An Unintentional Iron Fertilization Experiment
The idea of fertilizing the ocean with iron, an element essential for the growth of phytoplankton, is being considered as a climate change mitigation measure - when they die, phytoplankton take carbon into the deep ocean.
However, concerns have been raised about large scale intentional iron fertilization that may limit its effectiveness as a carbon dioxide removal strategy. A major one is nutrient robbing where iron added to one part of the ocean decreases nutrients that would have supported ecosystems (and natural carbon sequestration) elsewhere.
Nick Hawco, assistant professor at the University of Hawai‘i, is part of a team evaluating the potential for deliberate iron fertilization of the ocean, but he’s also researching an unintentional iron fertilization ‘experiment’ that has been on-going for decades.
Hawco is lead author of a new study that has revealed that iron released from coal combustion and steel making in East Asia 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.
Water and phytoplankton samples have now been studied during four different expeditions to the region.
The team found that the unintentional iron fertilization means 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, especially nitrate, leading to a crash in phytoplankton later in the season.
Industrial iron is therefore 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.
“To the best we can tell, the impacts of anthropogenic iron on ecosystems is most intense in the North Pacific. During our cruises, bad weather prevented us from reaching beyond 42 north, but we expect that impacts would reach up to Alaska.”
There are likely key areas of anthropogenic iron release in South America and Southern Africa that will grow over time, he says. It’s also likely that there were significant emissions during industrialization of the US east coast and throughout western Europe that would have added iron to the North Atlantic. These would have been most intense during the 20th century, and the impacts on the Atlantic resulting from this have probably come and gone.
“Our results indicate that a widespread unintentional iron fertilization experiment has been going on for decades as a result of industrial emissions from Asia. In this study, we were able to identify that iron input has coincided with increased chlorophyll in the northern transition zone region, while chlorophyll has decreased in the southern transition zone.
“That's the limit of what we can tell about how the ecosystem has changed based on data alone, especially because climate change is also impacting these ecosystems. One thing that struck me as our team was synthesizing our results was how - despite the best efforts of ocean scientists - we mostly have a few snapshots of these ecosystems to piece together how they might be changing due to human activities.”
There’s no way to measure iron without physically going to out to the open ocean, collecting a sample and analyzing it back in the lab, which can take months, he says. “There has been amazing progress in development of new sensors that can be used autonomously by ocean-going robots that can measure some nutrients (for example, the Biogeochemical Argo floats). Development of sensors to measure iron would represent a major leap forward.”
In other phytoplankton and climate change news, a fleet of ocean robots has been deployed to explore the depths of the Labrador Sea as part of a year-long experiment led by scientists from the UK’s National Oceanography Centre. They are studying the ocean processes that help remove carbon dioxide from the atmosphere and store it in the deep sea.