Nitrogen dynamics in the East Greenland Sea

This study established ammonium and nitrate uptake rates in the East Greenland Sea at both the Arctic and Polar Fronts in May of 1987 and at the Polar Front in June of 1987 and 1988. In addition, ammonium regeneration rates were measured at the Polar Front in 1987 and 1988. Field experiments carried out at the Polar Front in 1988 examined procaryotic versus eucaryotic uptake and the effects of substrate manipulation on uptake by both eucaryotic and procaryotic organisms. All uptake rates were measured using heavy isotope tracer techniques. The results of this study indicated total nitrogen uptake at the Arctic Front in May of 1987 was nearly one third that of the Polar Front at the same time. In addition, 70% of the nitrogen being assimilated at the Arctic Front was in the form of nitrate as compared to 28% at the Polar Front. Nitrate based or "new" production at the Polar Front was below the levels needed for flux from out of the euphotic zone without detremintal affects on the system. The low percentage of nitrate being assimilated was consistent for the Polar Front during June of 1987 and 1988. Uptake rates of ammonium were higher in June of 1987 than during any other period measured and absolute ammonium uptake rates were as much as 200 times that of nitrate absolute uptake at some stations. The most significant difference in physical parameters between the Arctic and Polar Fronts appeared to be the difference between depth of the pycnocline with relation to the euphotic zone with the Arctic Front having a much deeper mixed layer than the Polar Front. Correlation analysis of several parameters and nitrogen uptake rates showed a significant negative correlation between ammonium and the depth of the pycnocline. Results from inhibition and substrate manipulation experiments showed that changing substrate C:N ratios affected assimilation of ammonium by procaryotes. It appears therefore, from this study, that although primary production, as estimated from uptake rates, is consistent with open ocean production at the Arctic Front and that the majority of it is nitrate based. However, at the Polar Front total production is three to four times as high as at the Arctic Front and the majority is ammonium based. In addition bacteria appear to be utilizing ammonium as a source of nitrogen, and therefore may be competing with phytoplankton for this nutrient under certain circumstances.