Exploring a Mediterranean mesozooplankton 13 year time-series.

Zooplankton plays diverse crucial roles within the marine ecosystem and can also be used as bio- indicator of climate changes since it is very sensitive to environmental changes. Therefore it is essential to consider long-term plankton series. Given the analysis of plankton samples is time- consuming, it requires an effective and rapid analytical method. We have used in this work a supervised learning approach adapted for the semi-automatic classification of digital images of the mesozooplankton of the Bay of Calvi (Corsica, France) by using the Zoo/PhytoImage software. Together with a 11-years long zooplankton time-series, a set of nine environmental variables were monitored in order to identify controlling factors and determine whether the communities were sensitive to global environmental changes. The main components of the mesozooplankton community were characterized by both seasonal and inter-annual variability. Additionally, variation of holoplankton and meroplankton differentiated one from each other. The holoplanktonic community could be split into two subgroups according to its variation in function of the environment: cladocerans and appendicularians, and to a lesser extent, copepods on one hand, and cnidarians, chaetognathes and thaliaceans, on the other hand. Regarding inter-annual variation, one year (2007) showed particularly low production of total zooplankton which was also the case for all the different holoplanktonic taxa. Accounting for that phenomenon were identified some potentially underlying environmental factors. Finally, although water temperature increased significantly over the last years along with the frequency of marine heat wave events, no evident change in the global zooplankton composition was observed yet.STARECAPME

Corsican seagrass detritus : an opportune shelter or a copepod Eldorado?

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

The role of meiofauna in energy transfer in a Mediterranean seagrass bed (Calvi, Corsica)

Meiofaunal communities of the endemic Mediterranean seagrass, Posidonia oceanica, were sampled in five different habitats characterised by different degradation level of macrophytodetritus. In term of abundance, harpacticoid copepods represent half of the community followed by nematodes and polychaetes. Two meiofauna communities were distinguished: (1) a benthic community of meiofauna, living in the sediment or on highly fragmented macrophytodetritus, and (2) a foliar, epiphytal community associated with seagrass leaves and low fragmented macrophytodetritus leaves. They differed significantly in their harpacticoid copepod family composition. The benthic community consisted mainly of families like Tisbidae and Miraciidae, while the epiphytal community was dominated by families like Thalestridae and Laophontidae. These differences in composition may also imply a differential functional diversity. Trophic biomarkers (stable isotopes, fatty acids) were used to identify the major sources of organic matter contributing to the copepods diet and hence to gain insight in the overall carbon flux. Harpacticoid copepods showed preferences to feed upon the epiphytal biofilm community composed of bacteria, diatoms, fungi and microalgae. Copepods used the seagrass and detritus material merely as substrate, but were attracted to the biofilm rather than the plant material which is rich in structural carbohydrates difficult to assimilate by animals (i.e. lignin, cellulose, ...). Since harpacticoid copepods showed to use different sources of carbon, unravelling the contribution of each of them and the role of the degradation level of the detritus for food selectivity is the next step forward.Implications of Posidonia oceanica litter and its microbial and faunal associated communities in the carbon cycle of an coastal oligotrophic area (FRFC 2.4511.09) (2009-2013