The Future of Benthic Indicators : Moving up to the Intertidal

International audienceThe use of benthic indicators has increased dramatically during the last decades. The number of articles published on the subject, as well as the number of citations, has been particularly increasing since the early 90's, notably in relation with the implementation of directives for the management of aquatic/marine ecosystems such as the Water Framework Directive and the Marine Strategy Framework Directive. Current benthic indicators suffer from severe drawbacks and their practical use is still discussed and might have reached a dead end. Indicators based on species composition are not totally satisfactory, mainly because they exhibit a high spatio-temporal variability (e.g. variable at both seasonal and pluri-annual scales) and are user-dependent (e.g. divergent results from US orEuropeexperts.) In turn, modifications of behaviour, metabolism, phenotype or stable isotopes composition in invertebrates usually occur at short time scales, compared to detectable changes in community composition, and makes their use particularly relevant as indicators of perturbation. It is argued in this paper that these functional indicators might be relatively quickly implemented in the intertidal, and represent an effective alternative to current benthic indicators

Métabolisme benthique intertidal en milieu marin et estuarien : mesures in situ de la production et de la respiration, variabilité spatio-temporelle et bilans

In situ intertidal benthic metabolism (primary production and respiration) was mesured during emersed conditions on four according to a gradient from exposed to sheltered conditions corresponding to the typical sediments encountered along the French coasts of the English Channel. A closed-chamber was used to measure CO2 concentration via an infrared gas analyser. For each studied area (sandy beach of Wimereux, Bay of Somme, Bay of Seine and Bay du Mont Saint-Michel), temporal and spatial variability was assessed at different scales, and the factors controlling metabolic processes were then determinated. At both day and month scales, biological processes were mainly controlled by light (primary production) and temperature (primary production and respiration). At the seasonnal scale, local conditions (e.g. deposit/erosion cycle) may disrupt the system. In Wimereux, exposed conditions result in high instability where processes are difficult to predict. Within each site, the clear distinction of biosedimentary structures impacts on both chlorophylle a and primary production, but not on respiration. However at a larger spatial scale (inter-sites) this biosedimentary variability can not explain the observed discrepancies in metabolism measurements. Annual primary production budgets calculations show that production largely depends on seasonal changes in day length. On this seasonal trend are superimposed strong fornightly fluctuations expected from the superimposition of the tidal and day/night cycles. Finally, a spatialized budget was calculated for the Bay of Somme for three periods of the year when the benthic compartment of the bay appeared to be net heterotrophic.Le métabolisme benthique intertidal a été mesuré in situ au cours de l'émersion sur quatre sites s'inscrivant dans un gradient biosédimentaire (d'un estran sableux battu à des vases estuariennes). Les mesures ont été réalisées dans une chambre benthique couplée à un analyseur infra-rouge de la concentration en CO2. Sur chacun des sites étudiés, les variabilités temporelle et spatiale ont été explorées à différentes échelles et le contrôle des processus métaboliques par des facteurs biotiques et abiotiques a été déterminé. Il apparaît que les processus sont principalement régulés par la lumière et la température à l'échelle journalière et mensuelle dans les milieux estuariens. A l'échelle saisonnière s'additionnent les perturbations causées par les conditions locales propres à chaque site (cycle dépôt/érosion, par exemple). A Wimereux, les processus sont imprévisibles en raison de l'instabilité du milieu liée à l'exposition du site. La séparation entre les structures biosédimentaires propres à chaque site est bien marquée en termes de biomasse chlorophyllienne et de production, mais pas en terme de respiration. La variabilité spatiale à grande échelle du métabolisme ne peut pas s'expliquer seulement par la variabilité biosédimentaire. Des bilans de production annuelle calculés à l'échelle du m² montrent qu'elle dépend principalement des changements saisonniers de la durée du jour auxquels s'ajoutent des variations dues à la superposition des cycles tidaux et jour/nuit. Enfin, un bilan de production a été calculé à l'échelle de la Baie de Somme à trois périodes de l'année. Le compartiment benthique de la baie apparaît hétérotrophe net

Métabolisme benthique intertidal en milieu marin et estuarien : mesures in situ de la production et de la respiration, variabilité spatio-temporelle et bilans

