Seasonality in coastal macrobenthic biomass and its implications for estimating secondary production using empirical models

Macrobenthic secondary production is widely used to assess the trophic capacity, health, and functioning of marine and freshwater ecosystems. Annual production estimates are often calculated using empirical models and based on data collected during a single period of the year. Yet, many ecosystems show seasonal variations. Although ignoring seasonality may lead to biased and inaccurate estimates of annual secondary production, it has never been tested at the community level. Using time series of macrobenthic data collected seasonally at three temperate marine coastal soft-bottom sites, we assessed seasonal variations in biomass of macrobenthic invertebrates at both population and community levels. We then investigated how these seasonal variations affect the accuracy of annual benthic production when assessed using an empirical model and data from a single sampling event. Significant and consistent seasonal variations in biomass at the three study sites were highlighted. Macrobenthic biomass was significantly lower in late winter and higher in summer/early fall for 18 of the 30 populations analyzed and for all three communities studied. Seasonality led to inaccurate and often biased estimates of annual secondary production at the community level when based on data from a single sampling event. Bias varied by site and sampling period, but reached similar to 50% if biomass was sampled at its annual minimum or maximum. Since monthly sampling is rarely possible, we suggest that ecologists account for uncertainty in annual production estimates caused by seasonality.Agência financiadora EDF French Ministry of Higher Education, Research and Innovation French Ministry for the Ecological and Inclusive Transition through the Marine Strategy Framework Directive Agreement French Biodiversity Agency (Agence francaise pour la biodiversite) as part of the CAPANOUR projectinfo:eu-repo/semantics/publishedVersio

Estimating macrobenthic production and its consumption for understanding food limitation in coastal nurseries

Understanding the processes that regulate the abundance of wild populations is a primary goal in ecology, and a prerequisite for implementing effective conservation and management of exploited living resources. In many marine fish, there is strong evidence that density-dependent regulation occurs at the juvenile stage when numerous individuals concentrate in spatially-restricted coastal and estuarine nurseries. However, the underlying mechanisms remain poorly understood. In particular, whether competition for food limits fish growth and survival is still controversial. In this PhD thesis, we focused on the food limitation hypothesis for nursery-dependent marine fish, which states that food supply limits juvenile fish production and ultimately, recruitment to adult populations offshore. To do so, we largely relied on a new bioenergetics-based approach, which estimates the exploitation rate of macrobenthic prey by the epibenthic predator community (juvenile fish and predatory invertebrates), and infers from this rate to what extent juvenile fish are food-limited. Using that approach, we investigated temporal and spatial variations in food supply and consumption among several coastal and estuarine nurseries located along the West coast of France. Overall, this PhD thesis provides new findings which indicate that food supply may limit juvenile fish production in coastal nurseries, at least when food supply is low and/or food demand is high. We also highlight some key research gaps that need to be filled to improve our understanding of food limitation in coastal.Comprendre les processus qui régulent l’abondance des populations sauvages est un objectif de recherche majeur en écologie, et un prérequis pour la conservation des espèces et la gestion des ressources vivantes exploitées. Chez de nombreuses espèces marines, la régulation densité-dépendante des populations a lieu au stade juvénile, durant lequel les individus se concentrent dans des nourriceries côtières et estuariennes spatialement limitées. Cependant, les mécanismes sous-jacents restent largement incompris. Le rôle de la compétition trophique comme mécanisme de régulation est en particulier controversé. Dans le cadre de cette thèse, nous nous sommes intéressés à l’hypothèse de limitation trophique, qui stipule que les ressources alimentaires dans les nourriceries limiteraient la production des juvéniles de poissons et in fine leur recrutement. Pour cela, nous avons utilisé une nouvelle approche bioénergétique qui permet d’estimer la part de la production des proies macrobenthiques consommée par les juvéniles de poissons et les méga-invertébrés, et de déduire de cette proportion si la limitation trophique est effective ou non. L’application de cette approche à plusieurs cas d’étude suggère que les ressources trophiques pourraient limiter la production des juvéniles dans les nourriceries françaises, au moins lorsque l’offre alimentaire est faible et/ou la demande énergétique est forte. Nous soulignons également des lacunes qu’il parait indispensable de combler afin d’améliorer notre compréhension de la limitation trophique au sein des nourriceries, comme par exemple une meilleure quantification de l’accessibilité des proies

