Stoichiometric mismatch between phytoplankton and zooplankton consumers: Effects at contemporary, transgenerational, and evolutionary timescales

Abstract

In the field of ecological stoichiometry, there is a long tradition of studies exploring how P limitation of producers may affect the performance of consumers. However, many knowledge gaps still exist. In chapter 2, we experimentally disentangled the direct and indirect effects of P limited food, and found that both had a strong negative influence on the performance of the herbivorous rotifer Brachionus calyciflorus. Interestingly, the relative strength of these two types of effects varied among different life history traits. In chapter 3, We experimentally confirmed the existence of a ‘stoichiometric knife-edge’ by subjecting rotifer consumers to a broad C:P gradient, and showing that the highest growth was observed at an intermediate food C:P level of approximately 170. Above the optimal food C:P, the rotifers had to cope with P shortage and excess C simultaneously. They were able to remain homeostatic until a food C:P value up to 391, beyond which homeostasis broke down completely. Below the optimal food C:P, consumers had to deal with excess P. They remained homeostatic in the face of a three-fold C:P reduction by decreasing their food ingestion rates and increasing their P loss rates. In summary, these results indicate that rotifers deal with different challenges along opposite sides of the food stoichiometry gradient. In Chapter 4, we performed a life history experiment studying maternal effects related to the direct and indirect consequences of P limitation (see also Chapter 2). However, we found no fitness advantages of offspring hatched from larger eggs produced in inferior conditions. Furthermore, both direct and indirect maternal effects were important in determining consumer performance, and the relative strength varied with different life history traits. Surprisingly, maternal diets were found to be more important in determining the somatic growth of consumers than the contemporary diet, indicating that maternal effects have the potential to decouple the relationship between food stoichiometry and observed consumer performance. This highlights the need of taking maternal effects into account in ES. In chapter 5, we investigated how adaptation to P limitation may affect rotifer growth performance in the presence of an additional stressor, i.e. increased salinity, and vice versa. Interestingly, when fed P limited food, a simultaneous moderate increase in salinity had no impact on the growth of non-adapted reference populations. This can be explained by the idea that the excess C of LP food can potentially be used as extra energy to keep osmotic homeostasis and to neutralize the toxic effects of ions. Furthermore, we found that adaptation to P limitation reduced the ability of rotifer consumers to cope with increased salinity. Possibly, adaptation to P limitation may enable consumers to better eliminate excess C when fed P limited food. This ability may nevertheless be counter-adaptive under conditions of increased salinity given that less energy may be available to maintain osmotic homeostasis and mitigating ion toxicity. These results thus suggest that the evolutionary history of a population may strongly determine how it responds to different environmental stressors.