UH Mānoa study dives into the origins of the swirls of color appearing in the Pacific each summer

Vast swirls of color appear nearly every summer in the Pacific Ocean north of Hawaiʻi, and for years, the origins of these massive blooms of photosynthetic microbes remained a mystery.

That was until a study, led by the University of Hawaiʻi at Mānoa oceanographers, provided the first comprehensive look at the anatomy of these events.

“This paper represents a synthesis of many different observational perspectives which, only when evaluated together, allowed us to paint the whole picture,” said Rhea Foreman, lead author of the study and researcher at the Center for Microbial Oceanography: Research and Education in the Univeristy of Hawaiʻi at Mānoa School of Ocean and Earth Science and Technology.

“It required multiple people with a range of expertise to work together in order to see the overarching ecological processes,” Foreman continued.

The North Pacific Subtropical Gyre is described as an ocean desert due to its low levels of nutrients. However, in late summer, a unique partnership forms between diatoms (marine microbes that live inside a glass shell) and diazotrophs (bacteria that convert nitrogen gas into a biologically usable form).

Previous research established that summer blooms are often driven by this pairing, but beyond that, the causes of bloom initiation, sustenance, and collapse were unknown.

In summer 2022, oceanographers used the R/V Kilo Moana to try to catch a bloom event. When they noticed on satellite imagery that a bloom the size of Minnesota was within range of the expedition, a race was on to investigate.

The team investigated the bloom’s microbial community, nutrient dynamics, composition of particulate matter, rates of photosynthesis and nitrogen fixation, and abundances of specific functional genes.

Their study revealed that the blooms are likely triggered when the seed population of diatom-diazotroph associations experiences favorable conditions such as above-average concentrations of phosphate and silicate, and a shallower mixed layer at the surface ocean.

This shallow mixed layer acts to corral the photosynthetic microbes, keeping them near the surface where sunlight is abundant—something they require for efficient nitrogen fixation.

“This comprehensive expedition required careful planning, skillful execution, effective teamwork, and a bit of luck—we went four-for-four!” said David Karl, senior author on the study, Victor and Peggy Brandstrom Pavel Professor of Oceanography, and director of the Center for Microbial Oceanography.

The study also relied on the historical context provided by the University of Hawaiʻi at Mānoa Hawai‘i Ocean Time-series program that has conducted monthly monitoring of the physical, biological, and chemical characteristics at a nearby open ocean field station north of the Hawaiian Islands since 1988.

“By comparing the 2022 expedition data to the Hawaiʻi Ocean Time data, which shows baseline conditions at Station ALOHA, we were able to distinguish unique bloom characteristics from normal background conditions, and that helped us understand the lifecycle of the bloom,” said Foreman.

The researchers’ proposed lifecycle for the blooms included some predictions about ways in which they collapse. Some suggestions include the phytoplankton may either run out of some nutrient (phosphate, for example), the mixed layer may deepen and inhibit growth, or mortality may increase through parasites, viruses, or grazers that were originally in much lower abundances.

Because the diatom-diazotroph associations are heavy, they sink rapidly when they die and efficiently export carbon from the atmosphere to the deep ocean. Understanding the bloom’s lifecycle is key for modeling climate processes and predictions.