Benthic prey production index estimated from trawl survey supports the food limitation hypothesis in coastal fish nurseries

Coastal and estuarine habitats function as nurseries for many commercial marine species. In these ecosystems, the hypothesis that food supply limits juvenile fish density and survival has been widely debated. Direct approaches that test this hypothesis in temperate soft-bottom nurseries are data-intensive as they rely on beam trawl to collect juvenile fish and grab or core to collect their prey within the macrobenthic community. Thus, application has often been limited to a few sampling stations and temporal snapshots. However, scientific beam trawl surveys, conducted periodically in nurseries, sample, besides juvenile fish, benthic invertebrates including potential prey species. Using data collected solely from beam trawl surveys, we tested whether food supply limits juvenile fish densities in several French nurseries. First, we validated that data of benthic invertebrates from bottom trawl surveys could be used to estimate an index of benthic prey production, by comparing data collected by grabs and trawls at the same sampling locations. Using this index on an extended trawl dataset, we estimated inter-annual variability of benthic prey production among several nurseries along the coast of the Bay of Biscay. Estimates of benthic prey production index were similar among nurseries, although, these nurseries displayed different local hydrological patterns (currents and residence time). The index was finally used to investigate whether benthic prey production limits young-of-the-year fish density using quantile regressions. We found a significant and positive relationship between the benthic prey production index and young-of-the-year fish densities, including flatfish and round fish species. Hence, our study supports the hypothesis that trophic limitation occurs for juvenile fish in coastal and estuarine nurseries during their first year of life, although other factors likely limit them locally

Potentiel dendrochronologique des ligneux bas (R. ferrugineum, J. communis) pour la connaissance des changements environnementaux en milieu pyrénéen : le cas du Haut-Vicdessos

National audienceDepuis le maximum démographique du milieu du XIX siècle, les milieux montagnards connaissent des mutations très rapides liées aux impacts conjoints de la déprise agro-sylvo-pastorale et du réchauffement climatique. Dans l’observatoire Homme-Milieu Pyrénées-Haut Vicdessos, le caractère marginal des activités pastorales durant la phase de plein emploi liée à l’activité industrielle (XX° siècle) interfère avec la hausse des températures moyennes pour expliquer la fermeture rapide des paysages observée depuis le milieu du XX° siècle. Celle-ci se traduit par une afforestation rapide des versants dès la fin des années 1950. Dans ce contexte, sur la base d’une approche dendrochronologique mise en œuvre conjointement sur les peuplements forestiers de haute altitude mais également sur les ligneux bas, notre étude vise à (1) caractériser les patrons spatio-temporels de la recolonisation ligneuse avec une résolution annuelle par la datation - au moyen de méthodes dendrochronologiques - des ligneux hauts et bas ; (2) cerner l’influence de la déprise agro-sylvo-pastorale dans la dynamique de recolonisation ; (iii) mettre en évidence l’impact des fluctuations climatiques sur l’expansion et la croissance radiale des ligneux. Pour cela, plusieurs individus de rhododendrons, genévriers et pins ont été prélevés, datés et analysés.Nos résultats mettent en évidence que les ligneux haut et bas ont colonisé quasi simultanément le versant échantillonné en accord avec le pic de colonisation en 1950 mis en évidence par les études polliniques. La comparaison cerne-climat révèle que les genévriers sont plus sensibles aux températures estivales que les pins, ces deux espèces bénéficiant largement de la hausse des températures. Les ligneux bas, contrairement aux arbres, sont sensibles à l’enneigement, une date de déneigement tardive impactant négativement leur croissance. Cependant, le rhododendron se montre beaucoup moins sensible au climat régional mais est probablement davantage impacté par le microclimat et par les événements extrêmes de sécheresse ou de gel. Des résultats préliminaires issus d’analyses dendrométriques et microclimatiques montrent qu’à quelques mètres de distances, les conditions microclimatiques différent significativement ainsi que la croissance radiale des individus de rhododendrons instrumentés

