Biogeochemical anatomy and ecosystem dynamics of a large phytoplankton bloom north of the Hawaiian Islands

In the eastern portion of the oligotrophic North Pacific Subtropical Gyre (NPSG), summertime phytoplankton blooms are recurrent events whose frequency and spatial distribution are primarily known through satellite ocean color observations. Field sampling of blooms has been sparse, so their biogeochemical structure, ecosystem dynamics, and mechanisms of initiation have not been well described, except to show that they are commonly driven by diatom-diazotroph associations (DDAs). To better understand bloom dynamics, an oceanographic expedition in the summer of 2022 targeted a large (225,000 km²), long-lived (3 months) Hemiaulus-Richelia bloom north of the Hawaiian Islands for a comprehensive and multidisciplinary investigation into the bloom's microbial community composition, nutrient dynamics, suspended and sinking particulate matter, primary production and nitrogen (N₂) fixation, and abundances of genes catalyzing N₂ fixation and ammonia oxidation (nifH and amoA genes). These novel observations were interpreted together with previous bloom and non-bloom observations from Station ALOHA, the nearby field site of the Hawaii Ocean Time-series program, to gain a general understanding of bloom ecology and the biogeochemical conditions that regulate bloom initiation and demise. We found that a bloom is likely initiated from a ubiquitous summertime seed population of DDAs in the presence of (1) an above-average concentration of phosphate and silicate, (2) a shallow mixed layer that retains DDAs in high light, and (3) low mortality. The build-up of biomass in a bloom leads to a substantial increase in light attenuation; for example, in the 2022 bloom, the depth of the 1% surface light level shoaled by 50 m compared to non-bloom conditions. Decreased photon flux to the lower euphotic zone (>50 m) had significant biological and chemical consequences for the water column, including a diminished abundance of Prochlorococcus and an accumulation of ammonium due to net heterotrophic conditions. The collapse of a bloom can be caused by nutrient depletion (most likely phosphorus in our study region), a deepening mixed layer, and/or enhanced mortality (e.g., a rise in the abundance of grazers, viruses, or parasites). The average carbon export efficiency is high from DDA blooms (in large part because they are mineral-ballasted organisms), and the contribution from blooms to annual, gyre-wide export of organic matter is expected to be substantial.