Jean-Christophe Poggiale

Dynamic energy budget theory restores coherence in biology

We present the state of the art of the development of dynamic energy budget theory, and its expected developments in the near future within the molecular, physiological and ecological domains. The degree of formalization in the set-up of the theory, with its roots in chemistry, physics, thermodynamics, evolution and the consistent application of Occams razor, is discussed. We place the various contributions in the theme issue within this theoretical setting, and sketch the scope of actual and potential applications.

A New Approach for the Modelling of Sediment Reworking Induced by a Macrobenthic Community

A new model of bioturbation has been developed to describe short term sediment reworking induced by macrobenthic communities. The design of the model had to consider the mixing processes, firstly, at the organism level, and secondly, at community level. This paper describes the mixing mode of the four types of bioturbators defined by the authors: the biodiffusors, the upward-conveyors, the downward-conveyors and the regenerators. The mathematical formulation of these sub-models consists of ordinary differential equations. They take into account the size of the bioturbated zone, the output fluxes to the water column, tracer decay, physical mixing due to local currents and the type and intensity of the bioturbation processes. These sub-models make it possible to describe correctly the mixing events that have occurred in cores with each type of bioturbator. They also provide the basis for general bioturbation model, that will take into account the respective degrees of involvement of (i) the different bioturbation processes and their characteristics, (ii) the interference between the different processes, and (iii) make possible to predict the particle reworking in order to include it in studies of organic matter in early diagenesis.

Benthic biogeochemistry: state of the art technologies and guidelines for the future of in situ survey

Sediment and water can potentially be altered, chemically, physically and biologically as they are sampled at the seafloor, brought to the surface, processed and analysed. As a result, in situ observations of relatively undisturbed systems have become the goal of a growing body of scientists. Our understanding of sediment biogeochemistry and exchange fluxes was revolutionized by the introduction of benthic chambers and in situ micro-electrode profilers that allow for the direct measurement of chemical fluxes between sediment and water at the sea floor and for porewater composition. Since then, rapid progress in the technology of in situ sensors and benthic chambers (such as the introduction of gel probes, voltammetric electrodes or one- and two-dimensional optodes) have yielded major breakthroughs in the scientific understanding of benthic biogeochemistry. This paper is a synthesis of discussions held during the workshop on sediment biogeochemistry at the “Benthic Dynamics: in situ surveillance of the sediment–water interface” international conference (Aberdeen, UK—March 25–29, 2002). We present a review of existing in situ technologies for the study of benthic biogeochemistry dynamics and related scientific applications. Limitations and possible improvement (e.g., technology coupling) of these technologies and future development of new sensors are discussed. There are countless important scientific and technical issues that lend themselves to investigation using in situ benthic biogeochemical assessment. While the increasing availability of these tools will lead research in yet unanticipated directions, a few emerging issues include greater insight into the controls on organic matter (OM) mineralization, better models for the understanding of benthic fluxes to reconcile microelectrode and larger-scale chamber measurements, insight into the impacts of redox changes on trace metal behavior, new insights into geochemical reaction pathways in surface sediments, and a better understanding of contaminant fate in nearshore sediments.

Aggregation of Variables and Applications to Population Dynamics

1 IRD UR Geodes, Centre IRD de l’Ile de France, 32, Av. Henri Varagnat, 93143 Bondy cedex, France pierre.auger@bondy.ird.fr 2 Departamento de Matematicas, Universidad de Alcala, 28871 Alcala de Henares, Madrid, Spain rafael.bravo@uah.es 3 Laboratoire de Microbiologie, Geochimie et d’Ecologie Marines, UMR 6117, Centre d’Oceanologie de Marseille (OSU), Universite de la Mediterranee, Case 901, Campus de Luminy, 13288 Marseille Cedex 9, France Jean-Christophe.Poggiale@univmed.fr 4 Departamento de Matematicas, E.T.S.I. Industriales, U.P.M., c/ Jose Gutierrez Abascal, 2, 28006 Madrid, Spain esanchez@etsii.upm.es 5 IXXI, ENS Lyon, 46 allee d’Italie, 69364 Lyon cedex 07, France tri.nguyen-huu@ens-lyon.fr

Predator Migration Decisions, the Ideal Free Distribution, and Predator‐Prey Dynamics

The aim of the present work is to analyze the influence of optimal predator emigration decisions that can lead to the ideal free distribution (IFD) on the stability of predator‐prey systems. The assumption of optimal decisions is then relaxed to analyze the possible influence of different degrees of deviation from the IFD. The first migration rule we analyze is based on the marginal‐value theorem and assumes perfect knowledge of capture rate in the patch of residence and in the environment as a whole. When migration rates are high, this rule leads the predator population to the IFD. The results suggest that under these conditions predator migration plays no major role in the stability of the system. This is so because the systems naturally merge into a single patch. This result is independent of the particular functional response used. The other two rules we analyze take into account lower migration rates, the limitations in making optimal decisions by predators, and the possible constraints in the assessment of intake rate in the different patches. The results suggest that the processes that hinder the convergence of the populations to the IFD might make a major contribution to the stability of the system.

