The sea north of the Hawaiian Islands is part of the North Pacific Subtropical Gyre, a swirling current that spans the ocean from Japan to California. This vortex is often called a biological desert, as its waters contain low levels of nutrients, according to Science.

But satellites pointed towards the gyre catch something incredible nearly every summer: a swirling explosion of color across the water’s surface that blossoms and then dies away. This is a phytoplankton bloom.

Although these vibrant eruptions can cover hundreds of thousands of square miles of the ocean, much about how and why these large-scale biological shifts occur has been a mystery. A new study, published in Progress in Oceanography, has now revealed the biology of these blooms.

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What Makes Up Phytoplankton Blooms

Before the new research, scientists at least knew the main players in these blooms’ stories. They begin with two types of phytoplankton. The first are tiny marine microbes called diatoms, which partner with diazotrophs, bacteria that metabolize nitrogen gas into molecules that the blooms use as a nitrogen source. But beyond these basics, how the phytoplankton blooms spread and died out had remained unknown.

These blooms aren’t easy to study. They happen far from land and at unpredictable times during the summer. With a way of catching the blooms live, researchers at the University of Hawai‘i (UH) at Mānoa reserved ship time aboard the research vessel Kilo Moana in the summer of 2022. Luckily, a massive bloom northeast of Maui started up during their time aboard.

“This comprehensive expedition required careful planning, skillful execution, effective teamwork, and a bit of luck — we went four-for-four!” said David Karl, the study’s senior author and an oceanographer at the University of Hawai’i, in a press release.

On August 5, 2022, the Kilo Moana sailed into the bloom.

Finding the Perfect Conditions

Sampling from the waters, the team recorded a glut of information about the bloom. This included the microbes present in the bloom, nutrient dynamics, and the progression of biological processes such as photosynthesis and nitrogen fixation.

Their analysis showed that the partnership between diatoms and diazotrophs required additional conditions to kickstart a bloom. These include high levels of silicate and phosphate, a high survival rate among ocean microbes, and a shallow mixing region for the two microbial types, which exposes them to abundant light, helping them metabolize nitrogen and grow.

The researchers wanted to explore how the blooms affected the rest of the upper ocean. They found that the rapid, dense proliferation of phytoplankton blocked sunlight from reaching the depths, impacting marine life.

“The buildup of phytoplankton at the surface blocked enough light that there was a steep decline in photosynthesis beneath about 50 meters,” said Rhea Foreman, the study’s lead author and an oceanographer at the University of Hawai’i, in a press release.

The researchers combined their data with records from the University of Hawai’i’s Hawai‘i Ocean Time-series (HOT) program, which has monitored the ocean beside a local station for nearly 40 years.

“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.

How Blooms Eventually Die

This analysis suggested how the blooms may eventually die. The authors proposed that the explosions may run out of nutrient fuel, that the mixing region may descend into the ocean, reducing growth, or that increasing levels of parasites or viruses may increase mortality among the phytoplankton collaborators.

The team’s research could have important implications for our climate. The phytoplankton partnerships sink quickly when they die, taking large amounts of atmospheric carbon with them.

“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 concluded. “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.”

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Article Sources

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• This article references information from a study published in the journal Progress in Oceanography: Biogeochemical anatomy and ecosystem dynamics of a large phytoplankton bloom north of the Hawaiian Islands
• This article references information from a study published in the journal Science: The Ocean’s Biological Deserts Are Expanding
• This article references information from Hawai‘i Ocean Time-series (HOT): What’s Happening at Station ALOHA