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Scientists at Scripps Institution of Oceanography at the University of California San Diego have found that iron in deep ocean regions affects plankton growth, revealing a large, new area where iron influences life at the base of the food web.

The findings were published in the Proceedings of the National Academy of Sciences December 10, 2018.

Phytoplankton are photosynthetic microscopic organisms that form the base of the food chain in the ocean, and iron is a crucial element for phytoplankton growth. Scientists have known that iron levels affect the productivity of phytoplankton, but most of this previous research has focused on areas visible to satellites. Regions in the lower photic zone–below the surface that satellites cannot see–are also important areas for phytoplankton growth, particularly in the subsurface chlorophyll maximum layer, the region below the surface of the water that contains the most chlorophyll, an index of phytoplankton abundance. The location and size of this region varies with ocean depth; it can be found shallower in nearshore environments but much deeper in offshore areas.

This new research shows that low iron levels can limit phytoplankton growth in this deeper layer. Iron limitation in this region has consequences for the larger ecosystem, because plankton in this layer provide a significant amount of food and energy. Phytoplankton support grazers like krill, which in turn support a large marine food web, from fish to whales and even humans. Iron limitation could also impact the oceanic carbon cycle, the process of circulating carbon around the planet and controlling atmospheric carbon dioxide levels. Phytoplankton are crucial for moving carbon out of the atmosphere and deep into the ocean.

“In many areas of the ocean, the subsurface chlorophyll maximum layer is highly productive,” said Scripps researcher Katherine Barbeau, senior author of the study. “Understanding how iron limitation influences this important yet under-studied region enhances our ability to monitor changes in productivity in marine ecosystems.”

Aboard a research expedition with the California Cooperative Oceanic Fisheries Investigations (CalCOFI) on Research Vessel Sally Ride, the scientists collected water samples from various stations along a coastal to offshore trackline. The samples were then analyzed for iron.

One method involved looking at nutrient levels in the water. When diatoms - a common phytoplankton - are growing, they take up the minerals silicate and nitrate from the surrounding seawater. However, when they are stressed from limited iron they lose the ability to take up nitrate. Therefore a higher ratio of nitrate to silicate levels in the water is a sign that the diatoms are stressed and there is not sufficient iron. Looking at a 30-year nutrient dataset from the CalCOFI time series, the researchers found that iron limitation in nearshore subsurface chlorophyll maximum layers is most prominent during the summer, and has increased in frequency and extent over the last two decades. The reasons remain unclear, but could be related to large-scale climate systems like the North Pacific Gyre Oscillation. When they looked at nutrients globally, the researchers found indications of subsurface iron limitation in many upwelling regions.

“Iron limits primary production by phytoplankton in many open ocean environments,” said David Garrison, a program director for the National Science Foundation’s Long-Term Ecological Research program, which funded the project. “Now we also have evidence of this same iron process in coastal ecosystems.”

This research was supported by the National Science Foundation, including grants in support of  the California Current Ecosystem Long Term Ecological Research program. Additional support was provided by the U.S. Department of Energy. The CalCOFI program is supported by the California Department of Fish & Wildlife, the National Oceanic and Atmospheric Administration Fisheries Service, and Scripps Institution of Oceanography.