Search

During the 3rd Chinese National Arctic Research Expedition cruise in the summer of 2008, nutrients (NO3, NO2, SiO32, and PO43) and dissolved oxygen were measured in the western Arctic Ocean, to derive the vertical distribution of nutrient tracers and its relationship to water structure and biogeochemical processes. The nutrient data show that surface waters had the lowest NO3/PO43 (mean of 0.5) and SiO32/PO3 (mean of 2.8) values in the water column, suggesting an excess of phosphate. Winter Bering Shelf water (wBSW) had high Si* (16.7 µmol/L; Si*=[Si(OH)4]-[NO3]) with negative N* (−11.7 µmol/L; N*=[PO43]-16[PO43]+3.5 µmol/L) in the water column, indicating nitrate deficiency. The warm Atlantic layer had positive N* (0.8 µmol/L) and negative Si* (−5.4 µmol/L) compared with Pacific source water. The vertical distribution of nutrients indicates that wBSW can be characterized by N* minimum and Si* maximum. In contrast, minima of Si* and SiO32/PO43 below 200 m indicate the distribution of Atlantic warm water. [ABSTRACT FROM AUTHOR]

The nutrient-rich Pacific Ocean seawater that flows through the Bering Strait into the Chukchi Sea is generally considered to be the most important source of nutrients to the Arctic euphotic zone. The inflow is characterized by nitrogen deficit and low nitrate/phosphate (N/P) ratios; this is ascribed to sedimentary denitrification on the Chukchi shelf by preoccupant opinions. However, the Chukchi Sea also has high primary production, which raises the question of whether the biological pump may also significantly modulate nutrient properties of the throughflow. Here, we show that nitrate concentrations of the Pacific inflow gradually decrease northward in association with notable biological utilization. The phytoplankton N/P uptake ratio was 8.8±2.27, higher than the N/P ratio of Pacific inflow water (5–6). This uptake ratio, in combination with efficient vertical nitrogen export, serves to preferentially remove nitrogen (relative to phosphorus) from upper waters, thereby further intensifying the Arctic nitrogen deficit. Accordingly, as large as about 111.7×109 mol N yr−1 of nitrate was extra consumed, according to the real N/P uptake ratio rather than the ratio of the Pacific inflow, which may be as great as half the nitrogen loss ascribed to sedimentary denitrification. Our findings suggest that besides sedimentary denitrification, biological disproportionate utilization of nutrients in the Chukchi Sea upper water is another important contributor to the nitrogen limitation and excess phosphorus in the upper Arctic Ocean. In the rapid Arctic change era, the predicted reinforced biological carbon pump could further impact the nutrient dynamics and biogeochemical process of the Arctic Ocean. [ABSTRACT FROM AUTHOR]