Structure et fonctionnement des écosystèmes benthiques dans les lagons soumis aux perturbations anthropiques— Le lagon de Tahiti, Polynésie française —

Tahiti island is amongst tropical ecosystems subject to increasing anthropisation. Although of cultural and economical importance to human populations, the reef complex is quite poorly known, especially when dealing with soft bottom substrata. This study aims, on the one hand to identify taxa of benthic communities in lagoon sediments and to describe the community structure, on the other hand, to highlight functioning patterns, including effects due to anthropic disturbances. Comparison with either environmental variables of sediments (essentially organic matter rates, grain size and pelitic fraction, chlorophyll a and phaeopigments rates) or variables related to the macrobenthos (taxonomic richness S, abundance A, biomass B and trophic type) leads to identification of the nature and the intensity of disturbances.Channels in lagoon ecosystems appear to be settling areas where disturbance impacts are pronounced : deposit feeder populations exhibit a clear dominance. Fringing reef areas bring further information. At least, areas related to the barrier reef do not fit to assess disturbances such as they arise over Tahitian lagoons.S, A, B variables and trophic structure are influenced by increasing anthropic disturbances, which affect the system as organic and inorganic (pelitic fraction) particles. The defined variables reveal succession of opposite effects at the community level. First, with food limitation release — as food disponibility has proved to be restricted in non disturbed areas — the values for S, A, and B similarly increase, the trophic structure becomes complex. Steady anthropic stress induces particulate overload that reduces S, A, B values and simplify trophic structure with facilitation for deposit feeding organisms. Conclusions about the ecosystem dynamics give more influence to the intermediate disturbance hypothesis. These results for tropical area show some similarities with those given by the functioning models for disturbed temperate areas (Pearson & Rosenberg, 1978). Polychaetes and, to a lesser extent, molluscs are good indicators for disturbances in coastal systems. The Dasybranchus sp.1 (Polychaeta: Capitellidae) should be used as indicator species for anthropic perturbations. A diagnosis tool used for assessment of the state of health in lagoon ecosystems is derived from understanding of functioning patterns. The assessment is principally based on indices relating Chaetopterid and Capitellid presence in channels and fringing reefs.In conclusion, lagoons of Tahiti island appear to be disturbed, either by natural impacts due to terrigenous inputs or by anthropic effects mainly as various particulate inputs. In the harbour of Papeete, disturbance is strong but bioturbation by decapods activity prevents setting up of extreme stages of eutrophication.L’île de Tahiti fait partie de ces écosystèmes tropicaux soumis à une anthropisation croissante. Le complexe récifal, qui représente un élément culturel et économique d’envergure pour les populations humaines, est cependant l’objet d’une connaissance restreinte, notamment au niveau du compartiment benthique de substrat meuble. Cette étude tente donc, d’une part, d’identifier les taxons des communautés benthiques des sédiments lagonaires et de décrire la structure de ces ensembles, d’autre part de mettre en évidence les modalités du fonctionnement de ces peuplements, en prenant en compte les effets d’éventuelles perturbations anthropiques. La confrontation d’un ensemble de variables environnementales des sédiments — représentées essentiellement par les taux de matière organique, granulométrie générale et taux de particules fines, taux de chlorophylle a et phaeopigments — et d’un ensemble de variables relatives au macrobenthos — qui sont la richesse taxonomique (S), l’abondance (A), la biomasse (B) et le type trophique — et au microbenthos permet d’identifier la nature des perturbations et leur intensité.Les chenaux de l’écosystème lagonaire s’avèrent des zones de décantation où les effets des perturbations sont fortement marqués : les populations de déposivores dominent nettement. Les zones de récif frangeant apportent une information complémentaire moins riche. Enfin, les aires inféodées au récif barrière ne sont pas adaptées à la mise en évidence des perturbations anthropiques telles qu’elles apparaissent sur l’île de Tahiti.Sous l’influence de perturbations anthropiques croissantes, affectant les systèmes benthiques sous la forme d’apports particulaires organiques et inorganiques (particules fines), les variables S, A et B ainsi que la structure trophique, reflètent une succession d’effets contraires, au niveau des communautés. Dans un premier temps, une levée de la limitation nutritive affectant l’écosystème non perturbé permet un accroissement parallèle des valeurs des trois variables et une complexité de la structure trophique. L’effet anthropique continu provoque ensuite une surcharge particulaire qui réduit les valeurs des variables S, A, B et simplifie la structure trophique en favorisant les organismes déposivores. La réflexion finale sur la dynamique des écosystèmes favorise l’hypothèse « intermediate disturbance ». Ces résultats en milieu tropical possèdent une forte affinité avec ceux traduits par les schémas de fonctionnement des milieux perturbés établis pour la zone tempérée (Pearson et Rosenberg, 1978). Les polychètes et, dans une moindre mesure, les mollusques sont de bons indicateurs des perturbations des milieux côtiers. L’espèce Dasybranchus sp.1 (Polychaeta : Capitellidae) peut être définie comme espèce indicatrice de perturbations anthropiques. Le schéma de fonctionnement établi permet aussi la mise au point d’un outil de diagnostic de l’état de santé des écosystèmes lagonaires, surtout basé sur des indices liés à la présence de polychètes Chaetopteridae et Capitellidae dans les chenaux et récifs frangeants. Finalement les lagons de l’île de Tahiti apparaissent perturbés, soit par action naturelle sous forme d’apports terrigènes, soit par action anthropique, sous forme d’apports particulaires variés. Dans la station du port, fortement perturbée, le stade extrême de l’eutrophisation n’est pas atteint, repoussé essentiellement par l’activité de bioturbation de décapodes