Food limitation of juvenile marine fish in a coastal and estuarine nursery

International audienceDespite their importance for species conservation and sound management of exploited living resources, the density-dependent mechanisms that regulate wild populations are among the least understood process in ecology. In many marine fish species, there is strong evidence that regulation occurs at the juvenile stage, when individuals concentrate in spatially restricted nurseries. However, little is known about the underlying mechanisms. Whether competition for food resources determines fish growth and survival is particularly controversial. We investigated whether food supply may have limited juvenile fish production (integrating both growth and survival) in a coastal and estuarine nursery in western Europe. Using a recent bioenergetics-based approach, we calculated annual macrobenthic food production (FP) and annual food consumption (FC) by juvenile fish and predatory invertebrates for three consecutive years (2008–2010). We also calculated exploitation efficiency (FC:FP) and used it as an index of food limitation. Results revealed substantial interannual variations in FP (FP ~2–3 times higher in 2008 and 2010 than in 2009). FC by young-of-the-year fish followed a fairly similar pattern. In addition, predatory invertebrates consumed as much food as juvenile fish, highlighting the need to consider all dominant epibenthic predators when estimating the overall predation pressure on macrobenthic prey. Lastly, exploitation efficiency of the entire epibenthic predator community reached ~30% in 2009, which is relatively high despite the conservative modeling approach. Overall, these results suggest that food supply may have limited juvenile fish production during the study period, at least in 2009. Nonetheless, further studies based on longer time-series and/or other study sites are required to strengthen these findings

Changements climatiques et environnementaux à Bassiès : ce que nous révèlent les cernes de croissance des arbustes

National audienceMountain ecosystems are particularly sensitive to climate change, which in part causes encroachment of woody plants at the treeline ecotone, with repercussions on treeline advance and ecosystem carbon balance. Yet, studies investigating the long-term trends in radial growth as well as year-to-year response of several tree and shrub species to climate change are scarce, especially in the Pyrenees where dendroecological studies are hitherto critically lacking. Here, we estimate and comparethe long-term growth trends of two shrub (Rhododendron ferrugineum and Juniperus communis) and one tree (Pinus uncinata) species, and investigate their year-toyear growth response to changing climatic conditions and advancing snow melt-out timings. Overall, our results show that all species at our site, especially shrubs, have grown increasingly well over at least the last century, probably in response to increasing temperatures during the growing season and earlier snow melt-out dates. Nevertheless, the two shrub species differ quite strongly in their response to climate. Whereas the climate signal of J. communis has been relatively stable in recentdecades despite the persistent and significant warming trend, R. ferrugineum shows a strong shift in climate sensitivity and is increasingly affected negatively by climate change and by late-frost events. Altogether, our results address the different climate sensitivity of the two most common shrubs in the Pyrenees. They also contribute to a better understanding of vegetation dynamics in the Pyrenean treeline ecotone in the context of global change

Changements climatiques et environnementaux à Bassiès : ce que nous révèlent les cernes de croissance des arbustes

National audienceMountain ecosystems are particularly sensitive to climate change, which in part causes encroachment of woody plants at the treeline ecotone, with repercussions on treeline advance and ecosystem carbon balance. Yet, studies investigating the long-term trends in radial growth as well as year-to-year response of several tree and shrub species to climate change are scarce, especially in the Pyrenees where dendroecological studies are hitherto critically lacking. Here, we estimate and comparethe long-term growth trends of two shrub (Rhododendron ferrugineum and Juniperus communis) and one tree (Pinus uncinata) species, and investigate their year-toyear growth response to changing climatic conditions and advancing snow melt-out timings. Overall, our results show that all species at our site, especially shrubs, have grown increasingly well over at least the last century, probably in response to increasing temperatures during the growing season and earlier snow melt-out dates. Nevertheless, the two shrub species differ quite strongly in their response to climate. Whereas the climate signal of J. communis has been relatively stable in recentdecades despite the persistent and significant warming trend, R. ferrugineum shows a strong shift in climate sensitivity and is increasingly affected negatively by climate change and by late-frost events. Altogether, our results address the different climate sensitivity of the two most common shrubs in the Pyrenees. They also contribute to a better understanding of vegetation dynamics in the Pyrenean treeline ecotone in the context of global change