REBORN

Freshwater ecosystems are particularly affected by global change that threatens their ecological sustainability and subsequent services provided to humans (e.g., fishing, drinking water, recreational activities). In this context, predicting the response of aquatic populations to environmental stressors has become an urgent ecological problem that requires management policies aimed at minimizing the effects of global change on freshwater ecosystems. Yet, due to methodological constraints, most freshwater studies have focused on short-term responses of animals to environmental stress. While this approach is highly valuable for characterizing direct responses of organisms to stress, it does not allow for predicting their adaptation capacities to environmental change, thus impeding the establishment of long-term management strategies. In this research project, we aim to define the evolution of major zooplankton species (i.e., keystone organisms of aquatic food webs) in response to two main factors associated with global change: temperature and nutrient input. One main part of the project will focus on the Cladocera of the genus Daphnia, which produces resting eggs capable of remaining viable for decades in lake sediments. By extracting Daphnia resting eggs from different layers of sediment cores, we will resurrect individuals with a genetic background reflecting populations present in the sampled lakes over the last decades. The resurrected populations will be exposed to natural alterations of temperature and nutrient inputs to assess the long-term evolution of populations resistance capacities. We will especially assess how the individual growth rate is affected by two essential biological processes; i) the resting metabolic rate, reflecting the minimal energy required to sustain body functions, which is severely constrained by environmental temperature, and; ii) the ability to synthesize polyunsaturated fatty acids, essential dietary compounds whose availability strongly depends on nutrient inputs in lakes. By doing so, we will be able to model the evolutionary dynamics of these biological traits in zooplankton species experiencing environmental constraints and integrate such evolutionary processes into models predicting animal responses to global change. In parallel, we will sample zooplankton populations in multiple lakes with different thermal histories and nutrient loads to define how lake environmental history reverberates on the current population`s metabolic activity and polyunsaturated fatty acid synthesizing capacities. By aggregating our results regarding evolutionary processes and current populations responses, we will establish a large-scale model predicting further population dynamics under different scenarios of global change, thus improving our capacities to develop adequate management policies to preserve freshwater systems over longer time scales.

The project focused on the evolutionary adaptation of zooplankton metabolic rate versus environmental conditions. By using resurrection ecology approaches to bred daphnia magna resting eggs from up to 40 years old, we've been able to compare the physiological repsonse of different clonal lines and popuation (recent ones and ancient ones) to different thermal conditions. By doing so we showed that evolutionary adaptation in the field does not always match predictions of a reduction of resting metabolic rate with warming. We showed that suc differences in evolutionary adaptation may arise because evolution is not only driven by the gradual increase in temperature but also the extreme events such as heat waves, that can induce important mortality and thus, important selective conditions. In parrallel we also showed that despite modification of resting metabolic rate, thermal adaptation is strongly mediated by the energy use efficiency of organisms. This efficiency of energy use being able to mitigate the consequences of metabolic rate variations induced by fluctuating temperatures.