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See detailCorsican seagrass detritus: An opportune shelter or a copepod Eldorado?
Mascart, Thibaud ULg; Lepoint, Gilles ULg; Remy, François ULg et al

Poster (2014, March 07)

Seagrass ecosystems are extensive beds of marine flowering plants bordering tropical and temperate coastal regions. They play an important role in maintaining biological productivity and bio-geochemical ... [more ▼]

Seagrass ecosystems are extensive beds of marine flowering plants bordering tropical and temperate coastal regions. They play an important role in maintaining biological productivity and bio-geochemical cycles in the sea and support higher diversity and abundance of fauna in comparison to adjacent non-vegetated areas. The seagrass meadow primary production can be directly consumed through herbivory but the majority of the plant material falls on the sea floor during the autumnal leaf senescence. The leaf litter then degrades within the meadow or accumulates with other micro- and macrophytodetritus to form detritus accumulations on the adjacent non-vegetated sand patches. These exported accumulations are quite dynamic in relation to seafloor geomorphology and local hydrodynamics. Thus, the detritus accumulations are an easily disturbed ephemeral environment with one large influx a year. Consequently the physico-chemical characteristics can change very fast and impact the sheltering capacity and food supply present. Nonetheless, fishes, macrofauna and meiofauna are omnipresent throughout the year. In our study site along the shore of N-W Corsica, Posidonia oceanica seagrass meadows are characterised by substantial detritus accumulations. The present study aimed to analyse the biodiversity of the copepod species communities (Crustacea, Copepoda) in those detritus accumulations. The results showed that the copepod detritus community consisted of a mixture of species that are also found in adjacent habitats (seagrass meadow, sediment, epilithic habitats, water column). Each adjacent habitat is characterised by organisms that are morphologically adapted to the specific features of that habitat. The majority of copepods are epiphytic (order Harpacticoida), that occur typically on seagrass leaves and macroalgae. Other species are planktonic (orders Cyclopoida and Calanoida) and some were benthic (order Harpacticoida), known from the nearby sediment. A minority of the copepod community were parasitic on fish or invertebrate (order Siphonostomatoida). In order to clarify their origin, we assume that passive transport by currents plays a significant role next to the active migration from the anoxic sediments under the detritus. For sure they also reproduce within the detritus packages as we found many nauplii, copepodites and gravid females. The above mentioned suggestions cannot explain such high density of copepods by themselves. Other attraction mechanisms are needed to explain the important amount of planktonic and epiphytic species with good swimming ability, such as higher food accessibility. In the detritus no plant-defence mechanisms are present anymore and a lot of micro-organisms and thus potential food sources are present. Furthermore, the dense detritus package provides shelter and protection from potential predators. Subsequently we may consider the detritus accumulations as a copepod species-specific opportune Eldorado for sheltering, nursing and feeding. [less ▲]

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See detailTrophic and specific diversity of harpacticoid copepods associated to Posidonia oceanica macrophytodetritus
Mascart, Thibaud ULg; De Troch, Marleen; Remy, François ULg et al

Poster (2012, August 21)

Extended meadows of living Posidonia oceanica plants in the Mediterranean Sea produce large amounts of detritus of dead seagrass plants that are packed at the bottom of the sea. In spite of their large ... [more ▼]

Extended meadows of living Posidonia oceanica plants in the Mediterranean Sea produce large amounts of detritus of dead seagrass plants that are packed at the bottom of the sea. In spite of their large quantities, these phytodetritus are of low nutritional quality (high C:N:P ratio). However, these detritus are massively colonised by bacterial communities, fungi, diatoms, meiofauna and macrofauna. This leads to the assumption that those associated communities enrich the litter and play an important role in the energy transfer to higher trophic levels like macrofauna and juvenile fish that use these accumulations as nursery and feeding grounds. In these litter accumulations harpacticoid copepods (Crustacea) are the main meiofauna players (metazoans in the size range of 38µm – 1mm). Their families are characterised by different specialized morphologies (body form and appendages). Nonetheless their morphological differences they are all grazers and seem to feed on similar sources. Ecological theories state that diversity of trophic niches is an essential parameter to explain specific diversity. Therefore subtle trophic niches may occur among species assemblages, linked to the complexity of the phytodetritus. In order to unravel the ecological function, trophic relations, seasonal fluctuations and habitat interactions in these litter accumulations, a bulk stable isotope analysis (SIA) is conducted. The isotopic composition of C and N of the potential food sources and the most dominant harpacticoid copepod families are measured using an EA-IRMS coupling. The results are run in a SIAR Beyesian mixing model to calculate the approximate contributions of each potential food sources towards the composition of different families of harpacticoid copepods present in the macrophytodetritus. [less ▲]

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See detailLeaf fall impact on diversity and trophic ecology of vagile macrofauna associated with exported P.oceanica litter
Remy, François ULg; Mascart, Thibaud ULg; Dauby, Patrick ULg et al

Poster (2012, August 20)

In the Mediterranean Sea, Posidonia oceanica meadows produce a huge amount of detritus, evaluated up to 300 to 2000 g dry wt m-2 yr-1. This litter is mainly composed of dead leaves but also of uprooted P ... [more ▼]

