Effects of temporal fluctuation in population processes of intertidal Lanice conchilega (Pallas, 1766) aggregations on its ecosystem engineering

Ecosystem engineers contribute to ecosystem functioning by regulating key environmental attributes, such as habitat availability and sediment biogeochemistry. While autogenic engineers can increase habitat complexity passively and provide physical protection to other species, allogenic engineers can regulate sediment oxygenation and biogeochemistry through bioturbation and/or bioirrigation. Their effects rely on the physical attributes of the engineer and/or its biogenic constructs, such as abundance and/or size. The present study focused on tube aggregations of a sessile, tube-building polychaete that engineers marine sediments, <i>Lanice conchilega</i>. Its tube aggregations modulate water flow by dissipating energy, influencing sedimentary processes and increasing particle retention. These effects can be influenced by temporal fluctuations in population demographic processes. Presently, we investigated the relationship between population processes and ecosystem engineering through an <i>in-situ</i> survey (1.5 years) of <i>L. conchilega</i> aggregations at the sandy beach of Boulogne-sur-Mer (France). We (1) evaluated temporal patterns in population structure, and (2) investigated how these are related to the ecosystem engineering of <i>L. conchilega</i> on marine sediments. During our survey, we assessed tube density, demographic structure, and sediment properties (surficial chl-<i>a</i>, EPS, TOM, median and mode grain size, sorting, and mud and water content) on a monthly basis for 12 intertidal aggregations. We found that the population was mainly composed by short-lived (6-10 months), small-medium individuals. Mass mortality severely reduced population density during winter. However the population persisted, likely due to recruits from other populations, which are associated to short- and long-term population dynamics. Two periods of recruitment were identified: spring/summer and autumn. Population density was highest during the spring recruitment and significantly affected several environmental properties (<i>i.e.</i> EPS, TOM, mode grain size, mud and water content), suggesting that demographic processes may be responsible for periods of pronounced ecosystem engineering with densities of approx. 30 000 ind m^-2

Assessing the utility of fisheries-dependent data to support mixed fisheries management: a spatiotemporal simulation framework

Fishers exploit populations that are heterogeneously distributed in space and time without full knowledge of species distributions and with fishing gear that is not fully selective. The ability to change catch composition is limited by species mix at a particular location and time and the capture characteristics of the fishing gear. Models capturing the dynamics of the fisheries (‘fleet dynamics’ models) are often simplistic due to a lack of knowledge of the processes driving these catches, which occur both at large and small scales. We develop a simulation framework to investigate the importance of scaling on the interactions between fish populations and fisheries dynamics. The framework provides i) a realistic but tractable biological model of fish populations in space and time, including daily population processes (mortality, growth and recruitment) and population movement implemented as a combination of diffusive density-dependent processes and migrations; and, ii) a realistic fishing simulation model to capture how fishers may exploit heterogeneously distributed fish populations with different values and uncertain knowledge about the underlying spatial processes. We generate a model system where we investigate the consequences of scaling and data aggregation and validate this simulated data against data collected on fisheries operating in the Celtic Sea. The simulation allows a more in-depth understanding of factors important when using fisheries-dependent data to develop spatial management measures, from the micro- to the large-scale and individual to population processes, not otherwise possible due to the limitations in ‘real-world’ spatiotemporal data on fish distributions

Alternate Stable States Theory: Critical Evaluation and Relevance to Marine Conservation

In their 2023 book, “The Blue Compendium: From Knowledge to Action for a Sustainable Ocean Economy”, Lubchenko and Haugan invoked alternate stable (AS) states marginally as an undesired consequence of sources of disturbance on populations, communities and ecosystems. They did not provide detailed arguments, but considered the existence of AS states as a given. Conversely, May, in his 1977 Nature article, pointed out that, when applied to systems that are complex, “the [AS states] theory remains largely metaphorical”. This is the starting point of this critical review, which aims to re-examine the general theory behind AS states in ecological systems and its applications to marine ecology and conservation. The focus is first on theory, taking as examples communities that sustain competition and studying the relative importance of the fluxes of individuals between simple low-dimension, interconnected systems. We find that a minimal formulation of fluxes is sufficient to obtain a set of non-null multiple stable (MS) states and to trigger shifts between AS states when fluxes become large enough. This provides new insights into the theory of rescue and mass effects by distinguishing them through a threshold at which the system dynamics shift from one stable equilibrium to another. Then, we consider how the theoretical framework of AS states has been applied in marine environments. It appears that many applications have confounded shifts between AS states and changes in the structure of systems, particularly when the complexity of the systems increases. The main difficulty for any application remains that the concepts of MS and AS states can only be established and validated for low-dimension systems and simplified experiments. This is because the mathematical properties of models that describe large-dimension, complex systems deviate from the observed characteristics of their real-world counterparts. There are many intriguing scientific challenges around the plausible shifts between AS states, but a deeper understanding and characterization of their occurrence in nature would require a significant investment in modeling to formulate predictive ecosystem models

