Congratulations to Alain Fuménia (CYBELE team) who defended his thesis by videoconference on Wednesday, June 03, 2020

On the following topic: Spatial and temporal dynamics of particulate organic biomass synthesized in a region strongly influenced by atmospheric nitrogen fixation (OUTPACE oceanographic campaign in the tropical southwest Pacific)

The entire staff warmly congratulates Alain . In the photo, he is surrounded by Anne Petrenko, co-director and Thierry Moutin, thesis director.



Spatial and temporal dynamics of particulate organic biomass synthesized in a region influenced by the fixation of atmospheric dinitrogen (OUTPACE cruise in the western tropical south Pacific)


To date, the Western Tropical South Pacific (WTSP) is one of the least sampled oligotrophic regions of the world's ocean. My thesis was carried out in the framework of the Oligotrophy to UlTra-oligotrophy PACific Experiment cruise (OUTPACE, 160° E - 160° W, 18° - 20° S, March/April 2015), in which we investigated the biogeochemical cycles and ecosystems in this region. Based on both in situ measurements and Biogeochemical Argo floats’ data, the main objective of this study is to get insight on the different processes controlling the spatial and temporal dynamics of particulate organic matter (POM) in the light zone. The results obtained in this thesis highlighted that the dynamics of the POM derives from two distinct planktonic systems in this oligotrophic region.

In the light-sufficient but nutrient-limited mixed layer (<30 dbar) ([NO3-] <0.05μM), relatively high concentrations of nitrogen and particulate organic phosphate (PON and POP) (0.32-1.18 μM and 0.02-0.06 μM) are observed in the mixed layer (<30 dbar) of the Melanesian Archipelago (160 ° E - 170 ° W). This POM presents an excess of nitrogen with respect to the Redfield stoichiometry, with a N:P ratio of 19 ± 2. In this study, the high rates of biological dinitrogen fixation (N2) (631 ± 286 μmol N m-2 d-1 L) measured in this region has been pointed as the main source of new nitrogen, supporting the N-enriched POM production.

At the same time, a sharp increase in N* is observed in the thermocline waters (100-500 dbar) of the Melanesian Archipelago. The production of N-enriched POM supported by the fixation of N2, and the remineralization of this POM falling downward in the water column, results in an increase of N* (ΔN * diazotrophic) in the thermocline waters (up to 30 ± 19 mmol m-2 y-1 and 192 ± 87 mmol m-2 y-1, depending on the N:P ratio considered for the estimation).

By coupling particle backscattering coefficient (bbp) measurements with in situ measurements of PON and POP, statistically significant relationships between bbp and PON (and POP) concentrations were established (R² = 0.87 and 0.91, respectively), allowing to define, for the first time, optical proxies (PONopt and POPopt) of particulate organic biomass. During the stratified periods of the studied period (3 years), significant increases in PONopt (from 0.16 to 0.80 μM) are observed only in the Melanesian Archipelago. These recurrent increases of PON concentrations in this area can only be supported by nitrogen input via the N2 fixation process, suggesting that the WTSP region could play a relatively large role on the biological carbon pump.

In the vicinity of the euphotic depth (Z01), in the light-limited but nutrient-rich subsurface waters, the results obtained in this thesis show the permanent presence of a Subsurface Maximum of Chlorophyll-a (Chla) Layer (SCML). This SCML is permanently associated with a maximum of particulate organic biomass. A significant correlation (R² = 0.77) is observed between the Chla and PONopt values, especially between Z01 and the depth in which the measured PAR is reduced to 0.1% of its surface value (Z001). The significant correlation between PONopt and Chla in this layer highlights the phytoplanktonic origin of the particulate organic biomass maximum associated to the SCML. By using O2 data collected by the float FA on a seasonal time scale, the calculated Net Community Production (0.08 ± 0.01 mmol m-3 d-1 on average) exhibited a low but positive value within the SCML, in the dimly lit layers between Z01 and Z001.


Key words: oligotrophic regions, particulate organic matter, N2 fixation, particle backscattering coefficient, Net Community Production.