Vertical Mixing Effects on Phytoplankton Dynamics and Organic Carbon Export in the Western Mediterranean Sea

Abstract

A 3‐D high‐resolution coupled hydrodynamic‐biogeochemical model of the western Mediterranean was used to study phytoplankton dynamics and organic carbon export in three regions with contrasting vertical regimes, ranging from deep convection to a shallow mixed layer. One month after the initial increase in surface chlorophyll (caused by the erosion of the deep chlorophyll maximum), the autumnal bloom was triggered in all three regions by the upward flux of nutrients resulting from mixed layer deepening. In contrast, at the end of winter, the end of turbulent mixing favored the onset of the spring bloom in the deep convection region. Low grazing pressure allowed rapid phytoplankton growth during the bloom. Primary production in the shallow mixed layer region, the Algerian subbasin, was characterized by a long period (4 months) of sustained phytoplankton development, unlike the deep convection region where primary production was inhibited during 2 months in winter. Despite seasonal variations, annual primary production in all three regions is similar. In the deep convection region, total organic carbon export below the photic layer (150 m) and transfer to deep waters (800 m) was 5 and 8 times, respectively, higher than in the Algerian subbasin. Although some of the exported material will be injected back into the surface layer during the next convection event, lateral transport, and strong interannual variability of MLD in this region suggest that a significant amount of exported material is effectively sequestrated.

Preface to the Special Section: Dense Water Formations in the Northwestern Mediterranean: From the Physical Forcings to the Biogeochemical Consequences

“…This suggests that they locally have a great imprint on both physical and biogeochemical cycles. At larger scale, two distinct trophic regimes as High Bloom centered on the convection area and a Bloom bioregion located at its periphery were identified (Mayot, D'Ortenzio, Taillandier, et al, ; Mayot, D'Ortenzio, Uitz, et al, ), in agreement with regions mentioned above and defined by Kessouri et al ().…”

“…The DWF in the NWM impacts the nutrients distribution, matter export, and development of the trophic network, and it strongly contributes to the modification of the deep stocks of these components at the basin level. Three different areas of the western basin were identified by contrasting vertical mixing regimes: deep convection, shallow convection, and stratified area (Kessouri et al, ). Using a 3‐D high‐resolution coupled hydrodynamic‐biogeochemical model, these authors showed in all three regions that the mixed layer deepening during the destratification process induced an upward nutrients flux, triggering in turn, an autumnal phytoplankton bloom.…”

“…The authors estimated nutrient budgets for the entire year and showed that the convection region represented a sink of inorganic and a source of organic nitrogen and phosphorus. This process can efficiently export organic carbon below the photic layer (Kessouri et al, ;Kessouri et al, ; Severin et al, ).…”

“…The deep convection zone is likely an area of both enhanced energy transfer to higher trophic levels and organic matter export (Kessouri et al, ; Severin et al, ). The two trophic regimes observed on the primary producer level are also reflected on higher trophic levels.…”

Biogeochemical Argo: The Test Case of the NAOS Mediterranean Array

“…It is important to recall here that the main objective of the present analysis is to verify that the implementation plan (as defined in 2012) is effective under the metric of surface CHL seasonal cycle reconstruction. The growing number of research papers based on the NAOS array in recent years (among others de Fommervault et al, 2015b;Houpert et al, 2015;Lavigne et al, 2015;Mayot et al, 2016Mayot et al, , 2017aBosse et al, 2017;Kessouri et al, 2018;Terzić et al, 2018;Testor et al, 2018;Barbieux et al, 2019;Cossarini et al, 2019) further confirms the effectiveness of the implementation plan. Meanwhile, other assessment methods have emerged, like the use of Observing Simulation System Experiments (OSSE), given the recent advances in the assimilation scheme for BGC-Argo data in the Mediterranean Sea .…”