Arctic Ocean nitrate levels fall after 2009 sea-ice loss
The arctic ocean has shifted from a system mainly limited by light to one increasingly limited by nitrate availability after widespread sea-ice loss sharply reduced nitrate levels, researchers from the University of Edinburgh said. The study, published in Communications Earth & Environment, ties the change to a falling nutrient supply in waters leaving the Arctic and to a likely narrowing of what the food chain can support.
University of Edinburgh study
Marta Santos-García, a Ph.D. student in the University of Edinburgh's School of GeoSciences and co-lead of the study, said, "For years, sea-ice loss in the Arctic Ocean was expected to increase phytoplankton growth because more sunlight could reach surface waters." She added: "Our findings suggest that this relationship has changed: the Arctic Ocean appears to have shifted from a system mainly limited by light to one increasingly limited by nitrate availability, with far-reaching consequences for marine ecosystems, food chains and the role of the Arctic in Earth's climate."
The team analyzed more than two decades of sampling data from Fram Strait, the main gateway through which Arctic waters flow into the Atlantic. The clearest break came from 2009 onwards, when nitrate levels in waters leaving the Arctic began falling steadily. That drop lined up with a drastic reduction in Arctic sea ice that began around the same time.
Fram Strait nitrate decline
The study says the sea-ice loss exposed vast shallow regions of the ocean to sunlight, fueling a process that breaks down nitrate and removes it from seawater. It also says the extensive loss of ice ramped up benthic denitrification in shallow continental shelves that underlie nearly half of the Arctic Ocean. Nitrate is vital for plankton growth at the base of the Arctic food chain, so lower levels limit how much life the ecosystem can support.
Raja Ganeshram, professor in the University of Edinburgh's School of GeoSciences and study leader, worked on the analysis with researchers from the Norwegian Polar Institute, the Scottish Association for Marine Science, the Technical University of Denmark and Alfred-Wegener-Institut, Germany. The team said the shift to nitrate-limited conditions suggests the Arctic Ocean may only be able to support smaller species of plankton in the future.
Arctic Ocean food chain
The food-chain effect is only part of the change. The researchers said dwindling nitrate could also reduce the Arctic Ocean's capacity to store carbon, and they said it is very unlikely the Arctic Ocean will ever revert to its previous state because ongoing sea-ice loss is driving the change. Further research is needed on wider effects on marine populations in other parts of the world's oceans, including the North Atlantic, as the chemical shift in the Arctic spreads beyond the waters first measured in Fram Strait.
