VAriability of vertical and tropHIc transfer of fixed N2 in the south wEst Pacific and potential impact on the biological carbon pump – VAHINE

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 Pg 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 such as Trichodesmium constitutes one of the major sources of ‘new’ nitrogen for the surface ocean with a net input estimated at 100-200.1012 g.yr-1.

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 Trichodesmium bloom, and the evolution of stocks and fluxes of biogenic elements (C, N, P, Si)?

Question 3. Does the development of Trichodesmium increases 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 (Bonnet et al., In Rev.).
The overall strategy will combine experimental and modelling approaches. The experimental part consists in deploying triplicate large mesocosms (52 000 L, developed in the framework on a previous ANR project, DUNE) in New Caledonia during a Trichodesmium bloom (Task 1). To answer question 1, we will combine stable isotope labelling, cell sorting by flow cytometry, and NanoSIMS analyses (Task 2). To address question 2, we will determine the composition of the planktonic assemblage in the mesocosm (Task 3, action 1), and quantify stock and fluxes of biogenic elements (C, N, P, Si) (Task 3, action 2).
To address question 3 (carbon export), sediment traps will be collected every day for mass fluxes, particulate C, N, P, Si measurements (Task 4), and real time particle size distribution and abundance will be monitored using a camera immerged inside the mesocosms (UVP). Finally, a delta15N budget will be performed order to quantify the part of export production sustained by N2 fixation (Task 4).
The biogeochemical modelling approach which has already started in 2012 (Eco3M plateform, developed by the proposed team) (Task 5) will be used before the experiment as a prospective tool, and after, as a powerful tool to help answering the scientific questions.
Five project meetings will be organized (Task 0, Management) to synthesize the degree of advancement of each Task, and decide whether or not we validate the deliverable of a specific action and go further in the realization of the project.
Final end-products (new techniques, model) will be available to the scientific community (Task 6) (data base, publications, international scientific conferences), students, and to a broad audience through a documentary film realized by CANAL IRD.