In situ intertidal benthic metabolism (primary production and respiration) was mesured during emersed conditions on four according to a gradient from exposed to sheltered conditions corresponding to the typical sediments encountered along the French coasts of the English Channel. A closed-chamber was used to measure CO2 concentration via an infrared gas analyser. For each studied area (sandy beach of Wimereux, Bay of Somme, Bay of Seine and Bay du Mont Saint-Michel), temporal and spatial variability was assessed at different scales, and the factors controlling metabolic processes were then determinated. At both day and month scales, biological processes were mainly controlled by light (primary production) and temperature (primary production and respiration). At the seasonnal scale, local conditions (e.g. deposit/erosion cycle) may disrupt the system. In Wimereux, exposed conditions result in high instability where processes are difficult to predict. Within each site, the clear distinction of biosedimentary structures impacts on both chlorophylle a and primary production, but not on respiration. However at a larger spatial scale (inter-sites) this biosedimentary variability can not explain the observed discrepancies in metabolism measurements. Annual primary production budgets calculations show that production largely depends on seasonal changes in day length. On this seasonal trend are superimposed strong fornightly fluctuations expected from the superimposition of the tidal and day/night cycles. Finally, a spatialized budget was calculated for the Bay of Somme for three periods of the year when the benthic compartment of the bay appeared to be net heterotrophic.Le métabolisme benthique intertidal a été mesuré in situ au cours de l'émersion sur quatre sites s'inscrivant dans un gradient biosédimentaire (d'un estran sableux battu à des vases estuariennes). Les mesures ont été réalisées dans une chambre benthique couplée à un analyseur infra-rouge de la concentration en CO2. Sur chacun des sites étudiés, les variabilités temporelle et spatiale ont été explorées à différentes échelles et le contrôle des processus métaboliques par des facteurs biotiques et abiotiques a été déterminé. Il apparaît que les processus sont principalement régulés par la lumière et la température à l'échelle journalière et mensuelle dans les milieux estuariens. A l'échelle saisonnière s'additionnent les perturbations causées par les conditions locales propres à chaque site (cycle dépôt/érosion, par exemple). A Wimereux, les processus sont imprévisibles en raison de l'instabilité du milieu liée à l'exposition du site. La séparation entre les structures biosédimentaires propres à chaque site est bien marquée en termes de biomasse chlorophyllienne et de production, mais pas en terme de respiration. La variabilité spatiale à grande échelle du métabolisme ne peut pas s'expliquer seulement par la variabilité biosédimentaire. Des bilans de production annuelle calculés à l'échelle du m² montrent qu'elle dépend principalement des changements saisonniers de la durée du jour auxquels s'ajoutent des variations dues à la superposition des cycles tidaux et jour/nuit. Enfin, un bilan de production a été calculé à l'échelle de la Baie de Somme à trois périodes de l'année. Le compartiment benthique de la baie apparaît hétérotrophe net

Aliens eating aliens: an introduced amphipod as a potential prey of an invasive rocky shore crab in laboratory experiments

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Aliens eating aliens: an introduced amphipod as a potential prey of an invasive rocky shore crab in laboratory experiments

Behavioural interactions between introduced predators and introduced prey are still largely underestimated. The present work takes advantage of the co-occurrence of two introduced species, the Asian shore crab Hemigrapsus sanguineus and the amphipod Ptilohyale littoralis, respectively first recorded on rocky shores along the French coast of the eastern English Channel in 2005 and 2016. In this context, the predation by male and female H. sanguineus on P. littoralis was examined under controlled laboratory conditions, by presenting either juveniles of the blue mussel Mytilus edulis or adult P. littoralis to H. sanguineus. We subsequently assessed the potential prey preference of the Asian shore crab for P. littoralis and M. edulis by presenting the two prey items simultaneously in the same proportion. In the absence of choice, male H. sanguineus preyed significantly more on M. edulis than P. littoralis. In contrast, females preyed significantly less on M. edulis than P. littoralis; however, male and female H. sanguineus consumed similar numbers of P. littoralis. When choice was possible between P. littoralis and M. edulis, the crab did not exhibit preference stricto sensu for any type of prey. These results suggest that the Asian shore crab cannot be considered as a naive predator when confronted to a newly introduced prey. Our results also suggest that the amphipod P. littoralis did not exhibit any effective antipredator response towards the crab. These observations nevertheless warrant further work on the effects of abiotic factors (e.g. temperature) as well as other biotic interactions (e.g. presence of other prey or predators for H. sanguineus) may have on the observed prey-predator interactions between H. sanguineus and M. edulis and P. littoralis