Modélisation de la dynamique du microphytobenthos des vasières intertidales du bassin de Marennes-Oléron. Effets des synchroniseurs physiques sur la régulation de la production

The study aimed at quantifying the spatio-temporal evolution of microphytobenthic biomass in intertidal mudflats of Marennes-Oléron Bay. To reach this goal, a dynamic model has been proposed, including processes of primary production, active migration of epipelic diatoms and biomass lasses due to mortalities on one hand and resuspension in the water column during submersion on the other hand. The spatialisation is given by a hydrodynamic madel which calculates the water height everywhere. The photic environment of microphytobenthos is described in a discrete way, by means of two layers: the biofilm of epipelic algae « S »on the top of the sediment and the fust centimeter of the mud « F ». The endogenous vertical migration rhythm of epipelic algae controls the cell exchange between both layers: (i) an upward movement from F to S during the diurnal exposure period and a downward movement from S to F during night exposure and submersion periods. Because of this behavioural difference, two systems of ordinary differentiai equations have been formulated. During the diurnal exondation, only microalgae at the surface are able to photosynthesize at the level of the incident radiations. However, the number of cells in S is limited, and the exchanges from F to S are density-dependent. Production processes were studied experimentally; particularly, the shortterm temperature effect on the microphytobenthic photosynthetic capacity (Pmax) was quantified. In relation to it, a madel of the Mud Surface Temperature dynamics was developped to analyse the spatio-temporal dynamics of MST and Pmax. It was thus found out that microalgal photosynthesis exhibited thermo-inhibition during summer on mudflats. Loss terms were frrst considered to be global and were formulated as linear functions of the microphytobenthic biomass. It is shown that such a system, with fast commutations from one state to the other, reaches a cyclic equilibrium which was further validated by observed data series. In addition, the model reproduces the characteristics of the local seasonal dynamics and particularly the decrease of biomass in summer. However, the loss processes were not defmed weil enough to allow an accurate representation of the observed spatial structures. Sorne theoretical developments are proposed to improve the model formulation ; such new perspectives call for additional experiments in conjunction with the modelling approach. So, the existing system of equations can be used as a basis for future developments with the aim of testing hypotheses about microphytobenthos dynamics.L'objectif de l'étude est de quantifier la dynamique spatiotemporelle de la biomasse microphytobenthique des vasières intertidales de Marennes-Oléron. Un modèle dynamique local a été formulé dans ce but; il incorpore les processus de production, de migration active des microalgues épipéliques et de pertes de biomasse par mortalités et par remise en suspension dans la colonne d'eau. La spatialisation est conditionnée par un modèle hydrodynamique qui calcule la hauteur d'eau en tout point. L'environnement photique du microphytobenthos est décrit de façon discrète par une couche "S" de diatomées en surface du sédiment et une couche "F" représentant le centimètre superficiel de la vase. La migration verticale active des cellules définit les échanges entre ces deux couches, orientés du fond vers la surface en période  d'exondation diurne et de la surface vers le fond en périodes d'exondation nocturne et de submersion. A cause de cette différence de comportement migratoire, deux sous-sytèmes d'équations différentielles ont été  formulés. Pendant la période d'exondation diurne, seules les algues en surface peuvent photosynthétiser, en bénéficiant de la totalité de la lumière incidente; le nombre de places en surface est limité si bien que les échanges «du fond vers la surface» sont densité-dépendants. Les processus de production ont été étudiés expérimentalement ; en particulier, le contrôle à court-terme de la capacité photosynthétique par la température a été mis en équation. Par la suite, un modèle décrivant la dynamique de la température de surface de la vase a permis de caractériser le forçage des potentialités de production à l'échelle de l'écosystème et a mis en évidence un phénoméne de thermo-inhibition qui se généralise en été. Les processus décrivant les pertes de biomasses sont considérés comme globaux et sont des fonctions linéaires de la biomasse rnicrophytobenthique. Il a été montré qu'un tel système, commutant rapidement entre les deux états (exondation diurne et exondation nocturne+submersion), atteignait un équilibre cyclique, validé localement par des observations. Le modèle peut en outre expliquer la dynamique saisonnière, particulièrement la baisse locale de biomasse en été. Cependant, les processus qui décrivent les pertes de biomasse ne sont pas assez bien définis pour permettre de représenter correctement les structures spatiales observées. Des perspectives sont proposées pour y remédier, qui nécessitent de nouvelles expérimentations ; le modèle existant peut alors servir de base pour tester les hypothèses du fonctionnement du système

