NITRATE UTILIZATION BY PLANKTON IN THE EQUATORIAL PACIFIC MARCH 1988 ALONG 150-DEGREES-W

Rates of nitrate utilization were measured in March 1988, from 6-degrees-N to 8-degrees-S on longitude 150-degrees-W in the Pacific Ocean. Surface waters in the equatorial upwelling contained about 5-mu-M nitrate. Samples of seawater incubated on deck, as in primary production experiments, lost about 150-nM nitrate during the day and an equal amount at night at the equator. These rates exceeded by several fold the rates of nitrate incorporation into particulate matter. The carbon equivalent of the 24-hour nitrate consumption also exceeded the rate of C-14 primary production. The large nitrate consumption is not readily explained in terms of phytoplankton physiology but may serve to fuel speculation on the possible production of dissolved organic nitrogen and on the role of bacteria in processing nitrate in the equatorial Pacific upwelling region.