Évolutions des accumulations littorales et relations avec les dynamiques d'avant-plage dans l'archipel des Glénan (Sud-Finistère, France)

Au sein des systèmes littoraux, les relations de dépendance entre les accumulations littorales et leur avant-plage restent souvent délicates à cerner. Les besoins de connaissance demeurent donc importants car leur compréhension est à même d'alimenter la réflexion sur la gestion des évolutions littorales. C'est le cas pour les littoraux des îles de l'archipel des Glénan dans le sud du Finistère, en particulier au sein de la Réserve Naturelle, où des observations des évolutions côtières sont réalisées à différentes échelles de temps et d'espaces. Sur la base d'une analyse de cartes anciennes et de clichés aériens, les évolutions diachroniques du trait de côte de certaines des îles sont retracées. Elles sont mises en relation avec les modifications morphologiques des fonds sédimentaires, en particulier avec les migrations lentes de bancs et de dunes hydrauliques sableux. Les bilans qui en résultent montrent les liens de dépendance entre les plages adjacentes d'une même île, entre les accumulations d'îles voisines et, au final, entre la mobilité des accumulations sous-marines et celle des accumulations littorales de l'ensemble de l'archipel. Au sein de ce système littoral, si des événements météo-marins sont à l'origine de retraits rapides du trait de côte, les migrations lentes des accumulations sous-marines dans l'archipel définissent, sur des périodes plus longues, des conditions favorables à l'érosion ou l’accrétion des rivages. Ce sont là des conditions dont il est nécessaire de tenir compte dans la perspective de la gestion des évolutions des rivages des îles et des îlots de l'archipel.Within the coastal systems, the relationships between the coastal accumulations and the nearshore remain rather difficult to understand and define. The needs for knowledge are thus important because their understanding is needed to feed the reflection on the management of the coastal dynamics. This is the case for coastline of the islands of the Glénan Archipelago located in the South of Finistère (France), in particular within the Nature reserve, where observations of the coastal evolutions are realized, at various scales of time and spaces. The diachronic evolutions of the coastline are surveyed on the basis of an analysis of old maps and aerial pictures and also topographical surveys. They are then compared to the morphological modifications and the slow migrations of sandy subtidal dunes. The results show the links of dependence between the neighboring beaches of a same island, also between the accumulations of nearby islands and, finally, between the mobility of the submarine accumulations and the coastal accumulations of the whole archipelago. Within this coastal system, if weather and marine events are at the origin of the fast retreats of the coastline, the slow migrations of the submarine accumulations in the archipelago define alternatively, over longer periods, conditions for the erosion or the accretion of the coastline of the islands. These conditions need to be taken into account partially with the prospect of the management of the coastline evolutions of islands of the archipelago

Évolutions des accumulations littorales et relations avec les dynamiques d'avant-plage dans l'archipel des Glénan (Sud-Finistère, France)

International audienceWithin the coastal systems, the relationships between the coastal accumulations and the nearshore remain rather difficult to understand and define. The needs for knowledge are thus important because their understanding is needed to feed the reflection on the management of the coastal dynamics. This is the case for coastline of the islands of the Glénan Archipelago located in the South of Finistère (France), in particular within the Nature reserve, where observations of the coastal evolutions are realized, at various scales of time and spaces. The diachronic evolutions of the coastline are surveyed on the basis of an analysis of old maps and aerial pictures and also topographical surveys. They are then compared to the morphological modifications and the slow migrations of sandy subtidal dunes. The results show the links of dependence between the neighboring beaches of a same island, also between the accumulations of nearby islands and, finally, between the mobility of the submarine accumulations and the coastal accumulations of the whole archipelago. Within this coastal system, if weather and marine events are at the origin of the fast retreats of the coastline, the slow migrations of the submarine accumulations in the archipelago define alternatively, over longer periods, conditions for the erosion or the accretion of the coastline of the islands. These conditions need to be taken into account partially with the prospect of the management of the coastline evolutions of islands of the archipelago.Au sein des systèmes littoraux, les relations de dépendance entre les accumulations littorales et leur avant-plage restent souvent délicates à cerner. Les besoins de connaissance demeurent donc importants car leur compréhension est à même d'alimenter la réflexion sur la gestion des évolutions littorales. C'est le cas pour les littoraux des îles de l'archipel des Glénan dans le sud du Finistère, en particulier au sein de la Réserve Naturelle, où des observations des évolutions côtières sont réalisées à différentes échelles de temps et d'espaces. Sur la base d'une analyse de cartes anciennes et de clichés aériens, les évolutions diachroniques du trait de côte de certaines des îles sont retracées. Elles sont mises en relation avec les modifications morphologiques des fonds sédimentaires, en particulier avec les migrations lentes de bancs et de dunes hydrauliques sableux. Les bilans qui en résultent montrent les liens de dépendance entre les plages adjacentes d'une même île, entre les accumulations d'îles voisines et, au final, entre la mobilité des accumulations sous-marines et celle des accumulations littorales de l'ensemble de l'archipel. Au sein de ce système littoral, si des événements météo-marins sont à l'origine de retraits rapides du trait de côte, les migrations lentes des accumulations sous-marines dans l'archipel définissent, sur des périodes plus longues, des conditions favorables à l'érosion ou l’accrétion des rivages. Ce sont là des conditions dont il est nécessaire de tenir compte dans la perspective de la gestion des évolutions des rivages des îles et des îlots de l'archipel

