Spatial and temporal variability of the silicon cycle in diverse oligotrophic to mesotrophic environments

• Main Author: Leblanc, Karine
• Other Authors: Laboratoire d'océanographie et de biogéochimie (LOB), Université de la Méditerranée - Aix-Marseille 2-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS), Université de la Méditerranée – Aix-Marseille II, Quéguiner, Bernard
• Format: Doctoral or Postdoctoral Thesis
• Language: French
• Published: HAL CCSD 2002
• Subjects: biogeochemistry; diatoms; silicon; biogéochimie; diatomées; silicium; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; Indian; Southern Ocean; North East Atlantic
• Online Access: https://hal.science/tel-01783412; https://hal.science/tel-01783412/document; https://hal.science/tel-01783412/file/Leblanc-these-Hal.pdf

The silicon biogeochemical cycle was studied in diverse coastal and oceanic systems, oligotroph to mesotroph, in the Mediterranean Sea (SOFi, ALMOFRONT II and PROSOPE campains), in the North East Atlantic (POMME campaign) and in the HNLCLSi area of the Indian sector of the Southern Ocean (ANTARES 4 campaign). This work has evidenced the role of the availability of dissolved Si upon diatom distribution in the western Mediterranean, where silicic acid stocks can be inferior to nitrate stocks. Silicic acid concentrations can be very low and close to the detection limit in coastal systems (Gulf of Lions) as well as in open systems (Alboran Sea, Algerian Basin), contradicting the current assumption that dissolved silicon is never found below 1 μM in the Mediterranean. Si uptake kinetics carried out during the POMME and SOFi campaigns highlighted Si limitation at relatively high concentrations (≥ 1 μM). The half-saturation constants for Si uptake varied strongly depending on the specific structure of the diatom assemblage. Nutrient enrichment experiments performed during the POMME and ANTARES 4 campaigns have showned Si/Fe and Si/N co-limitation processes liable to limit the diatom spring bloom in the North East Atlantic and in the Southern Ocean HNLCLSi area. If dissolved Si availability does not seem to directly control primary production, its role appears crucial in structuring the specific composition of the phytoplankton assemblage, by favouring or not the dominance of diatoms, one of the main group responsible for rapid C export to depth. Results obained throughout the different campaigns illustrated the potential importance of Si dissolution in the euphotic layer, when silicic acid concentrations are low. This process may be of particular importance during the spring bloom, by allowing a replenishment of the depleted surface layer and inducing a production regime based on regenerated Si. Finally, production and exportation budgets established for each system studied allowed the comparison with the functionning ...