The North Pacific Transition Zone, just north of Hawai'i, is an important area for fisheries in the Pacific Ocean. Credit: Ryan Tabata, University of Hawai'i at Mānoa
Along with nutrients like nitrogen and phosphorus, iron is essential for the growth of microscopic phytoplankton in the ocean. However, a new study led by oceanographers at the University of Hawai'i at Mānoa with collaborators at the University of Washington revealed that iron released from industrial processes, such as coal combustion and steelmaking, is altering the ecosystem in the North Pacific Transition Zone. This region, just north of Hawai'i, is important for fisheries in the Pacific.
The study was published June 2 in the Proceedings of the National Academy of Sciences.
"We were able to see a connection between human activities and the location of key ecosystem boundaries in the ocean that are important for marine organisms," said co-author Randie Bundy, a UW associate professor of oceanography. "I hope this research highlights that human activities can impact the ocean in multiple ways, not just through changes in the climate. I think it also highlights the importance of tracking key ocean ecosystem boundaries over time, so we can better understand how this might impact marine organisms."
Iron from human activities billows into the atmosphere and can be carried to distant lands or oceans before it's scrubbed from the skies by rain. Industrial iron has previously been detected in the North Pacific Transition Zone, but it was unclear what effect the iron had on the ecosystem.
To piece together the seasonal cycle of iron input, phytoplankton growth and ocean mixing, the researchers analyzed water and phytoplankton samples and studied ocean dynamics during four different expeditions to this region of the Pacific Ocean. They also assessed the iron in these waters to determine whether it had the unique isotope signature of iron that is released from industrial processes.
The team found that phytoplankton in the region are iron-deficient during the spring, so an increase in the supply of iron boosts the spring phytoplankton bloom that is typical in the area. However, as a result of a booming bloom, they deplete other nutrients more quickly, leading to a crash in phytoplankton later in the season. Importantly, the iron isotope signature did, in fact, indicate the presence of industrial iron out in the Pacific, thousands of miles away from its source.
"The ocean has boundaries that are invisible to us but known to all sorts of microbes and animals that live there," said Nick Hawco, lead author and assistant professor at the University of Hawai'i at Mānoa School of Ocean and Earth Science and Technology. "The North Pacific Transition Zone is one of these boundaries. It divides the low-nutrient ocean gyres from the high-nutrient temperate ecosystems to the North. With more iron coming into the system, that boundary is migrating north, but we are also expecting to see these boundaries shift northward as the ocean warms."
That's not necessarily all bad, Hawco said. But unfortunately, the regions of the transition zone that are closer to Hawai'i are among those that are losing out.
"It's a one-two punch: Industrial iron is impacting the base of the food web and the warming of the ocean is pushing these phytoplankton-rich waters further and further away from Hawai'i," Hawco said.
The research team is developing new techniques to monitor the iron nutrition of ocean plankton. This will shed light on how changes in iron supply, from both natural or industrial sources, could impact ocean life.
"A project of this scale is truly the result of collaboration between scientists with diverse expertise," said co-author Sacha Coesel, a UW research scientist in oceanography. "Thanks to these collaborations, we were able to integrate satellite observations - which reveal large-scale, multi-year trends - with ship-based data collected over several years at the same locations. This integration allowed us to link broad environmental patterns with the fine-scale molecular details of gene expression in key organisms responding to iron availability. Individually, each dataset is valuable, but together, they provide the depth and resolution needed to generate robust, predictive insights into ecosystem dynamics."
Other UW co-authors are Ginger Armbrust and Mora Groussman. See the paper for a full list of authors.
This study was funded by the Simons Foundation and National Science Foundation.
This story is adapted from a release by the University of Hawai'i Mānoa School of Ocean and Earth Science and Technology.
