A time-series assessment of the nitrogen cycle at BATS

The assimilation of nitrate and ammonium in the euphotic zone of the Sargasso Sea at the US JGOFS Bermuda Atlantic Time-Series Study (BATS) site was measured from February, 1992 through April, 1994. Nitrate concentrations varied from <5 nM during the oligotrophic period to several hundred nM during the winter blooms in February and March of each year. Concentrations were frequently constant with depth until the nitracline, where they increased by 2 orders of magnitude over the 20 m sampling interval. Spatial and temporal variations in ammonium concentrations were less dramatic, ranging from 20 to nearly 200 nM during the bloom period and remaining generally constant with depth. Nitrate assimilation rates exhibited strong seasonality with rates <1 nM d−1 in the oligotrophic period, >10 nM d−1 during the blooms in 1993 and 1994, and reaching >150 nM d−1 during 1992. Nitrate assimilation by particles passing a GF/F filter was frequently a significant component of total nitrate use regardless of light level. Ammonium assimilation was more constant than nitrate assimilation over the year and small particles rarely contributed more than a few percent to the total assimilation. Nitrate assimilation was correlated with concentration, both at the highest concentrations during the 1992 bloom and at the lower concentrations during the oligotrophic periods. Integrated nitrate assimilation in 1992 was 0.94 mol N m−2 yr−1 compared to 0.14 mol N m−2 yr−1 in 1993, and was completely dominated by assimilation during the winter bloom. New production during the winter mixing period has not been previously assessed and can be a large contribution to the yearly value. Using the traditional definition of the f-ratio, values ranged from 0 to 0.8 with no pattern for nitrate concentrations between 1 and 50 nM. The constant, low nitrate concentrations in the water column, the lack of variation in the vertical profile and the observed regeneration of nitrate during the incubations all suggest that nitrate assimilation during the oligotrophic period does not contribute to “new” production and nitrate should be considered a regenerated nutrient in the oligotrophic ocean.