Simultaneous measurements of nitrate, oxygen, and carbon dioxide on oceanographic moorings: Observing the Redfield Ratio in real time

Johnson, Kenneth S.

Limnol. Oceanogr., 55(2), 2010, 615-627 | DOI: 10.4319/lo.2010.55.2.0615

ABSTRACT: Autonomous observations of dissolved nitrate, oxygen, and total inorganic carbon (TCO2, which is derived from pCO2 and estimates of alkalinity) from sensors on the M1 and M2 moorings, off Monterey Bay, California, are examined. These observations are used to assess the linkages between nitrogen, oxygen, and carbon cycling with the Redfield ratio as a framework for the analysis. Concentrations of TCO2, oxygen, and nitrate were highpass filtered to remove low-frequency signals driven by water mass changes. Daily cycles in each property are apparent with maxima or minima at the end of daylight. These daily cycles are consistent with biological production of oxygen or uptake of TCO2 and nitrate. Ratios of the changes in these diel cycles approach values expected from the Redfield ratio early in upwelling cycles. However, periods are frequently seen where the utilization of nitrate N is substantially lower than expected when compared to observed changes in TCO2 or oxygen. During these periods, fixed nitrogen must be supplied from other sources, such as ammonium or urea, or it is obtained from deeper waters by vertically migrating phytoplankton. These migrating phytoplankton must then return to the surface where inorganic carbon is consumed and oxygen is produced.

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