The smell of sex: water-borne and air-borne sex pheromones in the intertidal gastropod Littorina littorea

International audienceWe investigated whether individuals of the intertidal snail Littorina littorea may be attracted by water-borne and air-borne pheromones from conspecifics. The potential relative role of water-borne and air-borne pheromone in the behavioural and reproductive biology of L. littorea was assessed through chemotactic assays. These involved a series of choice experiments with snails, snail smell and snail mucus as stimuli in a T-maze under conditions of immersion and emersion, and during the nonmating and mating seasons. During the nonmating season, L. littorea did not choose the snail-conditioned side of the T-maze significantly more frequently than the control side under conditions of both immersion and emersion. During the mating season, under condition of immersion both males and females chose the side of the T-maze conditioned with individual of the opposite sex significantly more frequently than the control side, and did not show any preference for the side conditioned with individuals of the same sex. Under conditions of emersion, only males were significantly more attracted by cues from the opposite sex than by the control. These results indicate that L. littorea are not attracted by conspecifics during the nonmating season; the cues of conspecifics are then unlikely to play a role in the orientation or the aggregation behaviour of this species. The results also suggest that males and females are attracted by the odour of conspecifics of the opposite sex, which contains both water-borne and air-borne pheromones in females and only water-borne pheromones in males. Also the similar behaviours observed with snails, water-borne and air-borne snail smell, and snail mucus, as stimuli indicate that male and female sex pheromones are both present in mucus. Temperature manipulations conducted during both seasons did not affect any of the observed behavioural changes, hence suggesting that the chemoreceptive abilities of L. littorea may be driven by exogenous and/or endogenous factors that are decoupled from direct control by temperature. This is in sharp contrast to the locomotor activity of L. littorea, which consistently increased with temperature. We also report the possibility of mate-tracking in females, a behaviour that has yet to be reported in gastropods, since males are typically believed to be the active sex that searches for females. Our results build on previous evidence of the function played by mucus-trail following in the reproductive success of intertidal gastropods, through the ability of L. littorea to remotely detect the water-borne and air-borne sex-specific cues that are incorporated in their mucus

Are the eastern and western basins of the English Channel two separate ecosystems: get back in line with some cautionary comments

International audienceThe European Marine Strategy Framework Directive (MSFD 2008/56/EC-17/06/2008) will be implemented in regions/subregions determined by their hydrology, oceanographic and biogeographic features. In this framework, Dauvin (2012) claimed that the English Channel (EC) is characterized by hydrologic, oceanographic and biogeographic features that support its division in two main ecosystems: the Western Basin (WBEC) and the Eastern Basin (EBEC). We think that this statement is controversial and that the conclusions drawn by Dauvin (2012) were not supported by the data presented, especially regarding the use of the term 'ecosystem'. We therefore re-evaluated this viewpoint with regard to the data and terminology used...

Inter-annual variability of CO2 exchanges between an emersed tidal flat and the atmosphere

Carbon dioxide (CO2) exchanges between a tidal flat (Wadden Sea, Netherlands) and the atmosphere were measured during a three-year survey. CO2 exchanges were monitored during 1-2 days each month between September 2006 and September 2009 using a flux chamber. The flux of CO2 was separated into two fluxes: the dark flux and the gross light flux, with the dark flux representing the flux during darkness and the gross light flux the difference between the net CO2 flux (measured under light) and the dark flux. It was argued that dark and light fluxes may deviate from respiration and photosynthesis, as the fluxes between wet tidal sediment and the atmosphere are affected by the partial pressure of CO2 in pore water, which is only gradually changed by sources and sinks of CO2 in the sediment. Light and dark fluxes were empirically related to environmental parameters in order to interpolate between succeeding measurements. The dark flux appeared to increase with temperature and the light flux became more intense with increasing irradiance with signs of saturation at high light levels on many but not all measurement days. These relations with environmental parameters showed seasonal and inter-annual variability. Fluxes were negligible just after the site was emersed and it took up to 3 h of emersion until fluxes were adapted to environmental conditions. Dark and light fluxes both showed strong seasonality with high values in summer and low values in winter. The tidal flat appeared as a source of atmospheric CO2 in the first year of measurement (+0.35 mol CO2 m-2 yr-1) and a sink in the following two years (-1.59 and -0.72 mol CO2 m-2 yr-1). Since the source of CO2 was observed during an extremely warm year, we suggest that climate warming might influence the carbon budget of tidal flats.