Decreased thermal tolerance under recurrent heat stress conditions explains summer mass mortality of the blue mussel Mytilus edulis

Extreme events such as heat waves have increased in frequency and duration over the last decades. Under future climate scenarios, these discrete climatic events are expected to become even more recurrent and severe. Heat waves are particularly important on rocky intertidal shores, one of the most thermally variable and stressful habitats on the planet. Intertidal mussels, such as the blue mussel Mytilus edulis, are ecosystem engineers of global ecological and economic importance, that occasionally suffer mass mortalities. This study investigates the potential causes and consequences of a mass mortality event of M. edulis that occurred along the French coast of the eastern English Channel in summer 2018. We used an integrative, climatological and ecophysiological methodology based on three complementary approaches. We first showed that the observed mass mortality (representing 49 to 59% of the annual commercial value of local recreational and professional fisheries combined) occurred under relatively moderate heat wave conditions. This result indicates that M. edulis body temperature is controlled by non-climatic heat sources instead of climatic heat sources, as previously reported for intertidal gastropods. Using biomimetic loggers (i.e. 'robomussels'), we identified four periods of 5 to 6 consecutive days when M. edulis body temperatures consistently reached more than 30 °C, and occasionally more than 35 °C and even more than 40 °C. We subsequently reproduced these body temperature patterns in the laboratory to infer M. edulis thermal tolerance under conditions of repeated heat stress. We found that thermal tolerance consistently decreased with the number of successive daily exposures. These results are discussed in the context of an era of global change where heat events are expected to increase in intensity and frequency, especially in the eastern English Channel where the low frequency of commercially exploitable mussels already questions both their ecological and commercial sustainability.Funding Agency French Ministere de l'Enseignement Superieur et de la Recherche Region Hauts-de-France European Funds for Regional Economical Development Pierre Hubert Curien PESSOA Felloswhip Fundacao para a Ciencia e Tecnologia (FCT-MEC, Portugal) IF/01413/2014/CP1217/CT0004 National Research Foundation - South Africa 64801 South African Research Chairs Initiative (SARChI) of the Department of Science and Technology National Research Foundation - South Africainfo:eu-repo/semantics/publishedVersio

Relationships between biodiversity and the stability of marine ecosystems: comparisons at a European scale using meta-analysis.

The relationship between biodiversity and stability of marine benthic assemblages was investigated using existing data sets (n = 28) covering various spatial (m-km) and temporal (1973-2006) scales in different benthic habitats (emergent rock, rock pools and sedimentary habitats) through meta-analyses. Assemblage stability was estimated by measuring temporal variances of species richness, total abundance (density or % cover) and community species composition and abundance structure (using multivariate analyses). Positive relationships between temporal variability in species number and richness were generally observed at both quadrat (<1 m2) and site (100 m2) scales, while no relationships were observed by multivariate analyses. Positive relationships were also observed at the scale of site between temporal variability in species number and variability in community structure with evenness estimates. This implies that the relationship between species richness or evenness and species richness variability is slightly positive and depends on the scale of observation, suggesting that biodiversity per se is important for the stability of ecosystems. Changes within community assemblages in terms of structure are, however, generally independent of biodiversity, suggesting no effect of diversity, but the potential impact of individual species, and/or environmental factors. Except for sedimentary and rock pool habitats, no relationship was observed between temporal variation of the aggregated variable of total abundances and diversity at either scale. Overall our results emphasise that relationships depend on scale of measurements, type of habitats and the marine systems (North Atlantic and Mediterranean) considered

Microphytobenthic production estimated by in situ oxygen microprofiling : short-term dynamics and carbon budget implications

Purpose Short-term temporal variability of microphytobenthic primary production is suspected to be of the same magnitude as seasonal variability, but data remain very scarce due to methodological limitations. In this context, a 6-day in situ high-frequency survey was performed in a temperate intertidal mudflat using an automated microprofiling system. Materials and methods In situ microphytobenthic primary production was measured using an automated acquisition system for oxygen microprofiles. More than 900 microprofiles, acquired during six consecutive days in April 2008, allowed the establishment of robust relationships between oxygen production and irradiance. Moreover, simultaneous measurements of fluorescence parameters and oxygen microprofiles during two diurnal emersion periods led to significant correlations between relative electron transport rate (rETR) and gross oxygen production (GOP). Results and discussion The use of an automated system allowed the estimation of oxygen exchanges during both immersion and emersion periods, and to our knowledge, this is the first study presenting continuous measurements during six consecutive days. The intertidal mudflat studied here was characterized by a maximum net oxygen production of 6.74 mmol O-2 m(-2) h(-1). Evidence for microphytobenthic migration behavior was observed during several periods and induced important depletion in oxygen production while irradiance remained high. Consequently, estimations of GOP from fitted photosynthesis-irradiance curves (P-I curves) showed an overestimation of 31 % compared to the GOP actually measured during the whole deployment. Conclusions This study confirmed that oxygen microsensors may be used to record microphytobenthic primary production, as resulting dynamics agreed with fluorescence data, while production values were in accordance with those presented in the literature. High-frequency microprofiles acquisition may be an easy way to monitor short-term temporal changes in microphytobenthic primary production in order to calculate accurate carbon or oxygen budgets for intertidal areas

