Multifractal intermittency of Eulerian and Lagrangian turbulence of ocean temperature and plankton fields

International audienceIn this paper, we present evidence that intermittency of Eulerian and Lagrangian turbulence of ocean temperature and plankton fields is multifractal and furthermore can be analysed with the help of universal multifractals. We analyse time series of temperature and in vivo fluorescence taken from a drifter in the mixed coastal waters of the eastern English Channel. Two analysis techniques are used to compute the fundamental universal multifiractal parameters, which describe all the statistics of the turbulent fluctuations: the analysis of the scale invariant structure function exponent ?(q) and the Double Trace Moment technique. At small scales, we do not detect any significant difference between the universal multifiractal behavior of temperature and fluorescence in an Eulerian framework. This supports the hypothesis that the latter is passively advected with the flow as the former. On the one hand, we show that large scale measurements are Lagrangian and indeed we obtain for temperature fluctuations a ?2 power spectrum corresponding to the theoretical scaling of a Lagrangian passive scalar. Furthermore, we show that Lagrangian temperature fluctuations are multiscaling and intermittent. On the other hand, the flatter slope at large scales of the fluorescence power spectrum points out that the plankton is at these scales a "biologically active" scalar

New insights on the population genetic structure of the great scallop (Pecten maximus) in the English Channel coupling microsatellite data and demogenetic simulations.

International audienceThe great scallop (Pecten maximus) is a commercially important bivalve in Europe, particularly in the English Channel, where fisheries are managed at regional and local scales through the regulation of fishing effort. In the long term, knowledge about larval dispersal and gene flow between populations is essential to ensure proper stock management. Yet, previous population genetic studies have reported contradictory results. In this study, scallop samples collected across the main fishing grounds along the French and English coasts of the English Channel (20 samples with temporal replicates for three sites,n= 1059 individuals), and the population genetic structure was analysed using 13 microsatellite loci. Coupling empirical genetic data with demogenetic modelling based on a biophysical model simulating larval exchanges among scallop beds revealed a subtle genetic differentiation between south-west English populations and the rest of the English Channel, which was consistent with larval dispersal simulations. The present study provides a step forward in the understanding of great scallop population biology in the English Channel, underlining the fact that even in a context of potentially high gene flow and recent divergence times since the end of the last glacial maximum, weak but significant spatial genetic structure can be identified at a regional scale

Turbulence intermittency, small-scale phytoplankton patchiness and encounter rates in plankton: where do we go from here?

Turbulence is widely recognized to enhance contact rates between planktonic predators and their prey. However, previous estimates of contact rates are implicitly based on homogeneous distributions of both turbulent kinetic energy dissipation rates and phytoplanktonic prey, while turbulent processes and phytoplankton cell distributions have now been demonstrated to be highly intermittent even on small scales. Turbulent kinetic energy dissipation rates and intermittent (i.e. patchy) phytoplankton distributions can be wholly parameterized in the frame of universal multifractals. Using this framework and assuming statistical independence between turbulent kinetic energy dissipation rate and phytoplankton distributions, we evaluated the effect of intermittent turbulence and the potential effects of zooplankton behavioral responses to small-scale phytoplankton patchiness on predator-prey encounter rates. Our results indicated that the effects of turbulence on predator-prey encounter rates is about 35% less important when intermittently fluctuating turbulent dissipation rates are considered instead of a mean dissipation value. Taking into account zooplankton behavioral adaptations to phytoplankton patchiness increased encounter rates up to a factor of 60

Horizontal distribution and retention of <i>Owenia fusiformis</i> larvae (Annelida: Polychaeta) in the Bay of Seine

Between 8 May and 3 June 1987 horizontal distribution of Owenia fusiformis larvae (Annelida: Polychaeta) was studied in the eastern part of the Bay of Seine on two spatial scales to determine the role of larval dispersal by hydrodynamic factors on the population dynamics of a benthopelagic species. Larval dispersal occurred in two main directions, north-west and north-east, according to the residual tidal circulation. There was a substantial transport of larvae to a distance of 30–40 NM (48–65 km) from their emission point, resulting in a sizeable loss of larvae to the adult population. However, their densities were low in comparison with those observed in the eastern part of the bay, near the adult population. Larval densities remained highest near the adult population, principally under the influence of tidal processes. In contrast, there was a significant wind-induced larval movement at a time scale of a few days. The action of wind at a time scale of one month, the duration of larval life, appeared to be insignificant for dispersal in 1987 because of the frequent changes in wind direction. Climatological data collected between 1951 and 1980 showed that the wind effect observed in 1987 is probably typical of most years. The horizontal transport of Owenia fusiformis larvae, combined with the ontogenic vertical migration observed previously, causes a retention of these larvae in the eastern part of the Bay of Seine; this favours recruitment and may be one of the factors explaining the temporal stability of the adult population.</jats:p

Metamorphosis of <i>Pectin Aria Koreni</i> (Annelida: Polychaeta) and Recruitment of an Isolated Population in the English Channel

In the Bay of Seine (English Channel) the polychaete Pectinaria koreni exhibits interannual stability in its abundance, despite its isolated distribution and its bentho-planktonic life cycle. To test the hypothesis of a high level of recruitment related to high survival rate of the post-larvae due to metamorphosis in the water column (transformation of metatrochophore into aulophore), we undertook a study of the morphology and the dynamics of the aulophore stage during May and June 1987. The morphological study snowed that aulophore larvae do not possess all the morphological features needed for benthic life on their first contact with the substratum. Fifteen days are needed to complete this development, and during this time the mortality rate is high. The possible role of this delayed metamorphosis on the transport of aulophore larvae towards sites favourable to adults is also discussed.</jats:p

Size distribution and community structure of phytoplankton: influence of nutrient availability and sedimentary loss.

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Do hydrodynamic factors affect the recruitment of marine invertebrates in a macrotidal area? The case study of Pectinaria koreni (Polychaeta) in the Bay of Seine (English Channel)

International audienceFor marine benthic invertebrates exhibiting complex life cycles, changes in populations' distribution and abundance are governed by a large variety of physical, chemical and biological processes. From field observations in the Bay of Seine and laboratory experiments conducted since 1987 on the polychaete Pectinaria koreni, the present study highlights the relative importance of hydrodynamical and biological factors which affect individuals within both the planktonic and benthic phases at different scales of space and time in a macrotidal area. Pectinaria koreni is one of the main macrofaunal component of the Abra alba muddy fine sand community of the eastern Bay of Seine. Despite a highly advective and diffusive environment, a relative larval retention near adult population was reported due to some local hydrodynamics features (e.g. tidal residual circulation, Seine river plume front) and the interaction between the vertical current structure and the larval vertical migration. Although larval retention could be disrupted by wind induced currents, multiple spawning events over the reproductive period increase the likelihood that at least one larval cohort ensures a high recruitment during the life-span. Following a massive settlement whatever the sediment grain size, the newly settled larvae exhibited a high immediate decrease of their densities as a result of postlarval mortality and migration. Postlarval drifting was induced by a combination of physical factors (i.e. tidal currents and swell) and postlarval behaviour in response to sediment texture and adult/settlers interactions. According to the hydrodynamics of the bay, this process may generate a postlarval transport from offshore bottoms to coastal suitable habitats and counteract the demographic effects of larval dispersal. A conceptual model of factors governing the recruitment and population maintenance of Pectinaria koreni is proposed and discussed in comparison with results obtained on another polychaete, Owenia fusiformis, in the same area