Functional Effects of Parasites on Food Web Properties during the Spring Diatom Bloom in Lake Pavin: A Linear Inverse Modeling Analysis

This study is the first assessment of the quantitative impact of parasitic chytrids on a planktonic food web. We used a carbon-based food web model of Lake Pavin (Massif Central, France) to investigate the effects of chytrids during the spring diatom bloom by developing models with and without chytrids. Linear inverse modelling procedures were employed to estimate undetermined flows in the lake. The Monte Carlo Markov chain linear inverse modelling procedure provided estimates of the ranges of model-derived fluxes. Model results support recent theories on the probable impact of parasites on food web function. In the lake, during spring, when ‘inedible’ algae (unexploited by planktonic herbivores) were the dominant primary producers, the epidemic growth of chytrids significantly reduced the sedimentation loss of algal carbon to the detritus pool through the production of grazer-exploitable zoospores. We also review some theories about the potential influence of parasites on ecological network properties and argue that parasitism contributes to longer carbon path lengths, higher levels of activity and specialization, and lower recycling. Considering the “structural asymmetry” hypothesis as a stabilizing pattern, chytrids should contribute to the stability of aquatic food webs

Dynamique et fonctionnement des réseaux trophiques planctoniques dans la lagune de Bizerte (modélisation par la méthode inverse)

La lagune de Bizerte reçoit sur toute l'année des effluents de nature urbaine, industrielle et naturelle. De plus elle supporte une importante activité de pêche et de conchyliculture. L'objectif de ce travail est d'évaluer la variation des communautés planctoniques sur une échelle spatiale et saisonnière, l'état trophique de la lagune, la nature des réseaux trophiques planctoniques et l'impact des rejets et de la situation hydrologique sur les caractéristiques globales de la lagune. Afin de répondre à cet objectif, les échantillonnages sont menés en été 2004 au niveau de quatre stations : MA (influencée par les décharges urbaines) ; MB (influencée par des rejets industriels) ; MJ (à proximité d'un parc conchylicole) et R (centre de la lagune). Au cours de ces échantillonnages, facteurs abiotiques (températures, salinité, oxygène dissous, carbone organique dissous et particulaire et nutriments) et biotiques (bactérioplancton, pico, nano- et microphytoplancton, micro- et mésozooplancton) sont déterminés à trois profondeurs au niveau de chaque station. Parallèlement, des incubations in situ sont réalisées en été 2004 afin de mesurer 1) les taux de croissance des bactéries et du phytoplancton et de leur broutage par le microzooplancton et 2) les taux de sédimentation des particules. Au niveau de la station MJ, les échantillonnages et les incubations sont aussi conduits en automne 2004 et en hiver et au printemps 2005. En outre, les taux de production bactérienne et phytoplanctonique sont évalués à cette station pendant le printemps 2005. Les données des stocks de carbonne de chaque compartiment planctonique vivant et non vivant ainsi que les flux de carbonne recueillis sont intégrés dans une matrice de calcul. Cette dernière se base sur l'analyse inverse afin de quantifier les flux manquants et donner une image aux réseaux trophiques planctoniques. Une analyse des réseaux est appliquée par ma suite afin de mieux caractériser l'écosystème planctonique lagunaire de Bizerte.Bizerte lagoon, which supports intensive fishery activities and several aquaculture farms, is experiencing increasing anthropogenic pollutants loading from expanding urban, agricultural and industrial development along its shores. The focus of this thesis was to study the trophic structure and interactions within the planktonic food web in the lagoon, to assess the trophic status of the lagoon and the fate of biogenic carbon under different anthropogenic impacts. Four stations were investigated in summer 2004 (MA : impacted by urban discharge, MB : influenced by industrial activity, MJ : located in proximity of a shellfish farming sector, and R : in the lagoon central area). Station MJ was designed for a seasonal investigation. The sampling was carried out at three depths at each station for dissolved organic carbon (DOC), particulate organic carbon (POC), abundance, biomass and composition of phytoplankton, micro- and mesozooplankton. Dilution method was used to estimate growth of the three phytoplanktonic fractions and bacteria and their grazing by micrograzers. The sedimentation of carbon particles was also assessed (POC, phytoplankton, faecal pellets, and detrital matter). Primary and bacterial productions were determined in spring for MJ. Planktonic trophic food web of each station and season was modelled using the inverse method. The pattern of the estimated fluxes was characterized by network analysis.LA ROCHELLE-BU (173002101) / SudocSudocFranceF

Plankton food-web functioning in anthropogenically impacted coastal waters (SW Mediterranean Sea): an ecological network analysis

