Temporal variation tracer N*, implications for the biogeochemistry of nitrogen in Alfonso Basin, Gulf of California, Mexico

Abstract

Vertical and seasonal distribution of the biogeochemical tracer N* was estimated to delineate the key pathways that regulate nitrogen in the Alfonso Basin, Bay of La Paz, between February 2012 - March 2013. We used data of dissolved inorganic nitrogen (DIN) and phosphorus (DIP) concentrations from four levels in the euphotic zone (100, 33, 3, and 0.1% I0) and another four depths below it (150, 200, 250 and 300 m). For the calculation of N*, its constants were reestimated from the original equation based on historical data of nutrients compiled between December 2005 - December 2012. Detection of anomalies on the mean ratio of NID:DIP (13.16) indicated a predominance of negative values corresponding to a greater removal of NID, especially below the 150 m, under hypoxic conditions (< 1.4 ml l-1) and during periods of high
biological production for the area. The incidence of these values could be associated to the local denitrification process, as well as advection of water masses from the Tropical Eastern Pacific. Positive values were evident on surface from May until the end of the study in most part of the euphotic layer, with higher intensity between August - October, December and March 2013, suggesting an increase of the relative importance of biological N2 fixation on denitrification. The application of the tracer N* indicates that the participation of both N2 fixation and denitrification in the basin are active mechanisms of regulation of the nitrogen cycle. Future efforts will focus on the determination of rates of these physiological pathways.