Seasonal dynamics show prokaryotic community linkages between surface and deep ocean water

The biological carbon pump links surface to deep waters driving global oceanic carbon budgets. Sinking organic particles from surface waters provide key nutrients to the deep ocean, but could also serve as a 'seed bank' for the deep ocean. A recent synoptic study used large spatial scale sampling to link surface and deep microbial community composition via sinking particles. However, the temporal variability of this surface-deep connectivity remains unexplored. Here, a three year time-series of prokaryotic community composition from surface and deep (500 m) waters part of the Munida Microbial Observatory Time-Series (MOTS) was used to study the seasonality of epipelagic and mesopelagic communities. The goal was to establish how dynamic these two ecosystems are and linkages between them. Both surface and deep prokaryotic communities had seasonal patterns and high levels of variance from year to year, particularly in surface waters. Most of the seasonal variability was associated with phytoplankton bloom periods. Various surface heterotrophic bacteria growing in response to phytoplankton blooms had a lagged peak in relative abundance in deep waters post bloom. While detection of some surface prokaryotes in the deep ocean seemed seasonally linked to phytoplankton blooms, other copiotrophic or typically algal-associated groups that bloomed in the surface were not detected in the mesopelagic suggesting only specific populations were surviving the migration down the water column. We provide evidence showing transfer of organisms across depths is possibly not always unidirectional, with typically deep ocean microbes being seasonally abundant in surface waters. This indicates that the main mechanism linking surface and deep communities changes seasonally: sinking of organic particles during productive periods, and vertical convection during winter overturning.