The downward legacy of a phytoplankton bloom in the oligotrophic North Pacific Subtropical Gyre

Abstract

Ocean particles are a heterogeneous collection, including living and dead cells, feces, mucus‐rich aggregates, and mineral shells and frustules. In the well‐lit upper ocean, phototrophic processes fuel production of particulate matter; however, only a relatively small percentage (< 10% per day) of this material (mostly detrital) escapes the recycling‐intensive upper ocean, transiting downward by gravitational settling. In the North Pacific Subtropical Gyre, sinking particles are considered the major pathway supporting the vertical export of material to the mesopelagic zone. In the summer of 2021, an expedition explored a phytoplankton bloom, constrained within an anticyclonic eddy, that was detected from satellite chlorophyll products. Extensive near‐Lagrangian sampling of this bloom revealed a decaying population of diatoms containing diazotrophic symbionts imprinted over a relatively stable population of picocyanobacteria. Strong top‐down control appears to have led to the termination of this diazotrophy‐fueled growth, as evidenced by the prevalence of empty diatom frustules, incomplete drawdown of inorganic nutrients, and the presence and activity of protistan grazers/parasites coincident with the timing of bloom decline. Sinking rates of diatom frustules were ~ 50–100 m d ⁻¹ and drove elevated fluxes of carbon, nitrogen, and silicate into the mesopelagic zone, roughly 2–3× the regular summer fluxes measured at the nearby sampling station of the Hawaii Ocean Time‐series. While phytoplankton blooms are regular occurrences in this region, it is rare to sample their decay. The observations from the bloom demise described here highlight the significance of non‐steady‐state events in a region generally typified as a stable ecosystem.