Responses of zooplankton to variation in elemental composition of algae: Regulation at individual and ecosystem levels
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 (i.e. filtration rate) and/or physiological (i.e. excretion rate of digestive enzymes) responses designed to cope with dietary phosphorus (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 field observations on a Daphnia galeata population. Indeed, the filtration rate of lake Daphnia was positively correlated with lake seston carbon:phosphorus (C:P) ratios. So, if algae are deficient in P (high C:P ratios), 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. After an acclimation period when the growth rate must remains low, I predict that we can observe an increase of growth rate as soon as these functional responses are triggered. This is exactly what it is observed in a meta-analysis of published Daphnia growth rates. This analysis shows that the growth rate of juvenile Daphnia submitted to P-limited food first drops, then progressively increases. Moreover, we have conducted 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. This increase in respiration rate is, of course, the expression of enhanced metabolism. 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 nitrogen (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 community (C:N:P ratios) in the Esch-sur-Sûre reservoir (Grand-Duchy of Luxemburg) and C, N and P sedimentation rates. So, when e.g. the zooplankton community was composed of crustacean species with high bodily demand in P, the N:P ratio of small sedimenting particles increased, indicating high assimilation of P by zooplankton, and the corresponding N enrichment of faeces.
Life sciences, Aquatic sciences & oceanology, Sciences du vivant, Sciences aquatiques & océanologie
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