Open‐ocean convection process: A driver of the winter nutrient supply and the spring phytoplankton distribution in the Northwestern Mediterranean Sea

Abstract

This study was a part of the DeWEX project (Deep Water formation Experiment), designed to better understand the impact of dense water formation on the marine biogeochemical cycles. Here, nutrient and phytoplankton vertical and horizontal distributions were investigated during a deep open‐ocean convection event and during the following spring bloom in the Northwestern Mediterranean Sea (NWM). In February 2013, the deep convection event established a surface nutrient gradient from the center of the deep convection patch to the surrounding mixed and stratified areas. In the center of the convection area, a slight but significant difference of nitrate, phosphate and silicate concentrations was observed possibly due to the different volume of deep waters included in the mixing or to the sediment resuspension occurring where the mixing reached the bottom. One of this process, or a combination of both, enriched the water column in silicate and phosphate, and altered significantly the stoichiometry in the center of the deep convection area. This alteration favored the local development of microphytoplankton in spring, while nanophytoplankton dominated neighboring locations where the convection reached the deep layer but not the bottom. This study shows that the convection process influences both winter nutrients distribution and spring phytoplankton distribution and community structure. Modifications of the convection's spatial scale and intensity (i.e., convective mixing depth) are likely to have strong consequences on phytoplankton community structure and distribution in the NWM, and thus on the marine food web.

Deep sediment resuspension and thick nepheloid layer generation by open‐ocean convection

“…There is indeed evidence [see Séverin et al ., ] that nutrients concentrations are altered by the resuspension of deep sediment. Séverin et al .…”

“…Séverin et al . [] showed that during the winter cruise (DEWEX 1), in the absence of enhanced primary production, N and P concentrations for fully mixed stations are slightly larger than for stations where the mixing only affects part of the water column. This is notably expressed in the strong reduction of the N/P stoichiometric ratio.…”

Physical and Biogeochemical Controls of the Phytoplankton Blooms in North Western Mediterranean Sea: A Multiplatform Approach Over a Complete Annual Cycle (2012–2013 DEWEX Experiment)

“…Thus, the stronger and longer winter mixing in the “High Bloom” bioregion could have enhanced silicates availability, generating more favorable conditions for diatoms, and leading to the development of an enhanced diatom spring bloom. Severin et al [] showed that the DEWEX ship surveys stations performed in February 2013 inside the deep convection area presented higher surface [Si(OH) 4 ] and higher silicates to nitrates (Si:N) ratio ([Si(OH) 4 ] ∼ 7.1 µM and Si:N ∼ 0.86) than the ship surveys stations performed in the surrounding mixed area ([Si(OH) 4 ] ∼ 3.63 µM and Si:N ∼0.70).…”

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

“…In this context, and in the framework of the MerMex project, four cruises were carried out between July 2012 and June 2013 (Estournel et al, 2016b;Waldman et al, 2016). The data that were collected made it possible to describe the spatial distribution of phytoplankton in the deep convection northwestern Mediterranean during stratified, deep convection, and phytoplankton bloom periods (Mayot et al, 2017a;Severin et al, 2017). This unprecedented in situ observation offered a great opportunity to calibrate and validate a 3-D coupled physical-biogeochemical model (Estournel et al, 2016a;Kessouri et al, 2017).…”