Changes in phytoplankton and biomineral content of particles during episodic fluxes to abyssal depth

Large episodic pulses of particulate organic carbon (POC) at the deep‐sea (~ 4000 m) time‐series Sta. M in the Northeast Pacific Ocean have increased in frequency and magnitude over the past 32 years. We inferred the ecological drivers of these events by quantifying the phytoplankton and biomineral composition within particles collected by bottom‐moored sediment traps immediately before, during, and after 14 high‐flux events. Samples collected during high‐flux events contained a significantly different phytoplankton community from other sampling periods. These particles contained relatively fewer intact phytoplankton cells and a sustained contribution from fragmented diatom frustules from species typical in coastal blooms. Biomineral fluxes did not appear to be driving high‐flux events. We suggest that most of the observed high‐flux events were generated by offshore transport of coastal diatom blooms, but that these particles were also highly transformed by deep‐sea pelagic food webs before reaching bathypelagic depths.