Bio-optical characterization of subsurface chlorophyll maxima in the Mediterranean Sea from a Biogeochemical-Argo float database

Abstract

Abstract. As commonly observed in oligotrophic stratified waters, a subsurface (or deep) chlorophyll maximum (SCM) frequently characterizes the vertical distribution of phytoplankton chlorophyll in the Mediterranean Sea. Occurring far from the surface layer “seen” by ocean colour satellites, SCMs are difficult to observe with adequate spatio-temporal resolution and their biogeochemical impact remains unknown. Biogeochemical-Argo (BGC-Argo) profiling floats represent appropriate tools for studying the dynamics of SCMs. Based on data collected from 36 BGC-Argo floats deployed in the Mediterranean Sea, our study aims to address two main questions. (1) What are the different types of SCMs in the Mediterranean Sea? (2) Which environmental factors control their occurrence and dynamics? First, we analysed the seasonal and regional variations in the chlorophyll concentration (Chl a), particulate backscattering coefficient (bbp), a proxy of the particulate organic carbon (POC) and environmental parameters (photosynthetically active radiation and nitrates) within the SCM layer over the Mediterranean Basin. The vertical profiles of Chl a and bbp were then statistically classified and the seasonal occurrence of each of the different types of SCMs quantified. Finally, a case study was performed on two contrasted regions and the environmental conditions at depth were further investigated to understand the main controls on the SCMs. In the eastern basin, SCMs result, at a first order, from a photoacclimation process. Conversely, SCMs in the western basin reflect a biomass increase at depth benefiting from both light and nitrate resources. Our results also suggest that a variety of intermediate types of SCMs are encountered between these two endmember situations.

Profiling float observation of thermohaline staircases in the western Mediterranean Sea and impact on nutrient fluxes

“…Another CTD dataset has been collected by profiling floats, autonomous platforms that drift in the interior ocean and evenly surface for positioning and data transmission. The profiling floats considered in this study belonged to the NAOS Biogeochemical (BGC) Argo array (D'Ortenzio et al, 2019). The sampling strategy of this array is well suited for observations of thermohaline staircases, rather than the MedArgo array (Poulain et al, 2007).…”

“…We use the dataset acquired during the cruise PEACETIME (Guieu et al, this special issue), that carried out a large scale survey of the western Mediterranean Sea in May-June 2017, although data of earlier cruises are also considered. Shipboard data are combined with observations obtained by a unique array of Biogeochemical Argo floats deployed in the western Mediterranean Sea (D'Ortenzio et al, 2019). The progression of this study will be i) to provide a fine characterization of the staircases recorded by shipboard data, ii) to infer their spatial extension and temporal persistence using profiling float observations, iii) to confirm the salt fingering activity and iv) to assess the contribution of this mixing process in the progressive enrichment of LIW nutrients.…”

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 .…”

Influence of solar radiation on chlorophyll a concentration assessment using fluorescence measured by the submersible sensor in Lake Baikal

“…Correct assessment of C a based on the fluorescence measured by the submersible sensor requires the compensation of the PAR effects of on the chlorophyll a fluorescence. This problem remains relevant to date (Barbieux et al, 2019). To solve this problem, it was proposed to use chlorophyll a fluorescence profiles measured at night only when there is no PAR effect (Wojtasiewicz et al, 2018).…”