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\ndeep) chlorophyll maximum\xa0(SCM) frequently characterizes the vertical\ndistribution of phytoplankton chlorophyll in the Mediterranean Sea.\nOccurring far from the surface layer “seen” by ocean colour satellites,\nSCMs are difficult to observe with adequate spatio-temporal resolution and\ntheir biogeochemical impact remains unknown. Biogeochemical-Argo\xa0(BGC-Argo)\nprofiling floats represent appropriate tools for studying the dynamics of\nSCMs. Based on data collected from 36 BGC-Argo floats deployed in the\nMediterranean Sea, our study aims to address two main questions. (1)\xa0What\nare the different types of SCMs in the Mediterranean Sea? (2)\xa0Which\nenvironmental factors control their occurrence and dynamics? First, we\nanalysed the seasonal and regional variations in the chlorophyll\nconcentration (Chl\xa0 a ), particulate backscattering coefficient ( bbp ), a\nproxy of the particulate organic carbon\xa0(POC) and environmental parameters\n(photosynthetically active radiation and nitrates) within the SCM layer over the Mediterranean Basin. The\nvertical profiles of Chl\xa0 a and bbp were then statistically classified and the seasonal occurrence of each of the different types of SCMs\nquantified. Finally, a case study was performed on two contrasted regions\nand the environmental conditions at depth were further investigated to\nunderstand the main controls on the SCMs. In the eastern basin, SCMs result,\nat a first order, from a photoacclimation process. Conversely, SCMs in the\nwestern basin reflect a biomass increase at depth benefiting from both light\nand nitrate resources. Our results also suggest that a variety of\nintermediate types of SCMs are encountered between these two endmember\nsituations.