Viewed from space, vast swirls of color appear nearly every summer in the Pacific Ocean north of Hawaiʻi. For years, the origins of these massive blooms of photosynthetic microbes remained a mystery. Now, a study led by University of Hawaiʻi at Mānoa oceanographers provides 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 in the Center for Microbial Oceanography: Research and Education (C-MORE) in the UH Mānoa School of Ocean and Earth Science and Technology (SOEST). "It required multiple people with a range of expertises to work together in order to see the overarching ecological processes."
Race to sample the bloom
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, essentially creating fertilizer for the system). 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 and 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 experience 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 C-MORE.
Understanding lifecycles
The study also relied on the historical context provided by the UH Mānoa Hawaiʻi Ocean Time-series (HOT) 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 HOT 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.
