MOBYDICK – Marine Ecosystem Biodiversity and Dynamics of Carbon around Kerguelen: an integrated view

Description of the biodiversity

          Compared to previous studies, the originality of MOBYDICK is the concurrent investigation of pelagic biodiversity at all trophic levels and of every functional group from picoplankton to top predators, together with data acquisition of environmental drivers including dissolved and particulate nutrients.

          To achieve this objective, the diversity of procaryotes will be investigated at all depth layers, from surface waters to the mesopelagic realm. Diatoms are classically mentioned as the major primary producers in the SO and their diversity was described in detail in previous studies. By contrast, other phytoplankton groups such as haptophytes and dinoflagellates were far less studied, and MOBYDICK will pay special attention to these orga-nisms as recent findings suggest their importance for vertical C export efficiency even in the Southern Ocean (Rosengard et al., 2015; Balch et al., 2016; Rembauville et al., 2016). Focus will also put on potential parasites, organic matter decomposers or endosymbionts that represent other functional groups that were missed by classical approaches. Within micro– (20–200 μm) and mesozooplankton (200 μm–2 cm), organisms such as pteropods have also been overlooked although evidence from zooplankton community studies indicates that they are consistent, and sometimes dominant components of the Southern Ocean zooplankton (Hunt et al., 2008). Heterotrophic dino-flagellates were recently identified as the most important group of phytoplankton grazers in the Kerguelen region (Christaki et al., 2014), shedding new light on the roles of dinoflagellates and ciliates in the transfer of C to me-sozooplankton. Within macrozooplankton/micronekton (2–20 cm), copepods and krill were intensively studied (Murphy et al., 2007, 2016; Mayzaud & Pakhomov, 2014). By contrast, our knowledge on other macrozooplankton remains limited, and of particular interest in the SO are gelatinous zooplankton including salps and ctenopho-res (Chiba et al., 1998; Le Fèvre et al., 1998; Pakhomov et al., 2000). Finally, the lack of consideration for myc-tophids (lanternfishes) and other nektonic organisms such as squids, whereas they form a major component of fish biomass (Cherel et al., 2004; Collins & Rodhouse, 2006), has led to a simplistic vision of a general central role of the krill Euphausia superba in food webs of the Southern Ocean. MOBYDICK will fill this gap by providing a detailed description of of the biological diversity associated to these groups.

Carbon sequestration and transfer

During MOBYDICK, the combined information on taxonomy, C stock partitioning, and C flows will provide unprecedented information on key players and their roles in C-cycling in end-to-end food webs of contrasted ecosystems. This objective will be addressed through answers to the three following specific questions :

  • Which organisms are responsible for the input of organic carbon to the ecosystems ?

Besides the classical paradigm of strict autotrophs being the main vector for the transformation of DIC to POC and DOC, we will examine the novel emerging paradigm of mixotrophy in different marine organisms. Heterotrophic prokaryotes are recognized as the main degraders of organic matter in the ocean. The discovery that Archaea and Bacteria can also fix inorganic C (Herndl et al., 2005; Ingalls et al., 2006) challenges a restricted view of prokaryotes’ functions. In particular, a key issue will be to investigate the use of DIC to produce POC in the form of prokaryotic biomass and DOC in the form of prokaryotic- derived organic matter, a process with major implications for the microbial and possibly higher trophic levels.

  • Which organisms are responsible for the loss of organic carbon from the ecosystem?

We consider here the balance between community respiration and the production of detrital C. Respiration is the key biological process that returns organic C to DIC and therefore represents a major loss term for the food webs. The balance between production and respiration of organic matter is key to understand whether biological activity acts as a net source or sink of CO2 to the atmosphere. Regarding the production of detritus, besides the downward export by agregation/sedimentation through sinking material, vertical migration of organisms has been identified for a long time as a possible important mode of both upward and downward transport of C. MOBYDICK will dedicate efforts to properly characterize vertical migrations in space and time and relate this process to organic C partitioning between vertical fluxes and food web flows.

  • What are the trophic interactions that drive the fluxes of carbon among organisms?

MOBYDICK will pay attention to some interactions that all influence the C and energy fluxes within an ecosystem. In particular, we will focus on virus-host interactions since an increased viral activity could lead to a decrease of the efficiency of the biological carbon pump. Another key question is related to the ‘selective grazing’ issue through trophodynamics. In particular, MOBYDICK will investigate bacterial consumption by heterotrophic/mixotrophic nano- and picoplankton and also by phototrophic picoeukaryotes, which could also be important in this respect (e.g. Hartmann et al., 2013). The C flow mediated by higher trophic levels such as micronekton (including crustacean, gelatinous plankton, cephalopods and small fish) also requires a better description of their diets and of the spatio– temporal variability of their respective prey.

 

Climate change

This represents the most challenging issue that clearly cannot be solved based on a short-term project alone. However, by combining the results of the two first objectives and our present knowledge on the biogeochemistry and the ecology of top predators in the study region, MOBYDICK will provide key information on how the Kerguelen system could evolve in the future and on expected regional climate projections.

MOBYDICK will bring a milestone, by providing novel information on the concurrent description of biodiversity at all trophic levels and their integration into ecosystem functioning.

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