In the Mediterranean Sea, Posidonia oceanica meadows produce a huge amount of detritus, evaluated up to 300 to 2000 g dry wt m-2 yr-1. This litter is mainly composed of dead leaves but also of uprooted P.oceanica shoots and drift macro-algae from adjacent rocky bottoms. Although rich in refractory materials (lignin) and poor in P and N, these underwater accumulations of leaves are colonised by fungi, micro-algae (like diatoms), bacteria, but also by micro and macrofauna assemblages. These organisms could play an important role in leaf litter degradation and enrichment, but also in energy and carbon transfer from P.oceanica to higher trophic levels in adjacent coastal ecosystems. In this study we focus on the vagile macro-fauna (invertebrates with a size > 500µm) inhabiting the exported litter accumulations of the Calvi Bay (France). We took standardised samples at two different sites (a sheltered one and an exposed one) before and after leaf fall. We emphasised that crustaceans represent 65 – 85% of the biodiversity, followed by annelids and molluscs, representing respectively 10-20% and 10-15% of the diversity. That general pattern differs between sampling sites and we highlighted changes after leaf fall at both sites. In order to assess the impact of the autumn period litter input on the trophic structure of these invertebrates, we conducted gut contents observations and “bulk” stable isotope analysis. The isotopic compositions of C and N stable isotopes of the potential detritic food sources and of the most abundant invertebrate’s species were measured using EA-IRMS. We finally focused on the two most abundant Gammaridean Amphipoda species representing up to about 60% of the vagile macrofauna found in litter accumulations: Gammarella fucicola and Gammarus aequicauda. The results of their isotopic measurements were used in the “SIAR” Bayesian mixing model to calculate the potential contribution of their potential food sources. [less ▲]

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See detailA descriptive study of physico-chemical characteristics of Posidonia oceanica litter accumulation
Lepoint, Gilles ULg; Borges, Alberto ULg; Darchambeau, François ULg et al

Poster (2012)

The Mediterranean seagrass Posidonia oceanica meadow losses every fall the major part of its leaf biomass after senescing. These phytodetritus may decay within the meadow, be buried or be exported to ... [more ▼]

The Mediterranean seagrass Posidonia oceanica meadow losses every fall the major part of its leaf biomass after senescing. These phytodetritus may decay within the meadow, be buried or be exported to other habitats. They form large litter accumulations, notably on shallow water sand patches. Such accumulation host many organisms which participate to the degradation of this material. In a first step to understand the dynamics of these accumulations and of their associated biota, we have characterised their physico-chemical heterogeneity at different seasons. We measured the dissolved oxygen, nutrients and sulphide concentrations in interstitial waters from litter accumulations varying regarding their phytodetritus composition, fragmentation level and thickness. Results show that oxygen conditions were highly variable depending on litter thickness but also on local hydrodynamics. Anoxic conditions and presence of sulphide were sometimes measured, particularly in very thick litter or in degraded litter at the end of summer. Colonies of sulphur-oxidising bacteria were observed. Litter accumulations were also often enriched in ammonium and, sometimes, in dissolved phosphorus. It is not clear whether this results from the litter degradation within the accumulation or whether this is a consequence of a barrier effect between sediment and water column. Nevertheless, this makes litter accumulations particularly attractive for micro-phytobenthic producers. Litter accumulations appear as key habitats both to understand the dead-face of seagrass dynamics and its consequence for C cycle in coastal areas and to study the consequence of hypoxia on biodiversity in a natural context. [less ▲]

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See detailStructure trophique et diversité des macro-organismes associés aux litières de Posidonia oceanica (L.) Delile, dans la baie de Calvi en Corse.
Remy, François ULg

Master's dissertation (2010)

Le but de ce travail était d'évaluer la diversité et la variabilité à petite échelle des macro-invertébrésassociés à la litière de feuilles mortes de Posidonia oceanica, ainsi que de caractériser les ... [more ▼]

Le but de ce travail était d'évaluer la diversité et la variabilité à petite échelle des macro-invertébrésassociés à la litière de feuilles mortes de Posidonia oceanica, ainsi que de caractériser les relations trophiques existant entre ces organismes. Le volet trophique du travail a été étudié en utilisant deux méthodes : l'examen de contenus stomacaux et la mesure des rapports isotopiques du carbone et de l'azote. La campagne de prélèvements a eu lieu en novembre 2009, à deux stations situées dans la Baie de la Revellata à proximité de la station STARESO (Calvi, Corse). Les prélèvements ont été effectués sur deux accumulayions de litière, c'est-à-dire de feuilles mortes de posidonie associées à des macroalgues arrachées et à des pousses vivantes de posidonie. Les crustacés, et plus particulièrement les amphipodes gammaridés dominent largement les prélèvements. L'espèce Gammarella fucicola représente à elle seule 55% des organismes prélevés. Les espèces trouvées dans nos prélèvements sont également souvent présentes au sein de l'herbier de posidonie. Nous pensons que les différences existant entre les deux sites de prélèvements du point de vue de l'abondance, la densité et la diversité des organismes sont principalement dues aux caractéristiques physicochimiques de la litière et à son état de stabilité. En effet, il est probable que l'état de dégradation influence l'assemblage d'espèces présentes dans la litière. La combinaison des contenus stomacaux et des rapports isotopiques nous ont permis de confirmer le régime alimentaire détritivore des amphipodes et Idotea baltica, et donc leur rôle dans la fragmentation de la litière. Nous avons également pu mettre en évidence pour Athanas nitescens et Liocarcinus arcuatus, deux décapodes, un niveau trophique intermédiaire entre consommateur primaire et détritivore. Palaemon adspersus, un autre décapode, semble être le seul consommateur secondaire vrai que nous ayons pu analyser et les données suggèrent qu'il se nourrisse principalement d'Amphipodes. Bien qu'ils ne l'assimilent pas tous de la même façon, la plupart des organismes ingèrent une quantité non-négligeable de litière, ce qui suggère que ces animaux jouent tous un rôle plus ou moins important dans la fragmentation des feuilles, et donc dans le flux de carbone de la litière vers les niveaux supérieurs du réseau trophique. [less ▲]

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