The Arctic Ocean is experiencing rapid changes such as increased freshwater supply and summer sea ice melt. These changes can modify the amount and type of nitrogen and phosphorus nutrients available to primary producers—the microscopic plants at the base of the food chain. Such changes can influence ecosystem functioning.
The Atlantic and Pacific Oceans supply the majority of nutrients to the Arctic, but how much and what types of nutrients are supplied are not well understood. Previous research shows that phosphate (a phosphorus nutrient form) is exported from the Arctic and that the amount of nitrate (a nitrogen nutrient form) entering the Arctic is about the same as the amount leaving.
However, past research also shows that nitrate is also being consumed—the region is a nitrate sink. To maintain this balance between inflows, outflows, and consumption, scientists speculate that there must also be a source (or multiple sources) of nitrate inside the Arctic. The authors tested the idea that dissolved organic nitrogen and phosphorus could be those sources.
To get a better understanding of how organic phosphorus and nitrogen contribute to the wider Arctic Ocean nutrient budget, Torres-Valdés et al. measured nutrient loads along the paths of currents flowing into and out of the Arctic Basin. The team combined these data with seawater volume transports to calculate how much of each goes in and out and to find out whether these nutrients could explain the phosphate export and the nitrogen losses previously identified.
The team concluded that the Arctic Ocean exports slightly more organic nitrogen than it imports and that it exports much more organic phosphorus than it imports—which does not support the idea that organic nutrient forms can explain the balanced Arctic nutrient budget. They had suspected that the unknown source of interior inorganic nitrate might be dissolved organic nitrate, which is converted into inorganic nitrate through bacteria cycling or exposure to ultraviolet light. However, the amount of organic input was very close to the amount of organic output—so there was no large organic-to-inorganic conversation taking place. It was similar for phosphate, too: The authors were not able to explain the amounts of inorganic phosphate traveling in ocean waters by examining the organic phosphate.
However, the information gained tells us something about the complex interaction between physical, chemical, and biological processes within the Arctic Ocean. The results of this study also helped in identifying gaps in our knowledge, which will help focus further studies. (Geophysical Research Letters, doi:10.1002/2016GL069245, 2016)
—David Shultz, Freelance Writer