Deep phytoplankton biomass maxima in the global ocean: a Biogeochemichal-Argo floats investigation

Permanently or seasonally stratified systems are characterized by the presence of a deep chlorophyll maximum (DCM) often not detected by satellites. A DCM does not necessarily imply a deep phytoplankton (carbon) biomass maximum (DBM) because physiological responses to low irradiance result in higher chlorophyll a per phytoplankton carbon. The presence of a DBM likely reflects a combination of ample nutrient concentrations and a favorable light environment enabling phytoplankton to thrive at those depths, and an effective accumulation of organic carbon. In order to investigate the spatial and temporal variability in DBMs and DCMs in open ocean, we use a global dataset of 420 Biogeochemical (BGC)-Argo floats, located in 28 different oceanic regions, thus representing a wide range of open ocean biogeochemical conditions. We first develop a method to identify DCMs and DBMs based on the comparative vertical distribution of both chlorophyll a concentration and particulate backscattering coefficient, and then investigate the environmental parameters involved in the appearance and maintenance of those features. We show that the appearance and the depth of the DCMs/DBMs are primarily driven by the attenuation of light in the upper layer. We further highlight that the seasonal dynamics of DBMs and DCMs are clearly region-dependent, with high latitude environments (Subpolar Gyres and the Southern Ocean) being characterized by low occurrence of intense DBMs, while in oligotrophic regions (e.g. Subtropical Gyres), permanent DCMs essentially result from photo-acclimation with DBMs occurring preferentially in summer. The most permanent DBMs are observed in Subequatorial waters, where the DCM/DBM layer develops in both favorable light and nutrient conditions. We hypothesize here that cyclonic circulation patterns (e.g as in the Guinea Dome) are responsible for the favorable environmental conditions that stimulate phytoplankton growth, in a similar way to what is observed in the North western Mediterranean Sea around the summer solstice. Preliminary results from colocating BGC-Argo profiles with mescoscale eddies identified by altimetry reveal a positive influence of cyclonic eddies on the DBMs presence in stratified systems.