Anatomy of a phytoplankton bloom revealed north of Hawai‘i
The research team recovers a sediment trap from the back deck of the ship to capture sinking particles. Credit: Rhea Foreman
Large phytoplankton blooms north of the Hawaiian Islands have been seen in satellite imagery as vast swirls of color nearly every summer, but their origin and ecosystem dynamics have remained mysterious. A recent investigation led by University of Hawai‘i (UH) at Mānoa oceanographers revealed the confluence of conditions that support the explosion of photosynthetic microbes and the cascade of responses within the microscopic marine world. The highly multidisciplinary study offers the first comprehensive look at the anatomy of these massive biological 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 often described as an ocean desert due to its notoriously 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 are able to convert nitrogen gas into a biologically usable form, essentially creating fertilizer for the system. Previous research established that the summer blooms are often driven by this fruitful pairing, but beyond that, the causes of bloom initiation, sustenance, and collapse were unknown.
Summertime blooms are unpredictable in their exact timing and location, so they have not been sampled very often in the past. In the summer of 2022, oceanographers had secured ship time on the R/V Kilo Moana in hopes of catching a bloom event. When they noticed on satellite imagery that a bloom the size of Minnesota was within range of the expedition, the race was on to investigate the enormous feature.
“We were exceptionally fortunate to have support for a research cruise whose primary objectives focused on improving understanding of processes that govern particulate organic matter production and consumption in the upper ocean; and the timing of this summer bloom aligned perfectly with our scheduled cruise,” said Matt Church, chief scientist of the research cruise and professor of aquatic microbial ecology at University of Montana’s Flathead Lake Biological Station.
The multidisciplinary 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 are generally elevated in the presence of other favorable conditions: 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 C-MORE.
Shading the waters below
Microbes and their predators that live in surface waters are tuned to a certain level of sunlight. Blooms have an enormous impact on the biological and chemical structure of the deeper portion of the sunlit surface ocean because the sudden increase of phytoplankton in the bloom shades the waters below.
“We were surprised to see the huge impacts that resulted from higher light attenuation,” said Foreman. “The build up of phytoplankton at the surface blocked enough light that there was a steep decline in photosynthesis beneath about 50 meters. This changed the typical balance between phytoplankton and the bacteria that remineralize organic matter, which led to an unusual build-up of ammonium in the lower euphotic zone.”
Putting bloom and bust into context
The study also relied heavily on the historical context provided by the UH Mānoa Hawai‘i 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. The HOT program data offers nearly four decades of data, providing a benchmark of typical conditions.
“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.
The researchers’ proposed lifecycle for the blooms included some predictions about ways in which they collapse. They suggest that the phytoplankton may either run out of some nutrient (phosphate, for example), the mixed layer may deepen and inhibit growth, or mortality may increase though parasites, viruses or grazers that were originally in much lower abundances.
Implications for the global carbon cycle
Understanding these blooms is key for modeling climate processes and predictions. Diatom-diazotroph associations are heavy, so when they die, they sink rapidly and efficiently export carbon from the atmosphere to the deep ocean.
“What is mysterious is why nitrogen-fixing organisms are not in higher abundance in these waters, since they are essentially creating the nutrient that is most limited in this setting,” Foreman added. “Learning about what limits diazotroph growth tells us a lot about what limits the overall potential for photosynthetic production in low-nutrient gyres. This is important because the gyres play a large role in the ocean-atmosphere balance of carbon dioxide.”