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

Assessing the state of marine biodiversity in the Northeast Atlantic

The Northeast Atlantic, a highly productive maritime area, has been exposed to a wide range of direct human pressures, such as fishing, shipping, coastal development, pollution, and non-indigenous species (NIS) introductions, in addition to anthropogenically-driven global climate change. Nonetheless, this regional sea supports a high diversity of species and habitats, whose functioning provides a variety of ecosystem services, essential for human welfare. In 2017, OSPAR, the Northeast Atlantic Regional Seas Commission, delivered an assessment of marine biodiversity for the Northeast Atlantic. This assessment examined biodiversity indicators separately to identify changes in Northeast Atlantic biodiversity, but stopped short of determining the status of biodiversity for many species and habitats. Here, we expand on this work and for the first time, a semi-quantitative approach is applied to evaluate holistically the state of Northeast Atlantic marine biodiversity across marine food webs, from plankton to top predators, via fish, pelagic and benthic habitats, including xeno-biodiversity (i.e. NIS). Our analysis reveals widespread degradation in marine ecosystems and biodiversity, particularly for marine birds and coastal bottlenose dolphins, as well as for benthic habitats and fish in some regions. The poor biodiversity status of these ecosystem components is likely the result of cumulative effects of human activities, such as habitat destruction or disturbance, overexploitation, eutrophication, the introduction of NIS, and climate change. Bright spots are also revealed, such as recent signs of recovery in some fish and marine bird communities and recovery in harbour and grey seal populations and the condition of coastal benthic communities in some regions. The status of many indicators across all ecosystem components, but particularly for the novel pelagic habitats, food webs and NIS indicators, however, remains uncertain due to gaps in data, unclear pressure-state relationships, and the non-linear influence of some pressures on biodiversity indicators. Improving monitoring and data access and increasing understanding of pressure-state relationships, including those that are non-linear, is therefore a priority for enabling future assessments, as is consistent and stable resourcing for expert involvement

Large-scale regional comparisons of ecosystem processes: Methods and approaches

Large-scale regional marine ecosystems can be compared for various processes that include their structure and biodiversity, functioning, services, and effects on biogeochemical processes. The comparisons can proceed from data up, or from conceptual models down, or from a combination of models and data. This study proposes a typology of methods and approaches that are currently used, or could possibly be used for making large-scale ecosystem comparisons. The various methods and approaches are illustrated with examples drawn from the literature. © 2011 Elsevier B.V

Network analysis and inter-ecosystem comparison of two intertidal mudflat food webs (Brouage Mudflat and Aiguillon Cove, SW France)

Network analysis was used to analyse steady-state models of the food webs of two intertidal mudflat ecosystems: Aiguillon Cove and Brouage Mudflat, on the South-Western Atlantic Coast of France. The aim was to highlight emergent properties of food-web functioning in these two ecosystems and to compare these properties with other coastal ecosystems. Both ecosystems imported detritus in parallel to a high benthic primary production. They were characterised by a high diversity of resources. Both also exported living material, leading to a high quality production, quantified as export of Exergy. This export was mainly composed of cultivated bivalves during the cold season for Brouage Mudflat, and of the migration of grazing fish in Aiguillon Cove during the warm season. Their internal organization was characterised by short pathways, high recycling, high redundancy and low net ecosystem production, compared to the other systems selected. These characteristics, encountered in many estuaries, presented less extreme values. © 2007 Elsevier Ltd. All rights reserved

Planktonic food webs revisited: Reanalysis of results from the linear inverse approach

Identification of the trophic pathway that dominates a given planktonic assemblage is generally based on the distribution of biomasses among food-web compartments, or better, the flows of materials or energy among compartments. These flows are obtained by field observations and a posteriori analyses, including the linear inverse approach. In the present study, we re-analysed carbon flows obtained by inverse analysis at 32 stations in the global ocean and one large lake. Our results do not support two "classical" views of plankton ecology, i.e. that the herbivorous food web is dominated by mesozooplankton grazing on large phytoplankton, and the microbial food web is based on microzooplankton significantly consuming bacteria; our results suggest instead that phytoplankton are generally grazed by microzooplankton, of which they are the main food source. Furthermore, we identified the "phyto-microbial food web", where microzooplankton largely feed on phytoplankton, in addition to the already known "poly-microbial food web", where microzooplankton consume more or less equally various types of food. These unexpected results led to a (re)definition of the conceptual models corresponding to the four trophic pathways we found to exist in plankton, i.e. the herbivorous, multivorous, and two types of microbial food web. We illustrated the conceptual trophic pathways using carbon flows that were actually observed at representative stations. The latter can be calibrated to correspond to any field situation. Our study also provides researchers and managers with operational criteria for identifying the dominant trophic pathway in a planktonic assemblage, these criteria being based on the values of two carbon ratios that could be calculated from flow values that are relatively easy to estimate in the field. © 2013 Elsevier Ltd

Community structure of digenean parasites of sparid and labrid fishes of the Mediterranean sea: A new approach

The aim of this work was to study the structure of the parasite communities of Digeneans of 2 families of Teleost fishes (Sparidae and Labridae) of the Mediterranean sea. We tried to quantify the importance of both the microhabitat requirements of the parasite species and the effect of host biological factors on the parasite communities. We applied, for the first time in parasite community studies, the Canonical Correspondence Analysis (CCA) to analyse (i) the spatial distribution of parasite species within the digestive tract of the hosts; (ii) the host's biological factors (such as diet, host length, gregariousness and abundance) that may influence this spatial distribution of parasite species. Our results showed that potential microhabitats were vacant in the 2 host families studied revealing a lack of niche saturation because either there was little inter- and/or intraspecific competition or there were enough available space and resources within the host. Our results also indicated that the position of the parasite in the digestive tract is much more important than host biological factors for the structure of parasite community. Finally, we highlight the potential use of the CCA method for controlling for phylogenetic constraints in multi-species analyses

Preface

Editorial[No abstract available

Modeling the impact of oyster culture on a mudflat food web in Marennes-Oléron Bay (France)

International audienceWe used a carbon-based food web model to investigate the effects of oyster cultivation on the ecosystem of an intertidal mudflat. A previously published food web model of a mudflat in Marennes-Oléron Bay, France, was updated with revised parameters, and a realistic surface area and density of existing oyster cultures on the mudflat. We developed 2 hypothetical scenarios to estimate the impact of oyster cultivation on the food web structure of the ecosystem: one with no oysters, the other with a doubled area devoted to cultivated oysters in the bay. Oysters are direct trophic competitors of other filter feeders, and their presence modifies benthic-pelagic coupling by forcing a shift from pelagic consumers to benthic consumers. Increasing the surface area of cultivated oysters caused secondary production to increase, providing food for top predators (in particular juvenile nekton), reinforcing the nursery role of the mudflat in the ecosystem, and altering the species composition available to the top predators