The Summer Deep Phytoplankton Bloom, the one Missed by Satellites.

Traditionally the phytoplankton bloom has been considered as a surface « event », in particular because it can be easily detected through satellite Ocean color remote sensing. With the recent advance of autonomous platforms (floats, gliders), time-series of phytoplankton biomass resolved over the vertical dimension have become available as a complement to surface detection. Here, the analysis of such time series (more than 8000 profiles analyzed) in oceanic areas where surface waters are permanently (sub-tropical gyres) or temporary (temperate areas) nutrient-limited reveals, around the summer solstice, a conspicuous pattern at the level of the deep chlorophyll maximum (DCM). The DCM is associated with a concomitant increase in proxies of phytoplankton carbon (e.g. backscattering or attenuation coefficient), a co-variation that rules out the possibility that the DCM is only the consequence of photo-acclimation at this period of the year. Rather it seems that, thanks to surface waters becoming clearer (progressive removal of by-products of the spring bloom, phytoplankton photo-acclimation reducing Chla content) and to higher surface photon flux, light penetrates deeper and hence reaches the nutricline. A favorable environment for phytoplankton growth develops with signatures up to 4 mg Chla m^-3 for temperate areas. This early summer deep bloom has been neglected to date because of lack of relevant observations. Autonomous platforms reveal here a potentially significant mechanism of primary production at a period which, from the ocean surface, is generally considered as the most oligotrophic. A first assessment of the occurrence of such blooms at global scale is proposed.