Effects of chronic uranium exposure on life history and physiology of Daphnia magna over three successive generations

Daphnia magna was exposed to waterborne uranium (U) at concentrations ranging from 10 to 75 microgL(-1) over three successive generations (F0, F1 and F2). Progeny was either exposed to the same concentration as mothers to test whether susceptibility to this radioelement might vary across generations or returned to a clean medium to examine their capacity to recover after parental exposure. Maximum body burdens of 17, 32 and 54 ng U daphnid(-1) were measured in the different exposure conditions and converted to corresponding internal alpha dose rates. Low values of 5, 12 and 20 microGy h(-1) suggested that radiotoxicity was negligible compared to chemotoxicity. An increasing sensitivity to toxicity was shown across exposed generations with significant effects observed on life history traits and physiology as low as 10 microgL(-1) and a capacity to recover partially in a clean medium after parental exposure to <or=25 microgL(-1). Using a (14)C-labelled food technique, the study showed that uranium affected carbon assimilation in F0 at concentrations of 25 and 75 microgL(-1) (34 and 80% reduction respectively) and as low as 10 microgL(-1) in F1 and F2 (40 and 36% reduction respectively). Consequences were strong for both somatic growth and reproduction and increased in severity across generations. Maximum size was reduced by 12% at 75 microgL(-1) in F0 and 23% at 25 microgL(-1) in F2. Reduction in 21-day fecundity ranged from 27 to 48% respectively at 25 and 75 microgL(-1) in F0 and from 43 to 71% respectively at 10 and 25 microgL(-1) in F2. Growth retardation caused a delay in deposition of first brood of 1.3 days at 75 microgL(-1) in F0, of 1.9 days at 25 microgL(-1) in F1 and of 5 days at 25 microgL(-1) in F2. Differences in respiration rates and egg dry mass between the control and exposed daphnids were mainly an indirect result of uranium effect on body size. The observed increase in toxic effects across generations indicated the necessity of carrying out multigeneration tests to assess environmental risk of uranium in daphnids.

Imaging Oxygen Distribution in Marine Sediments. The Importance of Bioturbation and Sediment Heterogeneity

The influence of sediment oxygen heterogeneity, due to bioturbation, on diffusive oxygen flux was investigated. Laboratory experiments were carried out with 3 macrobenthic species presenting different bioturbation behaviour patterns: the polychaetes Nereis diversicolor and Nereis virens, both constructing ventilated galleries in the sediment column, and the gastropod Cyclope neritea, a burrowing species which does not build any structure. Oxygen two-dimensional distribution in sediments was quantified by means of the optical planar optode technique. Diffusive oxygen fluxes (mean and integrated) and a variability index were calculated on the captured oxygen images. All species increased sediment oxygen heterogeneity compared to the controls without animals. This was particularly noticeable with the polychaetes because of the construction of more or less complex burrows. Integrated diffusive oxygen flux increased with oxygen heterogeneity due to the production of interface available for solute exchanges between overlying water and sediments. This work shows that sediment heterogeneity is an important feature of the control of oxygen exchanges at the sediment–water interface.

Aggregation and emergence in systems of ordinary differential equations

The aim of this article is to present aggregation methods for a system of ordinary differential equations (ODEs) involving two time scales. The system of ODEs is composed of the sum of fast parts and a perturbation. The fast dynamics are assumed to be conservative. The corresponding first integrals define a few global variables. Aggregation corresponds to the reduction of the dimension of the dynamical system which is replaced by an aggregated system governing the global variables at the slow time scale. The centre manifold theorem is used in order to get the slow reduced model as a Taylor expansion of a small parameter. We particularly look for the conditions necessary to get emerging properties in the aggregated model with respect to the nonaggregated one. We define two different types of emergences, functional and dynamical. Functional emergence corresponds to different functions for the two dynamics, aggregated and nonaggregated. Dynamical emergence means that both dynamics are qualitatively different. We also present averaging methods for aggregation when the fast system converges towards a stable limit cycle.

Aggregation and emergence in ecological modelling: integration of ecological levels

Modelling ecological systems implies to take into account different ecological levels: the individual, population, community and ecosystem levels. Two large families of models can be distinguished among different approaches: (i) completely detailed models involving many variables and parameters; (ii) more simple models involving only few state variables. The first class of models are usually more realistic including many details as for example the internal structure of the population. Nevertheless, the mathematical analysis is not always possible and only computer simulations can be performed. The second class of models can mathematically be analysed, but they sometimes neglect some details and remain unrealistic. We present here a review of aggregation methods, which can be seen as a compromise between these two previous modelling approaches. They are applicable for models involving two levels of organisation and the corresponding time scales. The most detailed level of description is usually associated to a fast time scale, while the coarser one rather corresponds to a slow time scale. A detailed model is thus considered at the individual level, containing many micro-variables and consisting of two parts: a fast and a slow one. Aggregation methods allow then to reduce the dimension of the initial dynamical system to an aggregated one governing few global variables evolving at the slow time scale. We focus our attention on the emerging properties of individual behaviours at the population and community levels.

2-D optical quantification of particle reworking activities in marine surface sediments

Abstract Particle and solute transport by faunal activities may significantly influence rates and pathways of organic matter mineralization during early diagenesis in surface sediments. One of the most frequently utilized techniques to quantify benthic biological reworking activities involves the calculation of a biodiffusion coefficient ( D b ) estimated from model predictions of 1-D tracer distribution patterns. This technique is labor-intensive and time-demanding. Furthermore, it is normally used for measurements over several days and averages overall transport mechanisms from 3-D to 1-D on a cm scale. In the frame of this work, we developed a new technique based on the nondestructive screening of fluorescent particles (luminophores) using optical discrimination and CCD camera detection of fluorescence (2-D). At a site characterized by a dense population of the brittle star Amphiura filiformis and a high biodiffusion coefficient (obtained from 1-D distributions; D b =35.5±3.7 cm −2 year −1 ; n =3), the optical reworking coefficient (ORC), estimated from the 2-D luminophore distribution patterns, was calculated (ORC=27.4±9.1 Δ Q cm −2 h −1 ; n =24). A nondestructive 2-D approach to quantify particle reworking may provide a powerful and complementary tool to further understand particle transport by the benthic fauna in surface sediments. The optical technique for 2-D detection of luminophores is relatively fast and easy to perform, with the ability to detect small scale (mm) particle movements on a time resolution of minutes or less.