Sources of particulate organic matter at the ecosystem scale: a stable isotope and trace element study in a tropical coral reef

ABSTRACT: Spatial and seasonal variability of sources of particulate organic matter (POM) were evaluated in a coral reef ecosystem. Reef water POM was sampled monthly along a south-north gradient. The passage of tropical cyclone Gamede, which crossed Reunion Island in February 2007, provided an ideal opportunity to examine the impact of high river discharge. Stable isotope composition (δ13C, δ15N) was determined in oceanic, riverine and reef water POM, sedimentary organic matter (SOM), benthic primary producers and detritus. Trace elements (Ti, Fe, Cr, Ni, Cu, Zn) measured in SOM were used as additional terrestrial tracers. Stable isotope analysis of reef water showed that POM was affected by anthropogenic non-point nutrient sources, water circulation patterns and residence time. During the cold and dry season, the southern end of the reef received organic matter input derived from ocean primary production, while the middle and northern reef water POM originated primarily from SOM, reflecting an important bentho-pelagic coupling process. During the hot and wet season, reef water was enriched in benthic detritus due to higher autochthonous production. We found important spatial variability in POM sources, highlighting the importance of small-scale (hundreds of meters) studies when investigating ecosystem functioning. Although coastal tidal currents transported some river discharge material to the south of the reef, riverine POM input was limited, even after cyclone passage. We hypothesized that the major effect of the cyclone was the export of SOM and benthic detritus out of the system and that persistent disturbances such as groundwater discharges can be of greater importance than a cyclone in long-term degradation of ecosystems

Comparison of different segmentation approaches without using gold standard. Application to the estimation of the left ventricle ejection fraction from cardiac cine MRI sequences.

International audienceA statistical method is proposed to compare several estimates of a relevant clinical parameter when no gold standard is available. The method is illustrated by considering the left ventricle ejection fraction derived from cardiac magnetic resonance images and computed using seven approaches with different degrees of automation. The proposed method did not use any a priori regarding with the reliability of each method and its degree of automation. The results showed that the most accurate estimates of the ejection fraction were obtained using manual segmentations, followed by the semiautomatic methods, while the methods with the least user input yielded the least accurate ejection fraction estimates. These results were consistent with the expected performance of the estimation methods, suggesting that the proposed statistical approach might be helpful to assess the performance of estimation methods on clinical data for which no gold standard is available

Nonsupervised Ranking of Different Segmentation Approaches: Application to the Estimation of the Left Ventricular Ejection Fraction From Cardiac Cine MRI Sequences

International audienceA statistical methodology is proposed to rank several estimation methods of a relevant clinical parameter when no gold standard is available. Based on a regression without truth method, the proposed approach was applied to rank eightmethods without using any a priori information regarding the reliability of each method and its degree of automation. It was only based on a prior concerning the statistical distribution of the parameter of interest in the database. The ranking of the methods relies on figures of merit derived from the regression and computed using a bootstrap process. The methodology was applied to the estimation of the left ventricular ejection fraction derived from cardiac magnetic resonance images segmented using eight approaches with different degrees of automation: three segmentations were entirely manually performed and the others were variously automated. The ranking of methods was consistent with the expected performance of the estimation methods: the most accurate estimates of the ejection fraction were obtained using manual segmentations. The robustness of the ranking was demonstrated when at least three methods were compared. These results suggest that the proposed statistical approach might be helpful to assess the performance of estimation methods on clinical data for which no gold standard is available

