Science for All: Phytoplankton overfed on iron pollution are depleting nutrients faster
A new study has unearthed strong observational evidence that industrial pollution is altering nutrient cycles and ecosystem boundaries in the ocean.
According to the researchers, from across institutions in the U.S., iron released due to human activities enhances spring phytoplankton blooms as well as speeds up the rate at which nutrients are consumed. The changes may work in synergy with climate-driven ocean warming and stratification, speeding up the transition to nutrient-poor conditions across broader swaths of the ocean, they added.
The team published its findings in Proceedings of the National Academy of Sciences on June 2.
The authors conducted four oceanographic expeditions between 2016 and 2019 to measure dissolved iron and its isotopic composition. They also used satellite observations and gene expression analyses to understand the biological responses to the element's presence.
The team collected data during three springtime cruises in 2016, 2017, and 2019 and one autumnal cruise in 2018 to track seasonal changes in iron chemistry. Team members measured the amount of dissolved iron in the water and its isotopic composition using trace-metal clean techniques and mass spectrometry. They also analysed metatranscriptomic data from phytoplankton samples to look for the expression of genes associated with iron stress — bio-physiological changes induced by exposure to the element.
Satellite data were used to assess long-term changes in phytoplankton distribution and productivity. The team focused especially on chlorophyll-a levels, for which they used data 'produced by the Ocean Color Climate Change Initiative' of the European Space Agency, according to the paper. The Initiative 'integrates observations across multiple ocean colour platforms to form a continuous 26-year record'.
This way, the team reported that about 39% of surface ocean iron during spring comes from atmospheric pollution, particularly in emissions from East Asia. They found this 'anthropogenic iron' has a distinct isotopic signature that makes it traceable in seawater. Its input peaked during spring, when westerly winds have been known to carry aerosols across the Pacific Ocean.
The predictably higher iron concentration in spring appeared to make the phytoplankton more productive, especially that north of the Transition Zone Chlorophyll Front (TZCF) — the boundary between nutrient-rich and nutrient-poor waters.
The greater access to iron enhanced the growth of the phytoplankton, which consumed more nitrates. The team wrote in its paper, 'Over the past 25 years, increasing anthropogenic iron input appears to have stimulated springtime phytoplankton growth, ultimately leading to faster depletion of … nitrate. Thus, large-scale iron pollution may be increasing the prevalence of nitrogen limitation,' expanding the size of the nutrient-poor part of the ocean.
The retreat of an ecosystem boundary can have severe ecological and socioeconomic consequences. Phytoplankton are at the base of marine food chains, so a change in their distribution will affect zooplankton, fish, and larger predators like seabirds and whales. Species that can't migrate or adapt fast enough may decline or go extinct.
Equally, as productive regions retreat, unproductive areas will expand and the ocean will be able to take up less biological carbon, undermining an important natural buffer against climate change.
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