Presenter: Mara Freilich (Scripps Institution of Oceanography)

Description:
The low rates of primary production and the typically small phytoplankton communities in the deep chlorophyll maximum (DCM) of nutrient-limited subtropical gyres produce a sinking flux of particulate organic carbon (POC) that is too small to support the heterotrophic activity in the vast, mesopelagic (200-600 m depth region). Using biophysical observations from the Western Mediterranean Sea, an archetypical subtropical region, we reveal the presence of secondary chlorophyll maxima (SCM) beneath the DCM. Analysis of our high-resolution observations shows that these SCM are three-dimensional intrusions that are advectively transporting POC-enriched waters from the DCM along sloping isopycnal surfaces to depths exceeding 100 m. By targeted sampling of intrusions and analysis of their microbial community composition with flow cytometry and 16S rRNA gene V1-V2 sequencing, we find that such intrusions have about twice the POC content of surrounding waters at the same depth, and that the POC is dominated by non-photosynthetic bacteria. The phototrophic and heterotrophic microbial communities within the intrusions differ from their surrounding waters  and closely resemble the DCM from where the intrusions originate. These novel observations of both the physical process and microbial ecology show that advective transfer from the DCM is likely a major source of carbon and microbial diversity for the mesopelagic ocean.