Project funding

Project overview

The ocean is constantly stirred by currents that swirl and mix seawater creating fronts, filaments and eddies. These dynamic structures are known as ‘fine scales’, featuring spatiotemporal scales of 1-100 km and days-weeks. Fine scales alter biogeochemical gradients affecting phytoplankton productivity and carbon export, but their role in nitrogen cycling is unknown. The greatest source of bioavailable nitrogen in the ocean is nitrogen fixation performed by microbes called ‘diazotrophs’. Understanding the role of fine scales on diazotrophs requires dynamic sampling approaches at high-resolution, which is not possible with current discrete sampling methods. FIESTA/DEFINE2 will implement parallel measurements of nitrogen fixation and diazotrophs at a resolution >50 times higher than that possible today. This will allow more accurate quantifications of nitrogen inputs to the dynamic ocean, which is key to constrain its current and future role in primary production, CO2 withdrawal and climate change.

Objectives and approach

The overarching objective of FIESTA/DEFINE2 is to unveil the effect of fine scales on diazotrophy and their ultimate impact on nitrogen inputs to the ocean, through the following specific objectives:

1) To survey and characterize the physics of fine scale structures 

This project benefits from two oceanographic cruises in key regions featuring resident diazotroph communities and active fine scale dynamics: 

• The FIGURE cruise (funded by Eurofleets, R/V Atlantic explorer July 2022) in the Gulf Stream

• The SWOTAlis cruise (funded by CNES/NASA, R/V Alis April 2023) which will coincide with the fast-sampling phase of SWOT satellite in New Caledonia

Prior and during each cruise fine scale structures will be geolocated and monitored using the SPASSO software. During each cruise, two/three structures and a non-affected (control) area will be targeted for high-resolution sampling. Day and night samplings will be combined to capture different diazotroph circadian rhythms. 

2) To implement high-resolution measurements of diazotroph activity and abundance

Seawater pumped onboard from the ship’s underway system (5-7 m depth depending on the ship) will be bifurcated to measure:

• N2 fixation using the argon-induced hydrogen production (AIHP) developed by our collaborator Sam Wilson

• Diazotroph abundance/diversity using an automated filtration device called OCE-5 which retains DNA used to quantify main diazotroph groups by quantitative PCR and amplify the nifH gene amplicon sequencing 

3) To statistically relate physical, biogeochemical and diazotrophy data to estimate the role of fine scales in controlling nitrogen inputs and diazotroph distributions in the ocean 

The different datasets will be interpolated using Kriging methods. Interpolated data will be clustered in different fine-scale structure types and control areas using k-means and spectral clustering methods . To reveal relationships between the different data types, spectral analysis and geostatistical methods such structure function analysis (variograms) and patchiness analyses within coherent spatiotemporal frames.

Bridges members involved

Cora Hörstmann

Project collaborators

Stéphanie Barrillon, Andrea Doglioli, Anne Petrenko, Gérald Grégori, Olivier Grosso (MIO, France)

Sam Wilson (Newcastle University, UK)

Sophie Cravatte (LEGOS, France)

M. Dolores Pérez-Hernández (ULPGC, Spain)

Borja Aguiar-González (ULPGC, Spain)