A First Individual-Based Model to Simulate Humpback Whale (Megaptera novaeangliae) Migrations at the Scale of the Global Ocean

Whale migrations are poorly understood. Two competing hypotheses dominate the literature: 1. moving between feeding and breeding grounds increases population fitness, 2. migration is driven by dynamic environmental gradients, without consideration of fitness. Other hypotheses invoke communication and learned behaviors. In this article, their migration was investigated with a minimal individual-based model at the scale of the Global Ocean. Our aim is to test if global migration patterns can emerge from only the local, individual perception of environmental change. The humpback whale (Megaptera novaeangliae) meta-population is used as a case study. This species reproduces in 14 zones spread across tropical latitudes. From these breeding areas, humpback whales are observed to move to higher latitudes seasonally, where they feed, storing energy in their blubber, before returning to lower latitudes. For the model, we developed a simplified ethogram that conditions the individual activity. Then trajectories of 420 whales (30 per DPS) were simulated in two oceanic configurations. The first is a homogeneous ocean basin without landmasses and a constant depth of −1000 m. The second configuration used the actual Earth topography and coastlines. Results show that a global migration pattern can emerge from the movements of a set of individuals which perceive their environment only locally and without a pre-determined destination. This emerging property is the conjunction of individual behaviors and the bathymetric configuration of the Earth’s oceanic basins. Topographic constraints also maintain a limited connectivity between the 14 DPSs. An important consequence of invoking a local perception of environmental change is that the predicted routes are loxodromic and not orthodromic. In an ocean without landmasses, ecophysiological processes tended to over-estimate individual weights. With the actual ocean configuration, the excess weight gain was mitigated and also produced increased heterogeneity among the individuals. Developing a model of individual whale dynamics has also highlighted where the understanding of whales’ individual behaviors and population dynamic processes is incomplete. Our new simulation framework is a step toward being able to anticipate migration events and trajectories to minimize negative interactions and could facilitate improved data collection on these movements

Can the intertidal benthic microalgal primary production account for the "Missing Carbon Sink"?

available : https://www.obs-vlfr.fr/Journal/index.php/JORD/article/view/41/17International audienceThis communication emphasizes the role of intertidal benthic primary production in the global carbon cycle. Despite an increasing number of publications about benthic diatoms in specialized marine biology and ecology journals, this ecological compartment has not yet caught the attention of global biogeochemical cycle investigators. The idea that a large number of small contributions can be responsible for the estimated 'missing carbon sink' at a global scale has been suggested, although not addressed explicitly. In this context, the study focussed on the benthic microalgal compartment (the so-called "microphytobenthos") of shallow-water ecosystems. It is a very productive compartment in terms of carbon assimilation (several orders of magnitude higher than the phytoplankton one). This study demonstrated that resuspension of this biomass by tidal oscillations over intertidal flats, even in low hydrodynamic conditions, generates a net carbon flux from the littoral zones to open coastal basins. Measurements allowed us to estimate that this microphytobenthic compartment, which colonizes a small fraction of the total Earth surface (smaller than the accuracy of the ecosphere surface estimate), could however generate a flux corresponding to the "missing carbon sink"

Designing the Next Generation of Condition Tracking and Early Warning Systems for Shellfish Aquaculture

Early detection of environmental disturbances affecting shellfish stock condition is highly desirable for aquaculture activities. In this article, a new biophysical model-based early warning system (EWS) is described, that assesses bivalve stock condition by diagnosing signs of persistent physiological dysfunctioning. The biophysical model represents valve gape dynamics, controlled by active contractions of the adductor muscle countering the passive action of the hinge ligament; the dynamics combine continuous convergence to a steady-state interspersed with discrete closing events. A null simulation was introduced to describe undisturbed conditions. The diagnostic compares valve gape measurements and simulations. Indicators are inferred from the model parameters, and disturbances are assessed when their estimates deviate from their null distribution. Instead of focusing only on discrete events, our EWS exploits the complete observed dynamics within successive time intervals defined by the variation scales. When applied to a valvometry data series, collected in controlled conditions from scallops (Pecten maximus), the EWS indicated that one among four individuals exhibited signs its physiological condition was degrading. This was detected neither during experiments nor during the initial data analysis, suggesting the utility of an approach that quantifies physiological mechanisms underlying functional responses. Practical implementations of biological-EWS at farming sites are then discussed

Modelling the Dynamics of Outbreak Species: The Case of <i>Ditrupa arietina</i> (O.F. Müller), Gulf of Lions, NW Mediterranean Sea

