On the following subject: Spatial and temporal dynamics of particulate organic biomass synthesised in a region strongly influenced by atmospheric nitrogen fixation (OUTPACE oceanographic campaign in the south-western tropical Pacific)
The entire staff congratulates Alain chalheureusement. In the photo, he is surrounded by Anne Petrenko, co-director and Thierry Moutin, thesis director.
Summary of the thesis:
To date, the Southwest Tropical Pacific Ocean (WTSP) is one of the least explored oligotrophic regions of the global ocean. This PhD thesis work is part of the Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE, 160° E - 160° W, 18° - 20° S, March/April 2015) campaign, and concerns biogeochemical cycles and ecosystems in this region. Based on in situ measurements made during the campaign and data from the Biogeochemical Argo floats (BGC-Argo), the main objective of this study is to define the different processes that control the spatial and temporal dynamics of particulate organic matter (POM) synthesised in the illuminated layer. The results show that the dynamics of POM are controlled by two planktonic systems that are a priori distinct in this oligotrophic region.
In the highly illuminated but nutrient-limited mixed layer (<30 dbar) ([NO3-] <0.05µM), relatively high concentrations of nitrogen and particulate organic phosphate (NOP and POP) (0.32-1.18 µM and 0.02-0.06 µM respectively) were observed in the Melanesian Archipelago (160° E - 170° W). The MOP produced has an excess of nitrogen compared with the value expected by Redfield stoichiometry, with an N:P ratio of 19±2. Our results demonstrate that the intense biological fixation of diazote (N2) (631±286 μmol N m-2 d-1 L on average) measured in this region is the main source of new nitrogen capable of sustaining the production of N-enriched MOP. At the same time, a strong increase in N* was observed in the waters of the thermocline (100-500 dbar) of the Melanesian Archipelago. The production of N-enriched MOP supported by the fixation of N2, and the remineralisation of this MOP falling in the water column are at the origin of an increase in N* (ΔN*diazotroph) in the waters of the thermocline (between 30 ± 19 mmol m-2 a-1 and 192 ± 87 mmol m-2 a-1, depending on the N:P ratio considered for the estimation).
Based on measurements of the particle backscatter coefficient (bbp) combined with in situ measurements of NOP and POP, statistically significant relationships were observed between bbp and NOP (POP) values (R²=0.87 and 0.91, respectively), making it possible to define optical proxies (PONopt and POPopt) for particulate organic biomass. Over the entire study period (3 years), significant increases in PONopt (from 0.16 to 0.80 µM) were observed only in the Melanesian Archipelago and during stratified periods. It was shown that this production of autotrophic biomass (and associated particles) could only be sustained by the input of nitrogen via the N2 fixation process, strongly suggesting that the WTSP region could play a relatively important role in the biological carbon pump.
Near the bottom of the euphotic layer (Z01), in subsurface waters that are poorly lit but sufficiently rich in nutrients, the results obtained in this thesis show the permanent presence of a subsurface chlorophyll-a (Chla) maximum (SCML). This SCML is permanently associated with a maximum of particulate organic biomass. A significant correlation (R²=0.77) was observed between Chla and PONopt values, particularly between Z01 and the depth at which PAR is reduced to 0.1% of its surface value (Z001), thus highlighting the phytoplanktonic origin of the maximum particulate organic biomass associated with the SCML. Using O2 data collected by the FA float on a seasonal time scale, the results show that a relatively low but positive net community production (0.08 ± 0.01 mmol m-3 d-1 on average) is associated with SCML in the low-light layers between Z01 and Z001.
Key words: oligotrophic regions, particulate organic matter, N2 fixation, particulate backscatter coefficient, net community production.