Nutrients like nitrate and phosphate in the sea are both indicators of biological activity and indicators of the identity and mixing of ocean water masses. Their role as indicators derives from the fact that biological processes remove nutrients from much of the surface ocean and deliver nutrients to the deep sea due to the remineralization of organic matter. However, due to the insensitivity of standard nutrient techniques, field investigations on the role of nutrients as tracers have had to focus on the deep sea where the highest nutrient concentrations are located. Nutrients in the upper, sunlit portion of the ocean have been much less studied because easily usable analytical techniques have not been available. Now however, with easy-to-use high sensitivity techniques we are following the fates of nitrate, nitrite, and ammonia in the surface of the ocean. The most interesting result so far is moderately (i.e., 200 nanomolar vs 10 nanomolar) concentrations of ammonia in the surface ocean can persist for periods of up to five days when estimates of ammonia utilization rates in the surface ocean suggest that phytoplankton should readily consume the ammonia. Thus ammonia in the surface ocean can, in effect, serve as a water mass tracer. We in my laboratory are pursuing this interesting question.