Variabilité saisonnière et spatiale des flux métaboliques (respiration, excrétion) des communautés benthiques à Abra alba (Wood, 1802) et Ophiothrix fragilis (Abildgaard, 1789) du bassin oriental de la Manche et du sud de la mer du Nord

National audiencedes sédiments superficiels. L'estimation des flux métaboliques à l'interface eau-sédiment de deux communautés (communautés des cailloutis et des sables fins envasés) du bassin oriental de la Manche montre un lien très étroit entre les paramètres faunistiques et les processus biogéochimiques rencontrés dans le sédimen

Approche fonctionnelle des communautés benthiques à Abra alba et Ophiothrix fragilis du bassin oriental de la Manche et du sud de la Mer du Nord

Les espèces benthiques ingénieurs de la macrofaune sont des acteurs importants du fonctionnement chimique des sédiments superficiels. L'estimation des flux métaboliques à l'interface eau-sédiment de deux communautés (communautés des cailloutis et des sables fins envasés) du bassin oriental de la Manche montre un lien très étroit entre les paramètres faunistiques et les processus biogéochimiques rencontrés dans le sédiment

A closed-chamber CO2-flux method for estimating intertidal primary production and respiration under emersed conditions

This paper describes a closed-chamber method for measuring CO2 fluxes in intertidal soft sediments during periods of emersion. The method relies on closed-circuit incubations of undisturbed sediment and measurement of CO2 exchanges using an infrared gas analyser. The method was assessed during field experiments, both in light and dark conditions, on an exposed sandy beach and in an estuary. The rates of gross community production measured under moderate diance (4.2 mg C m(-2) h(-1) on the exposed sandy beach and 35 mg C m(-2) h(-1) in the estuary) are in good agreement with rates reported in the literature. In conjunction with appropriate sampling strategies, this method can be useful for estimating and comparing production of intertidal areas or for assessing factors that influence production

Additional records and distribution (2011-2012) of Hemigrapsus sanguineus (De Haan, 1835) along the French coast of the English Channel

International audienceThe invasion process can be described as a succession of stages initiated by the transport of organisms from their native range to a new area where they persist, proliferate and spread. It is important to monitor the demographic development of invaders for management purposes. This study focuses on the different stages of population development during the invasion process and underlines the importance of understanding and monitoring the 'persistence phase'. The distribution of Hemigrapsus sanguineus (Asian shore crab) in the English Channel, along the French coast, was first undertaken in 2008. In 2010, 35 sites were surveyed and it appeared that the abundance of this species had already established a 2-5 fold increase since 2008. The present study presents the geographical distribution of H. sanguineus in 2011 and 2012 which includes a further 39 sites (72 sampling stations in 2012). All populations observed during previous years persisted in 2011 and 2012. In 2012, H. sanguineus was detected at 61 sites; 36 intensely colonised (including 3 newly colonised sites compared to 2011); 22 had trace numbers and 3 sites had 'proven presence'. In addition to males with carapaces up to 39 mm width (CW), abundances increased by a factor of 2 since 2010, which testifies for the naturalized status of the species along the French coast of the English Channel. Since 2008, La Hougue proved to be the most abundantly colonised site along the French coast. By 2011 it had an average density of 101±19 ind.m-2, with an abundance of 258±54 individuals (under 30 boulders). Populations were subsequently halved in 2012. Increasing densities and abundances recorded between 2008 and 2011 at la Hougue suggest that H. sanguineus had reached the 'expansion phase', but the dynamics of H. sanguineus populations at the most colonised sites (12 sampling sites with abundance >200 individuals under 30 boulders), suggest that maximum values had already peaked and that the 'persistence phase' was probably reached. The implementation of pluri-annual surveys seems of prime importance to correctly evaluate population dynamics of alien species