Claire Duchet

Anthropogenic eutrophication poses a significant threat to freshwater environments globally. It also influences the population dynamics of invasive and native species, yet the cumulative effects of eutrophication and invasive species on native organisms are not well understood. We used invasive gibel carp (Carassius gibelio Bloch, 1782) and native crucian carp (Carassius carassius Linnaeus, 1758) as model taxa to investigate how eutrophication influences their interspecific competition. Carassius carassius is being outcompeted by invasive C. gibelio across most of its native range, and eutrophication may play a role in the competitive displacement of C. carassius. We explored how varying eutrophication levels influence growth and survival of both species in laboratory and mesocosm experiments, where we exposed them to different feeding rations and nutrient levels, respectively. We hypothesized that (1) C. gibelio benefits more from increased nutrient levels than C. carassius—which favors invasive C. gibelio over native C. carassius in interspecific competition—and that (2) the growth and survival of C. carassius deteriorate under interspecific competition with C. gibelio compared to intraspecific competition. Our experiments revealed that excess nutrient availability was more beneficial to the invasive C. gibelio. Overall, C. gibelio individuals grew more than C. carassius, and the difference between the two species increased with nutrient levels in both experiments, especially in the weight of smaller individuals. Moreover, interspecific competition led to lower survival in C. carassius but not in C. gibelio in the mesocosm experiment. This study shows that eutrophication may modulate interspecific competition, and cultural eutrophication of freshwater habitats may enhance the success of invasive species. Graphical abstract

Temperature is a key driver of individual physiology and ontogeny in ectotherms, with important consequences for individual fitness and population dynamics. Freshwater ectotherms typically grow and develop faster but reach smaller adult sizes at higher temperatures, a pattern known as the “temperature-size rule” (TSR). However, temperature-size responses can vary across ontogeny. Some freshwater arthropods also adjust the number of larval moults, offering additional developmental flexibility that could modulate temperature-size responses. We investigated the temperature dependence of the larval development of Sympetrum striolatum , a widespread univoltine dragonfly with variable number of larval instars. Larvae were reared from egg to final larval instar at six constant temperatures (9, 13, 17, 21, 25, and 29 °C). We assessed number of moults, developmental rate and body size of selected instars, and measured the metabolic rate at the last larval instar (F0). No hatching occurred at 9 °C, and all larvae died early at 29 °C, indicating a narrow thermal window for development (< 20 °C range). Development rate increased with temperature between 13 °C and 25 °C. While early instars followed TSR expectations, with smaller size at higher temperatures, final body size increased with temperature due to more larval moults and larger size increments in later instars at higher temperatures. Metabolic rate at 21 °C was significantly lower in individuals reared at 25 °C, indicating thermal acclimation under thermal stress. These findings reveal complex, stage-specific responses to temperature and highlight the developmental plasticity of S. striolatum . Our results provide insights into how freshwater ectotherms with flexible development strategies may cope with climate change.

Multiple anthropogenic stressors influence the functioning of ponds and lakes, but their combined effects are often little understood. We ran two mesocosm experiments to evaluate the effects of warming (+4°C above ambient) and environmentally relevant concentrations of a mixture of commonly used pharmaceuticals, an emerging class of chemical contaminants, on tri-trophic food webs representative of pelagic communities in ponds and other small standing waters. We quantified the main and interactive effects of warming and pharmaceuticals on each trophic level and attributed them to the direct effects of both stressors and the indirect effects arising through biotic interactions. Warming and pharmaceuticals had stronger effects in the summer experiment, altering zooplankton community composition and causing delayed or accelerated emergence of top insect predators. In summer, both stressors and top predators reduced filter-feeding zooplankton biomass, while warming and pharmaceuticals had opposing effects on phytoplankton. In the winter experiment, the effects were much weaker and primarily limited to a positive effect of warming on phytoplankton biomass. Overall, we show that pharmaceuticals can exacerbate the effects of climate warming in freshwater ecosystems, especially during the warm season. Our results demonstrate the utility of community-level studies across different seasons for the risk assessment of multiple emerging stressors in freshwater ecosystems.

Neonicotinoid insecticides represent nearly a quarter of the global market and are widely used in agriculture but also for lawn, garden care, and pest control. They are highly water-soluble, persistent in soil, and may enter the aquatic compartment via spray drift, runoff, or leaching, and contribute to downstream aquatic toxicity. Although insects appear to be the most sensitive group to neonicotinoids, other groups, such as crustaceans and birds, may also be affected. Furthermore, most studies focus on single-insecticide exposure and very little is known concerning the impact of neonicotinoid mixtures on aquatic invertebrates. The present study was designed to test potential toxicological effects of an environmentally relevant mixture of imidacloprid, clothianidin, and thiamethoxam on populations of Ceriodaphnia dubia and Daphnia magna under controlled conditions. Chronic toxicity tests were conducted in the laboratory, and survival and reproduction were measured for both species under exposure to nominal concentrations of imidacloprid (0.256 µg/L), clothianidin (3.11 µg/L), thiamethoxam (1.49 µg/L), and a mixture of the three compounds at the same concentrations of the individual compounds. The neonicotinoids did not affect the survival of C. dubia and D. magna founders. Reproduction of C. dubia was affected only by the mixture. All three individual insecticides as well as the mixture caused a significant reduction in the reproduction of D. magna . Our results highlight the complexity of pesticide toxicity and show that traditional toxicological approaches such as acute mortality studies, especially tests with single compounds, can underestimate negative impacts that occur in the environment. Highlights: Neonicotinoids are currently the most frequently used insecticides worldwide. An environmentally relevant mixture of three neonicotinoids was evaluated on two daphniid species. The mixture negatively affected the reproduction of C. dubia and Daphnia magna . Traditional toxicological approaches with single compounds may underestimate the effects occurring in the environment at low concentrations.

1. Hydroperiod duration has been identified as the main factor determining the faunal composition and structure of aquatic communities in temporary habitats. It is hypothesised that desiccation will positively affect mosquito oviposition habitat selection during the post‐drought period due to the lack of antagonists. 2. An experiment was carried out in outdoor mesocosms to assess whether desiccation events have post‐drought effects on community richness and mosquito oviposition. Three different treatments were randomly assigned to the mesocosms: (i) eight mesocosms were left dried for a week and then reflooded; (ii) eight mesocosms stayed dry for 4 weeks before they were reflooded; (iii) eight mesocosms were maintained at a constant water volume of 30 litres during the entire experiment as controls. Mosquito oviposition and invertebrate community richness were monitored in every mesocosm, along with environmental parameters [water temperature, pH, conductivity, dissolved oxygen, total suspended solids (TSS) and chlorophyll a concentration]. 3. Post‐drought mosquito oviposition and larval abundance were higher in the short‐drought and long‐drought pools than in the control. Desiccation negatively affected the biomass of the filter feeder invertebrates in both desiccation treatments. Chlorophyll a concentrations were higher in the long‐drought pools than in controls. The negative impact of desiccation on zooplankton led to a post‐drought increase in algae, associated with an increase in mosquito oviposition. 4. Despite immediate negative effect on mosquitoes, pulsed disturbances can benefit mosquitoes as they favour oviposition during the post‐disturbance recovery period due to a lower abundance of mosquito antagonists and higher food resources for their offspring.