygen deficiencies.