The aim of this work was to improve our knowledge about the responses and adaptations of zooplankton, and especially Daphnia, when faced with food sources that are deficient in essential elements.

First, we have modelled the constraints of feeding, digestion and growth in Daphnia. We have hypothesized that Daphnia possesses the intrinsic ability to initiate behavioural and/or physiological responses designed to cope with dietary P deficiency. This first stoichiometrically explicit model of Daphnia filtration and digestion shows that an increase in feeding rate is an appropriate response to cope with P-deficiency. This mathematical prediction is in agreement with our field observations on a Daphnia galeata population. The filtration rate of lake Daphnia was positively correlated with lake seston C:P ratios. So, if algae are deficient in P, Daphnia filter more water, and so feed more intensively. This behaviour may be seen as a use of C assimilated in excess from the algae, this to generate more energy that can then be used to obtain more P, through increased feeding and digestion rates.

Behavioural and physiological adaptations taken into account in the model are however not immediate. A meta-analysis of published growth rate of juvenile Daphnia submitted to P-limited food shows that, after an acclimation period where the growth rate remains low, we observe an increase of growth rate as soon as these functional responses are triggered.

We have also conducted some laboratory experiments to demonstrate enhanced respiration in daphniids fed with P-depressed algae. Once again, this observation is in total agreement with the predictions of the model. From a stoichiometrical perspective, the increase in respiration allows the disposal of excess C with respect to P. In the same study, we have also shown the higher excretion of organic C by P-limited Daphnia.

Beyond individual responses, this work was also concerned with the global impact of these adaptations to fluxes of matter in the ecosystem. As specific needs for N and P of each zooplankton species probably lead to species-specific regulations of both elements, the relative composition in C, N and P of faeces produced by zooplankton at a certain time must reflect the average elementary requirements of the entire zooplankton community. We have indeed observed a good correlation between the stoichiometrical characteristics of the zooplankton in the Esch-sur-Sûre reservoir and C, N and P sedimentation rates.