The study is the first attempt to (i) model spring food webs in three SW Mediterranean ecosystems which are under different anthropogenic pressures and (ii) to project the consequence of this stress on their function. Linear inverse models were built using the Monte Carlo method coupled with Markov Chains to characterize the food-web status of the Lagoon, the Channel (inshore waters under high eutrophication and chemical contamination) and the Bay of Bizerte (offshore waters under less anthropogenic pressure). Ecological network analysis was used for the description of structural and functional properties of each food web and for inter-ecosystem comparisons. Our results showed that more carbon was produced by phytoplankton in the inshore waters (966–1234 mg C m-2 d-1) compared to the Bay (727 mg C m-2 d-1). The total ecosystem carbon inputs into the three food webs was supported by high primary production, which was mainly due to >10 µm algae. However, the three carbon pathways were characterized by low detritivory and a high herbivory which was mainly assigned to protozooplankton. This latter was efficient in channelling biogenic carbon. In the Lagoon and the Channel, foods webs acted almost as a multivorous structure with a tendency towards herbivorous one, whereas in the Bay the herbivorous pathway was more dominant. Ecological indices revealed that the Lagoon and the Channel food webs/systems had high total system throughput and thus were more active than the Bay. The Bay food web, which had a high relative ascendency value, was more organized and specialized. This inter–ecosystem difference could be due to the varying levels of anthropogenic impact among sites. Indeed, the low value of Finn’s cycling index indicated that the three systems are disturbed, but the Lagoon and the Channel, with low average path lengths, appeared to be more stressed, as both sites have undergone higher chemical pollution and nutrient loading. This study shows that ecosystem models combined with ecological indices provide a powerful approach to detect change in environmental status and anthropogenic impacts

A new type of plankton food web functioning in coastal waters revealed by coupling Monte Carlo Markov Chain Linear Inverse method and Ecological Network Analysis

International audiencePlankton food webs (PFW) typology is based on different categories of functioning, according to the dominant processes and the role played by heterotrophic bacteria, small vs large phytoplankton, and small vs large zooplankton. Investigating the structure and the function of planktonic foodn webs in two SW Mediterranean waters (inshore and marine sites) at four seasons, using inverse (LIM-MCMC) and ecological network (ENA) analyses, we identified a new type of food web, called the “bacterial multivorous food web”. This food web adds to the conventional trophic continuum as previously reported. The “bacterial multivorous food web” present in winter showed the lowest primary production among seasons, but highest bacterial production. Several food web ratios characterized this new typology e.g. picophytoplankton net primary production to total primary production varied from 0.20 to 0.28; bacterial to primary production ratio is higher than values reported in global scale (≅1); bacterial net production to the potential protozoan prey net production was high (> 0.2). In this special food web, carbon was mostly recycled, with a moderate fraction channeled to deep waters, which lead to a higher retention of carbon inside the ecosystem. This winter PFW also seemed to be the most organized, specialized, stable and mature, as related to common interpretations of ENA. The spring was characterized by herbivorous food web, with highest activity coinciding with low stability. Although less usual, the herbivorous pathway was also observed during summer, in inshore waters. The autumn food webs, which functioned as multivorous or microbial food webs, appeared to be stable and mature. Finally, our study demonstrates the usefulness of food web models derived ratios combined with ecological network analysis indices to conduct evaluation of the structure and functioning of ecosystems and potentially to support management decisions in marine environment

The effect of physical drivers on ecosystem indices derived from ecological network comparison across estuarine ecosystems

The structure and function of estuarine food webs change in response to both natural and anthropogenic stresses. The construction of quantitative food webs and their analysis by means of Ecological Network Analysis provides outputs that have been used in many studies to assess system development, stress, robustness, resilience and maturity. Here we attempt to relate to the physical characteristics of the environment, ecosystem indices derived from Ecological Network Analysis. Ten models of food webs were gathered, across a selection of soft-bottom estuaries representative of a large morphological and hydrodynamic diversity, from wave-dominated to mixed energy tide-dominated systems. The selection allowed the comparison of their derived Ecological Network Analysis indices, because of similarities of accuracy in the representation of detritus and bacteria, and because models took into account all trophic levels up to top-predators. In order to obtain comparable physical characteristics, global models were used for a homogeneous description of tide and tidal prisms. Spearman correlations, hierarchical ascendant clustering and Redundancy Analysis were applied to examine the relationship between Ecological Network Analysis indices and physical characteristics. The set of four physical variables selected (catchment area, tidal range at neap tide, index of tide-wave domination and latitude in absolute value) explained 67% of the structure of the Ecological Network Analysis indices. This implies that the physical forcing related to climate, hydrodynamics and morphology is essential for determining the ecological emergent properties of the food web. In the European policy context of determining the 'good ecological status' of coastal ecosystems, it implies that the use of Ecological Network Analysis indices for basing the determination of operational indicators should be done, taking into account this context of a strong influence of physical factors. © 2012