Lipid-free tuna muscle samples are suitable for total mercury analysis

Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, but also representing the main exposure to methylmercury, a potent neurotoxin that biomagnifies along food webs. The combination of ecological tracers (nitrogen and carbon stable isotopes, δ15N and δ13C) to mercury concentrations in tunas is scarce yet crucial to better characterise the influence of tuna foraging ecology on mercury exposure and bioaccumulation. Given the difficulties to get modern and historical tuna samples, analyses have to be done on available and unique samples. However, δ13C values are often analysed on lipid-free samples to avoid bias related to lipid content. While lipid extraction with non-polar solvents is known to have no effect on δ15N values, its impact on mercury concentrations is still unclear. We used white muscle tissues of three tropical tuna species to evaluate the efficiency and repeatability of different lipid extraction protocols commonly used in δ13C and δ15N analysis. Dichloromethane was more efficient than cyclohexane in extracting lipids in tuna muscle, while the automated method appeared more efficient but as repeatable as the manual method. Lipid extraction with dichloromethane had no effect on mercury concentrations. This may result from i) the affinity of methylmercury to proteins in tuna flesh, ii) the low lipid content in tropical tuna muscle samples, and iii) the non-polar nature of dichloromethane. Our study suggests that lipid-free samples, usually prepared for tropical tuna foraging ecology research, can be used equivalently to bulk samples to document in parallel mercury concentrations at a global scale

Reduced n-3 highly unsaturated fatty acids dietary content expected with global change reduces the metabolic capacity of the golden grey mullet

In this study, we hypothesised that a reduction in n-3 HUFA availability for higher consumers, as expected with global change, would negatively impact the physiological performances of fish. The aim was to experimentally evaluate the effect of n-3 HUFA dietary content on cardio-respiratory performances of the golden grey mullet (Liza aurata), a microalgae grazer of high ecological importance in European coastal areas. These performances were evaluated in terms of critical swimming speed U (crit), associated oxygen consumption MO2, post-exercise oxygen consumption and calcium fluxes in cardiomyocytes. Two replicated groups of fish were fed on a rich (standard diet, SD diet: 1.2 % n-3 HUFA on dry matter basis, DMB) or a poor n-3 HUFA (low n-3 HUFA diet, LD diet: 0.2 % n-3 HUFA on DMB) diet during 5 months and were called SD and LD groups, respectively. The results showed that the LD diet reduced growth rate as well as the aerobic capacity of L. aurata at 20 A degrees C, suggesting that fish may have to save energy by modifying the proportion of energy allocated to energy-demanding activities, such as digestion or feeding. In addition, this LD diet induced higher levels of haematocrit and plasma osmolality, indicating a stress response at the second and third levels in that group. However, the LD diet caused a massive increase in swimming efficiency. This should improve the capacity of L. aurata to migrate and to forage over a wide area. In turn, these could then compensate for the reduction in growth rate and aerobic metabolism

Ocean warming combined with lower omega-3 nutritional availability impairs the cardio-respiratory function of a marine fish

Highly unsaturated fatty acids of the omega-3 series (HUFA) are major constituents of cell membranes, yet poorly synthesised de novo by consumers. Their production, mainly supported by aquatic microalgae, has been decreasing with global change. Understanding the consequences of such reductions is essential for ectotherm consumers, since temperature tightly regulates the HUFA content in cell membranes, maintaining their functionality. Integrating individual, tissue and molecular approaches, we examined the consequences of the combined effects of temperature and HUFA depletion on the key cardio-respiratory functions of the golden grey mullet, an ectotherm grazer of high ecological importance. For four months, fish were exposed to two contrasting HUFA diets (4.8% ecosapentaenoic acid (EPA)+docosahexaenoic acid (DHA) on dry matter (DM) vs. 0.2% EPA+DHA on DM) at 12°C and 20°C. Ventricular force development coupled with gene expression profiles measured on cardiac muscle suggest that combining HUFA depletion with warmer temperatures leads to (1) a proliferation of sarcolemmal and SR Ca2+ channels and (2) a higher force-generating ability by increasing extracellular Ca2+ influx via sarcolemmal channels when the heart has to sustain excessive effort due to stress and/or exercise. At the individual scale, these responses were associated with a relatively greater aerobic scope, maximum metabolic rate and net cost of locomotion, suggesting the higher energy cost of this strategy. These impaired cardiac performances could have wider consequences on other physiological performances such as growth, reproduction or migration, all greatly depending on heart function