Research axis 2 : complexity and efficiency of the biological pump
Coordinators : Olivier AUMONT (IRD/ LPO) and Philippe PONDAVEN (LEMAR)
Laboratoire de Physique des Océans), CNRS, Ifremer, IRD, UBO(
Laboratoire des sciences de l'environnement marin), UBO, CNRS, IRD(
Concentrations of total dissolved carbon dioxide are about 20% higher in the deep ocean than in the surface ocean and ocean sediments contain up to several weight percent organic carbon. The set of processes that transports particulate organic carbon (POC) to the deep sea and sediments is collectively known as the "biological pump" and includes the primary production of POC, the packaging of POC, via coagulation or other processes, into large, rapidly sinking particles versus its entrainment into marine food webs. With a quantitative understanding of these processes, we could better evaluate the role of the biological pump in the oscillation of atmospheric CO2 concentrations from 280 µatm to 190 µatm over interglacial and glacial cycles and how the biological pump will respond to (or could be harnessed to sequester) the CO2 added to the atmosphere by humankind.
At present, there is no consensus on the mechanisms that control the efficiency of the biological pump, something which varies both spatially and temporally, at different scales (Boyd and Trull, 2007). In this project, we will address the complexity involved in the production and export of biogenic materials by working at different scales (from individual particles and organisms up to ecosystems) and combining novel experimental and modeling approaches (micro- and mesocosms, Dynamic Energy Budget, inverse modeling, virtual reality).
Objectives and expected results
The end goal is to make significant headway with the challenging endeavor of incorporating complexity at the individual level into ecosystem scales models.
Individual life history traits have been poorly accounted for in models, although they play a fundamental role in the quality of the biogenic matter produced by phytoplankton and bacteria, which in turn strongly affects the transfer of carbon to higher trophic levels and to the deep sea. The trade-offs between growth rate, susceptibility to grazing, and sedimentation losses; and the stoichiometric mismatch between phytoplankton and zooplankton in terms of their biomass carbon to nutrient (P, N, Si, Fe) ratio will mainly determine the proportion of production channeled into higher trophic levels. Functional diversity of primary producer communities, arising from individual life histories, affects the resource use efficiency and thereby the amount of carbon which can be fixed by phytoplankton (Striebel et al. 2009) In light of this, our first objective will be to investigate the ecological controls on the quantity, quality and character of biogenic materials produced in relation to primary production, and how this quality impacts the export of carbon to the deep sea. This will be achieved through a combination of laboratory and field mesocosm experiments in which we can manipulate the life history traits of organisms, and/or the complexity of the food web.
It is well recognized that interaction between sinking particles (aggregates, fecal pellets) and the organisms that inhabit the mesopelagic layer (100 – 1000 m) strongly determines the depth at which carbon and associated biogenic elements will be remineralized (Ragueneau et al., 2006; Buesseler et al., 2007). However, these interactions are yet poorly known. They depend in part on the quality of the sinking particles, something which evolves with depth due to remineralization processes, and in part on the ecological structure of the mesopelagic zone. Therefore, our second objective will be to determine the fate of biogenic materials sinking as aggregates and fecal pellets by studying degradation processes inside sinking particles and the interaction between these particles and mesopelagic organisms. Degradation experiments will follow stoichiometric changes during degradation, while interactions with the mesopelagic organisms will be explored through laboratory experiments and the development of a virtual plankton + particles model.
Through this unique combination of suspended field mesocosms, novel experimental setups, and innovative modeling approaches (inverse modeling, DEB, virtual reality), we will be able to improve the parameterization of complexity in ecosystem models and large scale marine biogeochemical models.