Nitrogen Uptake by Phytoplankton in the Discontinuity Layer of the Eastern Subtropical Pacific Ocean1

In the eastern subtropical Pacific Ocean off the continental shelf of Mexico, a pronounced discontinuity layer often exists within the euphotic zone. Phytoplankton in this layer used nitrate as their major inorganic source of nitrogen, while those in the overlying nutrient‐impoverished water used ammonium ion. The uptake of 14 C‐HCO 3 − generally exhibited two maxima, one above the discontinuity layer and the other within it. Chlorophyll a concentrations usually increased with depth and reached maximum concentrations in the discontinuity layer. The chlorophyll a found at the upper boundary of the discontinuity layer exhibited low or undetectable 14 C uptake.