Protists represent the majority of the eukaryotic diversity in the
oceans. They have different functions in the marine food web, playing
essential roles in the biogeochemical cycles. Meanwhile the available
data is rich in horizontal and temporal coverage, little is known on
their vertical structuring, particularly below the photic zone. The
present study applies DNA metabarcoding to samples collected over three
years in conjunction with the BATS time-series to assess marine protist
communities in the epipelagic and mesopelagic zones. The protist
community showed a dynamic seasonality in the epipelagic, responding to
hydrographic yearly cycles. Mixotrophic lineages dominated throughout
the year; however, autotrophs bloomed during the rapid transition
between the winter mixing and the stratified summer, and heterotrophs
had their peak at the end of summer, when the base of the thermocline
reaches its deepest depth. Below the photic zone, the community,
dominated by Rhizaria, is depth-stratified and relatively constant
throughout the year, mirroring local hydrographic and biological
features such as the oxygen minimum zone. The results suggest a dynamic
partitioning of the water column, where the niche vertical position for
each community changes throughout the year, likely depending on nutrient
availability, the mixed layer depth, and other hydrographic features.
Finally, the protist community closely followed mesoscale events
(eddies), where the communities mirrored the hydrographic uplift,
raising the deeper communities for hundreds of meters, and compressing
the communities above.
Main hydrographic features of the Sargasso Sea in Spring 1979
