Institut Méditerranéen d’Océanologie

VAHINE

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The aim of the VAHINE project is to study the fate of fixed nitrogen in the oceanic pelagic food web and its potential impact on carbon export. The field campaign of VAHINE took place in the South West Pacific (New Caledonia) in Jan.-Feb. 2013, involving 16 scientists from France, Israel, Germany and the USA. This project is funded by INSU-LEFE, GOPS, IRD and ANR-JCJC (PI : Sophie Bonnet, IRD/MIO Noumea/Marseille)

The ocean provides many provisioning services, such as food, employment, water, energy. It also provides regulating services. The most important one is the regulation of climate. The ocean absorbs 2 Petagramms (10^15 g) of carbon annually, representing 1/3 of the annual anthropogenic inputs of CO2 to the atmosphere (IPCC, 2001).

The availability of nitrogen is one of the main factors controlling primary productivity and carbon sequestration by the ocean. Biological N2 fixation by diazotrophic organisms (or N2-fixing organisms) constitutes one of the major sources of ‘new’ nitrogen for the surface ocean with a net input estimated at 100-200 Teragramms.yr-1 (10^12 g).

A critical question that remains unanswered so far is the fate of nitrogen newly fixed by diazotrophs in oceanic food webs. The objective of VAHINE is to answer this central question, that can be detailed by the following questions :

Question 1 . What is the primary route of transfer of newly-fixed N in the planktonic ecosystem, i.e. is N preferably transferred to the classical food web (phytoplankton, zooplankton, fish larvae), or to the microbial food web ?

Question 2 . What is the evolution of heterotrophic prokaryotes, pico-, nano-, microphytoplankton, and zooplankton assemblage before, during and after a bloom of diazotrophs, and the evolution of stocks and fluxes of biogenic elements (C, N, P, Si) ?

Question 3 . Does the development of diazotrophs increase the efficiency of carbon export ?

To date, this lack of knowledge is essentially due to two scientific and technical bottlenecks including 1/ the lack of techniques which allow us to trace the passage of this element through the different compartments of the food web, 2/ the logistical difficulties to follow the dynamic of a diazotroph bloom and co-occurring plankton for several weeks.
We propose here to overcome these difficulties by using a combination of new powerful techniques developed by the proposed team, including 1/ high-resolution nanometer scale secondary ion mass spectrometry (NanoSIMS) coupled to 2/ flow cytometry cell sorting and 3/ 15N2 isotopic labelling.

The overall strategy combines experimental and modelling approaches. The experimental part consists in deploying triplicate large mesocosms (52 000 L) for one month in New Caledonia during a diazotrophs bloom (Task 1). To answer question 1, we have combined stable isotope labelling, cell sorting by flow cytometry, and NanoSIMS analyses (Task 2). To address question 2, we have determined the composition of the planktonic assemblage in the mesocosms (Task 3, action 1), and quantified stocks and fluxes of biogenic elements (C, N, P, Si) (Task 3, action 2).
To address question 3 (carbon export), sediment traps have been collected every day for mass fluxes, particulate C, N, P, Si measurements (Task 4). Finally, a delta15N budget will be performed order to quantify the part of export production sustained by N2 fixation in the mesocosms (Task 4).
The biogeochemical modelling approach (Eco3M plateform, developed by the proposed team) (Task 5) has been used before the experiment as a prospective tool, and is currently being improved (model validation) in order to help answering the scientific questions of the project.

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