Conference Agenda

Ponds represent the most abundant freshwater lentic systems on Earth and provide essential ecosystem services including water supply, quality improvement, food provisioning or global biodiversity support. Despite their small size, they play a disproportionately important role in global carbon and nitrogen cycles, including fluxes of highly potent greenhouse gas (GHG) such as CO₂, CH₄ and N₂O. However, ponds suffer from a critical lack of consideration in management plans and legislations, leaving them especially vulnerable to anthropogenic disturbances such as alien species introduction. Alien fish introduction in naturally fishless ponds has been identified as a major issue for ponds biodiversity, but there is a lack of assessment of their consequences on food web dynamics and ecosystem functioning. By inducing different trophic cascades, alien fish have the potential to profoundly alter food webs in recipient ecosystems, with complex consequences on the services they deliver. In particular, there is a critical lack of data on the potential influence of biotic components such as predators on the dynamic of GHG fluxes in ecosystems. Here, we present findings from an ongoing project aiming to shed light on the impact of alien fish invasions on pond food webs and ecosystem functioning by relying on a wide array of techniques, including bulk δ¹³C and δ¹⁵N isotope analysis of biotic and abiotic ecosystem components, and field measurements of CO₂, CH₄ and N₂O fluxes. Data from 38 ponds in France, Slovenia and Croatia, collected in late Spring, reveal that alien fish collapse food web structures, shift primary producers equilibria toward phytoplankton domination, degrade water quality and significantly increase CO₂ partial pressure and CH₄ concentration in water relative to fishless ponds, but without apparent effect on N₂O fluxes. We discuss the implications of these results in a context of biological conservation and global changes.

In the short‐ or mid‐term, the variation of leaf‐respired δ¹³CO₂ has important consequences for δ¹³C of CO₂ in air in terrestrial ecosystems. Therefore, the isotope composition of plant respired CO₂ is of crucial importance for understanding plant and ecosystem carbon balance. It has previously been shown in tobacco (Nicotiana tabacum) that the balance between ammonium and nitrate has an influence on δ¹³C of leaf‐respired CO₂. However, uncertainty remains as to whether (i) the effect of N nutrition is observed in all species, (ii) N source also impacts on respired CO₂ in roots, and (iii), there is a relationship or equation predicting δ¹³C of respired CO₂ that can be applied regardless of N conditions, species, and represents a hurdle in plant ¹³C budget modelling.

Here, we carried out isotopic measurements of respired CO₂ and various metabolites using two species (spinach, French bean) grown under different NH₄⁺:NO₃^- ratios. Both species showed a similar pattern, with a progressive ¹³C-depletion in leaf respired CO₂ as the ammonium proportion increased, while δ¹³C in root-respired CO₂ showed little change. Supervised multivariate analysis showed that δ¹³C in respired CO₂ was mostly determined by organic acid (malate, citrate) metabolism, in both leaves and roots. We then took advantage of non-stationary, two-pool modelling that explained 73% of variance in δ¹³C in respired CO₂. It demonstrates the critical role of the balance between the utilization of respiratory intermediates and the remobilization of stored organic acids, regardless of anaplerotic bicarbonate fixation by phosphoenolpyruvate carboxylase and the organ considered. These findings underscore the role of nitrogen availability in shaping plant carbon metabolism and stable isotope signatures, with broader implications for modeling plant contributions to atmospheric CO₂.

Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) are progressively more employed to study the foraging ecology of ectotherms predators like crocodilians. However, accurate and precise estimations of trophic discrimination factors between diet and crocodile tissues (Δ¹³C and Δ¹⁵N, respectively) from captive experiments under controlled conditions are necessary to quantify the contribution of their preys in their feeding habits reliably. The issues of isotope equilibrium and isotopically constant diet are important factors that influence accurate estimations of diet-tissue discrimination factors. These assumptions influence the accuracy and reliability of ecological inferences based on stable isotopes. We raised Nile crocodiles (Crocodylus niloticus) fed with two isotopically distinct (but constant) diets and under controlled experimental conditions. They grew enough to precisely estimate the trend patterns that ultimately reach isotopic equilibrium. We sampled blood (plasma and red blood cells), scute keratin and collagen and nail tissues during the experiment and diet-tissue discrimination factors were estimated at isotope equilibrium with diet. Overall, our estimations of average diet-tissue discrimination factors (in ‰) were +0.1 for plasma, +0.1 for red blood cells (RBC), +0.3 for keratin, +1.9 for collagen, and +1.2 for nail tissue for δ¹³C, and +0.2 for plasma, +1.9 for RBC, +1.6 for keratin, +2.3 for collagen, and +1.8 for nail tissue for δ¹⁵N. We found no clear significant effect of crocodile size on these tissue estimates, but a slight influence for plasma Δ¹⁵N. Understanding these differences in ectotherm isotope ecology is crucial for interpreting trophic relationships within food webs that include animals like reptiles.

[This abstract is dedicated to the memory of Prof. Keith A. Hobson, who united us in the quest of stable isotope ecology.]

Some Sacoglossa sea slugs feed on macroalgae and sequester chloroplasts in the cells of their digestive diverticulum. In some species, mostly within the genus Elysia, the stolen chloroplasts – kleptoplasts – remain photosynthetically competent for weeks to months. These sea slugs, like other gastropods, produce a viscous secretion or mucus involved in protection, locomotion, and reproduction. In this study, we profiled the carbohydrate composition of the mucus of the kleptoplast-bearing sea slug Elysia crispata and tracked the incorporation of carbon and nitrogen via stolen chloroplasts. Mucopolysaccharides were mainly composed by uronic acids and galactose with 1→4 glycosidic linkages. Using inorganic ¹³C-labelled sea water, incorporated carbon was found in the secreted mucus only in the presence of light, mainly in the form of 3-O-methylgalactose, galactose, and fucose. On the contrary, additional inorganic ¹⁵N in the sea water could be traced in the mucus in both dark and light conditions. Results show that inorganic carbon was fixed through photosynthesis in the kleptoplasts, translocated to the animal tissue and incorporated in the secreted mucopolysaccharides. The present study pinpoints the biological relevance of photosynthesis to the metabolism of these remarkable animal-chloroplast symbiotic associations.