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 (UH) at Mānoa revealed that iron released from industrial processes, such as coal combustion and steel making, is altering the ecosystem in the North Pacific Transition Zone, a region just north of Hawai'i that is important for fisheries in the Pacific. The study was published today in the Proceedings of the National Academy of Sciences USA.
"This is an example of the large-scale impact that human pollution can have on marine ecosystems that are thousands of miles away from the source," said Nick Hawco, lead author of the study and assistant professor in the Department of Oceanography at the UH Mānoa School of Ocean and Earth Science and Technology .
Iron from human activities billows into the atmosphere and can be carried to distant lands or oceans before being scrubbed from the skies by rain. Industrial iron has previously been detected in the North Pacific Transition Zone, however, it was unclear what effect iron was having on the ecosystem.
To piece together the season cycle of iron input, phytoplankton growth, and ocean mixing, Hawco and collaborators from several universities 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 Hawco. "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 shared. 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.
