Using preformed nitrate to infer decadal changes in DOM remineralization in the subtropical North Pacific

The preformed nitrate distribution throughout the subtropical North Pacific is characterized by a negative anomaly between 100 m and the 25.4 sigma(theta) surface. Its presence indicates that nitrogen remineralization in the upper thermocline deviates significantly from Redfield stoichiometry. It has been previously suggested that this feature is created during the degradation of nitrogen-poor dissolved organic matter (DOM) (Emerson and Hayward, 1995). Here we present evidence from two transects in the eastern subtropical North Pacific that degradation of DOM with a high C:N ratio is quantitatively responsible for negative preformed nitrate. We develop a simple isopycnal mass balance to determine preformed nitrate using apparent oxygen utilization (AOU), dissolved organic carbon (DOC), and dissolved organic nitrogen (DON) data. These results agree well with the traditional preformed nitrate calculation and indicate that the intensity of the anomaly is proportional to the magnitude of DOM remineralization and its C: N remineralization ratio. An analysis of preformed nitrate distributions in the eastern subtropical North Pacific between 1980 and 1997 reveals a significant increase in the negative anomaly. While a portion of this change may be attributable to a reduction in ventilation of the gyre, the majority of the signal is due to an increase in the magnitude and/or C:N ratio of DOM remineralization. This conclusion supports recent hypotheses that DOM production and cycling may have increased in this region in the past 2 decades as a result of changes in the phytoplankton community structure.