Incorporating food-web parameter uncertainty into Ecopath-derived ecological network indicators

International audienceEcological network analysis (ENA) provides numerous ecosystem level indices offering a valuable approach to compare and categorize the ecological structure and function of ecosystems. The inclusion of ENA methods in Ecopath with Ecosim (EwE) has insured their continued contribution to ecosystem-based management. In EwE, ENA-derived ecological conclusions are currently based on single values of ENA indices calculated from a unique input flow matrix. Here, we document an easy-to-use routine that allows EwE users to incorporate uncertainty in EwE input data into the calculation of ENA indices. This routine, named ENAtool, is a suite of Matlab functions that performs three main steps: (1) import of an existing Ecopath model and its associated parameter uncertainty values in the form of uncertainty intervals into Matlab; (2) generation of an ensemble of Ecopath models with the same structure as the original, and with parameter values varying based on the prescribed uncertainty limits; and (3) calculation of a set of 13 ENA indices for each ensemble member (one set of flow values) and of summary statistics across the whole ensemble. This novel routine offers the opportunity to calculate ENA indices ranges and confidence intervals, and thus to perform quantitative data analyses. An application of ENAtool on a pre-existing Ecopath model of the Bay of Biscay continental shelf is presented, with a focus on the robustness of previously published ENA-based ecological traits of this ecosystem when the newly introduced uncertainty values are added. We also describe the sensitivity of the ENAtool results to both the number of ensemble members used and to the uncertainty interval set around each input parameter. Ecological conclusions derived from EwE, particularly those regarding the comparison of structural and functional elements for a range of ecosystem types or the assessment of ecosystem properties along gradients of environmental conditions or anthropogenic disturbances, will gain in statistical interpretability

Effects of experimental sediment resuspension on protozooplankton grazing activity: implication for the planktonic food web structure

International audienceThe effects of sediment elutriate, prepared from a sediment resuspension simulation process, on the growth of bacterioplankton and of different sized phytoplankton as well as on their grazing by protozooplankton were investigated in Mediterranean coastal ecosystems during spring phytoplankton bloom. In parallel, the response of bacterioplankton, phytoplankton and protozooplankton to sediment elutriate enrichment was assessed in microcosm experiments. Elutriate addition provoked a high enrichment in nutriments of the overlying water, particularly in ammonium, dissolved organic matter but also in trace metal elements and herbicides. Overall, elutriate addition significantly increased the biomass of bacterioplankton and of total phytoplankton, but the phytoplankton size fractions responded differently to elutriate enrichment. The picophytoplankton showed a significant strong enhancement in both growth and production rates, relatively to control, while these rates significantly decreased for nano- and microphytoplankton. Accordingly, the phytoplankton size structure shifted in elutriate microcosm toward a dominance of the picophytoplankton concomitant with a significant reduction in larger size fractions. The elutriate addition caused also a clear shift in the taxonomic composition of protozooplankton, associated with a significant modification of its functional diversity, with the dominance of pico-sized cell consumers (such as aloricate ciliates, heterotrophic nanoflagellates and mixotrophic dinoflagellates) in the detriment of grazers of nano- and microphytoplankton (such as heterotrophic dinoflagellate). The protozooplankton grazing on small prey (bacterioplankton and picophytoplankton) increased, while feeding on nano- and micro-sized phytoplankton decreased after elutriate addition. These results have implication for the carbon transfer pathway, which could change from an herbivorous food web to a microbial food web, influencing so the exportation of biogenic carbon through the other trophic compartments

Network analysis of the planktonic food web during the spring bloom in a semi-enclosed lagoon (Arcachon, SW France)

International audienceThe structure and functioning of the food web in Arcachon Bay (Bay of Biscay, Northeast Atlantic Ocean) was analyzed during the spring bloom period to evaluate the sensitivity of this ecologically and economically important ecosystem perturbation. Differences in the timing of the peaks in phytoplankton and zooplankton populations occur due to a mismatch between primary production and grazing. Using an inverse approach based on in situ experimental data, an ecological network analysis was carried out to characterize emergent properties of the food web and to estimate carbon flows. The data set was composed of rate measurements for net primary production, import and export of dissolved organic carbon, and grazing of heterotrophic nanoflagellates and ciliates by metazoan micro- and mesozooplankton. Ecological network analysis indices were calculated on the estimated fluxes and compared to values from plankton models built with exactly the same method. The largest activities in the resulting model came from the nano- and microphytoplankton. The detritivory/herbivory ratio, the recycling rates and the relative redundancy of the system were very low compared to other planktonic systems, even in similar periods of bloom. These values indicate a transitional system with poor resilience that exports a large quantity of carbon either to the benthos where it is consumed by non-planktonic consumers such as oysters, or else to systems outside of the bay (outwelling)