Biological nitrogen fixation is the conversion of nitrogen gas (N2) to fixed nitrogen (e.g. nitrate). N2 fixation is a crucial component of global ocean biogeochemical cycles. It provides the major source of nitrogen necessary to balance nitrogen loss via denitrification and annamox and thus controls the magnitude of fixed nitrogen in the ocean, with consequences for the cycling and storage of carbon and other nutrients.
Until recently, N2 fixation was thought to be restricted to the warm surface waters (20 to 30dC) of the low latitude subtropical gyres. This perception has focused decades of research on nitrogen fixation to the low latitude ocean, where other nutrients such as iron and phosphorus limit the activity of nitrogen fixers. However, there is new evidence that nitrogen fixing organisms are present and active in the cold surface waters of the Arctic Ocean. The nitrogen fixing gene has been detected in the western and central Arctic Ocean and observed rates of nitrogen are comparable to warm water nitrogen fixation. This unexpected result means that our current understanding of global nitrogen fixation is incomplete as it neglects to account for nitrogen fixation in cold waters. In addition, we do not know how temperature, light, nutrients and iron control the distribution and activity of nitrogen fixing organisms in the Arctic Ocean. There is an urgent need to better understand nitrogen fixation in the Arctic Ocean for three reasons. Firstly, the primary productivity of the Arctic Ocean is already limited by the availability of nitrate. Warming of the Arctic Ocean has caused sea ice to decline by 9% per decade since the 1970s, causing primary productivity to increase by 25%. As the Arctic Ocean warms and becomes ice free, primary production is predicted to increase. To support increased primary production, there must be an additional source of nitrate, but this source remains elusive. Inputs of nitrate from rivers, the atmosphere and dissolved organic nitrogen do not meet the nitrate demand associated with increased primary production. Nitrogen fixation may therefore be a crucial source of nitrogen in the contemporary and future Arctic Ocean and provide the fixed nitrogen necessary to support the future increase in primary production. Secondly, there is ongoing debate on the status of the nitrogen budget in the Arctic Ocean. Although the amount of nitrate entering and leaving the Arctic Ocean is equal, there is a large nitrate sink in the sediments on the Arctic shelves. This implies that either there is a large deficit in nitrate in the Arctic Ocean, or there is currently a source of nitrate that is unaccounted for. Nitrogen fixation may be the missing source required to balance the Arctic Ocean nitrogen budget. However, the lack of observations means that pan-Arctic estimates of nitrogen fixation are rudimentary. Finally, global numerical models that represent the oceanic nutrient cycling and the marine ecosystem currently ignore nitrogen fixation in the Arctic Ocean. This means that the ability of these models to predict how the Arctic ocean will respond to increased warming and increased productivity is inaccurate because they don’t account for the additional nutrient source or the effect nitrogen fixation has on nutrients such as carbon, phosphorus and iron. Overall, our lack of knowledge on nitrogen fixation in the Arctic Ocean means we do not know how important this process is to global nitrogen fixation or nitrogen budget. N-ARC proposes to conduct the first holistic study of nitrogen fixation in the eastern Arctic Ocean. We chose this region for two reasons; (a) new data from N-ARC team has detected the gene responsible for nitrogen fixation in this region, providing new evidence that nitrogen fixation is occurring in the eastern Arctic Ocean and (b) there are strong gradients in temperature and nutrients, allowing us to explore how the ocean conditions control nitrogen fixation in the Arctic Ocean.