A time-series assessment of the nitrogen cycle at BATS

The assimilation of nitrate and ammonium in the euphotic zone of the Sargas so Sea at the US JGOFS Bermuda Atlantic Time-Series Study (BATS) site was m easured from February, 1992 through April, 1994. Nitrate concentrations var ied from < 5nM during the oligotrophic period to several hundred nM during the winter blooms in February and March of each year. Concentrations were f requently constant with depth until the nitracline, where they increased by 2 orders of magnitude over the 20m 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 w ith depth. Nitrate assimilation rates exhibited strong seasonality with rat es < 1nMd(-1) in the oligotrophic period, <greater than> 10nMd(-1) during t he blooms in 1993 and 1994, and reaching > 150 nMd(-1) during 1992. Nitrate assimilation by particles passing a GF/F filter was frequently a significa nt component of total nitrate use regardless of light level. Ammonium assim ilation was more constant than nitrate assimilation over the year and small particles rarely contributed more than a few percent to the total assimila tion. Nitrate assimilation was correlated with concentration, both at the h ighest concentrations during the 1992 bloom and at the lower concentrations during the oligotrophic periods. Integrated nitrate assimilation in 1992 w as 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 prod uction during the winter mixing period has not been previously assessed and can be a large contribution to the yearly value. Using the traditional def inition of the f-ratio, values ranged from 0 to 0.8 with no pattern for nit rate concentrations between 1 and 50 nM. The constant, low nitrate concentr ations in the water column, the lack of variation in the vertical profile a nd the observed regeneration of nitrate during the incubations all suggest that nitrate assimilation during the oligotrophic period does not contribut e to "new" production and nitrate should be considered a regenerated nutrie nt in the oligotrophic ocean. (C) 2001 Elsevier Science Ltd. All rights res erved.