An outbreak species exhibits extreme, rapid population fluctuations that can be qualified as discrete events within a continuous dynamic. When outbreaks occur they may appear novel and disconcerting because the limiting factors of their dynamics are not readily identifiable. We present the first population hybrid dynamic model that combines continuous and discrete processes, designed to simulate marine species outbreaks. The deterministic framework was tested using the case of an unexploited benthic invertebrate species: the small, serpulid polychaete Ditrupa arietina. This species is distributed throughout the northeast Atlantic Ocean and Mediterranean Sea; it has a life cycle characterised by a pelagic dispersive larval stage, while juveniles and adults are sedentary. Sporadic reports of extremely high, variable densities (from 10,000 ind.m−2) have attracted attention from marine ecologists for a century. However, except for one decade-long field study from the Bay of Banyuls (France, Gulf of Lions, Mediterranean Sea), observations are sparse. Minimal formulations quantified the processes governing the population dynamics. Local population continuous dynamics were simulated from a size-structured model with a null immigration–emigration flux balance. The mathematical properties, based on the derived hybrid model, demonstrated the possibilities of reaching an equilibrium for the population using a single number of recruits per reproducer. Two extrapolations were made: (1) local population dynamics were simulated over 180 years using North Atlantic Oscillation indices to force recruitment variability and (2) steady-state population densities over the Gulf of Lions were calculated from a connectivity matrix in a metapopulation. The dynamics reach a macroscopic stability in both extrapolations, despite the absence of density regulating mechanisms. This ensures the persistence of D. arietina, even when strong, irregular oscillations characteristic of an outbreak species are observed. The hybrid model suggests that a macroscopic equilibrium for a population with variable recruitment conditions can only be characterised for time periods which contain several outbreak occurrences distributed over a regional scale

Flux dissous à l'interface eau-sédiment sous des écoulements oscillants

La diagénèse précoce résulte des transferts de matières à travers l'interface eau-sédiment. Ce processus est contraint par la disponibilité en oxygène, par l'environnement physique de l'interface eau-sédiment (porosité et perméabilité du sédiment) et par le forçage hydrodynamique. Lorsque ce dernier est instationnaire, un couplage fort peut s'établir avec la dynamique biochimique. Dans les sédiments non-perméables, les échanges à l'interface se produisent via diffusion. Un modèle de diffusion-réaction, décrivant la dynamique des transferts diffusifs d'oxygène lorsque la colonne d'eau est soumise à un écoulement oscillant, montre une stimulation de l'oxygénation du sédiment sous des écoulements oscillants énergétiques. La compétition entre le temps de diffusion à travers la couche limite diffusive, le temps accordé à cette diffusion pendant une période de l'onde, et le temps de consommation de l'oxygène dans le sédiment, régule la dynamique non-linéaire des flux diffusifs à l'interface eau-sédiment. Dans les sédiments perméables, les processus advectifs jouent un rôle prédominant dans les échanges à travers l'interface. Ainsi, les réservoirs de sels nutritifs peuvent être rendus disponibles pour la production primaire pélagique lors d'épisodes de vagues énergétiques (pompage subtidal). Un suivi hebdomadaire des concentrations en sels nutritifs de l'eau pélagique et de l'eau interstitielle a été réalisé en deux stations de la baie de Banyuls. De plus, une exploration spatiale de la perméabilité, de la granulométrie et des stocks en sels nutritifs a été menée en baie de Banyuls et dans le golfe du Lion. Un bilan de la contribution des apports par pompage subtidal est proposé.PARIS-BIUSJ-Sci.Terre recherche (751052114) / SudocSudocFranceF

An inference procedure for behavioural studies combining numerical simulations, statistics and experimental results

The technical difficulties of performing underwater observation mean that marine ecologists have long relied on behavioural experiments to study reactions of marine organisms. In this article, we examine the underlying complexity of assumptions made in raceway experiments and we propose a statistical inference procedure tailored to this type of experimental protocol. As an example, experiments were performed to test if light of two different intensities affects the proximal behaviour (i.e. direct, local and immediate) of two species of crustaceans, the hermit crab (Pagurus bernhardus), and the green crab (Carcinus maenas). Individuals were collected in the vicinity of the Sven Loven Marine Center in Tjarno (Sweden). Their movements in raceways were recorded and the statistical distance between the resulting experimental distribution and a simulated null distribution was used to compare their behaviour in two situations: dim (when they were expected to feed) and bright light (when they were expected to shelter). Initial tests indicated no differences of behaviour between dim and bright light for the two species. However, when compared with the reference state (here, a null distribution) the behaviour in dim light deviates significantly from the null distribution suggesting non-random behaviour. Our results suggest that efforts should be made to understand the behaviours of the individuals of these two species to establish a comprehensive reference state as a basis for comparison. This fundamental information should be a prerequisite before implementing experiments testing how potential disturbances affect individual organisms in behavioural ecology