Protist plankton seasonality and community structure in the epi- and mesopelagic at BATS

The composition, functionality and role in the oceanic biogeochemical cycles of protist plankton has been the subject of intense study in the last few decades. Most of these studies are limited to the epipelagic (0-200 m) with only a few studies focusing on the deeper layers. We are presenting the seasonality and vertical structure of the protist planktonic community in the subtropical North Atlantic Ocean at the Bermuda Atlantic Time-series Study site (BATS) in the epi- and mesopelagic, by means of metabarcoding. DNA samples were collected monthly from the surface to 1000 m depth as part of the BIOS-SCOPE program, and the V4 region of the 18S gene was sequenced. Community composition of the protist plankton was compared between months and depths and put into the context of hydrographic and biological forcings in the water column. A strong seasonality was observed in the epipelagic, especially at the deep chlorophyll maximum. This seasonality gradually weakens with depth, with little signal below 500 m. The protist community showed a strong structure with depth, controlled by density layers. However, within the epipelagic, the protist community responded to other environmental variables such as nutrient availability and the depth of the mixed layer.