Improved estimation of the left ventricular ejection fraction using a combination of independent automated segmentation results in cardiovascular magnetic resonance imaging

—This work aimed at combining different segmenta-tion approaches to produce a robust and accurate segmentation result. Three to five segmentation results of the left ventricle were combined using the STAPLE algorithm and the reliability of the resulting segmentation was evaluated in comparison with the result of each individual segmentation method. This comparison was performed using a supervised approach based on a reference method. Then, we used an unsupervised statistical evaluation, the extended Regression Without Truth (eRWT) that ranks different methods according to their accuracy in estimating a specific biomarker in a population. The segmentation accuracy was evaluated by focusing on the left ventricular ejection fraction (LVEF) estimate resulting from the LV contour delineation using a public cardiac cine MRI database. Eight different segmentation methods, including three expert delineations, were studied, and sixteen combinations of the five automated methods were investigated. The supervised and unsupervised evaluations demonstrated that in most cases, STAPLE results provided better estimates of the LVEF than individual automated segmentation methods. In addition, LVEF obtained with STAPLE were within inter-expert variability. Overall, combining different automated segmentation methods improved the reliability of the segmenta-tion result compared to that obtained using an individual metho

Seagrass macroinvertebrate bycatches support mosquito net trawl fishery in Madagascar

The use of mosquito nets, primarily for targeting small and/or juvenile fish, has rapidly increased in Western Indian Ocean shallow seagrass beds and coral reefs over the last 20 years. However, to date, invertebrate by-catch by locally-made fishing gear has not been reported. We studied the mosquito net trawl fishery in seagrass areas in the Bay of Toliara, Southwest Madagascar through the GPS tracking of fishers from August 2018 to February 2019. Catches were monitored through monthly landing surveys to characterize spatial temporal patterns in the abundance and diversity of macroinvertebrates in the seagrass beds. Fishing was carried out at low tide, mostly at night, by fishers pulling a hand-made trawl net of varying dimensions. Overall, 43 macroinvertebrate taxa were identified out of 217,080 individuals collected from 109 catch samples. Catches were generally composed of crustaceans (mainly Portunidae, Processidae, Penaeidae, and Alpheidae). The crab Thalamita mitsiensis largely dominated the abundance and biomass of the macroinvertebrate assemblage (from 6% to 100% and from 5 to 100% of the overall density and biomass, respectively). Macroinvertebrates composed 1.5% to 100% of the total catch per trip (i.e., 4–55 kg trip−1). They were sold for human consumption or animal feed, which provided additional income to fishers (USD 1–24 trip−1 and 1–72% of catch income per trip). This study revealed that macroinvertebrate resources provide valuable by-catch to small-scale fishers in Southwest Madagascar. This bycatch generates income that further encourages the use of mosquito net trawls and exacerbates their negative effects on coastal seagrass ecosystems and fisheries

Differential predictors for alcohol use in adolescents as a function of familial risk

Abstract: Traditional models of future alcohol use in adolescents have used variable-centered approaches, predicting alcohol use from a set of variables across entire samples or populations. Following the proposition that predictive factors may vary in adolescents as a function of family history, we used a two-pronged approach by first defining clusters of familial risk, followed by prediction analyses within each cluster. Thus, for the first time in adolescents, we tested whether adolescents with a family history of drug abuse exhibit a set of predictors different from adolescents without a family history. We apply this approach to a genetic risk score and individual differences in personality, cognition, behavior (risk-taking and discounting) substance use behavior at age 14, life events, and functional brain imaging, to predict scores on the alcohol use disorders identification test (AUDIT) at age 14 and 16 in a sample of adolescents (N = 1659 at baseline, N = 1327 at follow-up) from the IMAGEN cohort, a longitudinal community-based cohort of adolescents. In the absence of familial risk (n = 616), individual differences in baseline drinking, personality measures (extraversion, negative thinking), discounting behaviors, life events, and ventral striatal activation during reward anticipation were significantly associated with future AUDIT scores, while the overall model explained 22% of the variance in future AUDIT. In the presence of familial risk (n = 711), drinking behavior at age 14, personality measures (extraversion, impulsivity), behavioral risk-taking, and life events were significantly associated with future AUDIT scores, explaining 20.1% of the overall variance. Results suggest that individual differences in personality, cognition, life events, brain function, and drinking behavior contribute differentially to the prediction of future alcohol misuse. This approach may inform more individualized preventive interventions