Nitrite distribution and its regulating processes in the equatorial Pacific Ocean

Abstract

The distribution of nitrite and nitrate in the tropical Pacific Ocean along the 155°W meridian was investigated during the KH-69-4 Cruise of the R. V. Hakuho Maru (Aug. 12–Nov. 13, 1969). The accumulation of very high concentrations of nitrite was observed in the subsurface waters on both sides of the equator. South of the equator, between 2·5°S and 7·5°S, the nitrite maximum was centered at a depth of about 150 m on the σ t surface 24·0–24·5 and coincided with the salinity maximum of the South Pacific Subtropical Subsurface Water. The highest value, 2·92 μg at.N/1., was observed at 148 m depth at 2·5°S. North of the equator, the nitrite maximum was located above the σ t surface 24·0. Other characteristics of this water mass differ to some extent from those of the subsurface water of the South Pacific. Capacities for nitrite production of the nitrite-maximum waters in the equatorial regions were of the order of 0·06–0·08 μg at.N/1./day. The residence time of nitrite was estimated to be about 30 days. A survey of the bacterial population in the subsurface waters showed that nitrate-reducing bacteria were abundant in the tropical South Pacific, whereas the numbers of nitrifying and denitrifying (N 2-producing) bacteria were extremely small. The contribution of the nitrate-reducing bacteria is probably significant in the South Pacific area. However, circumstantial evidence suggests that, north of the equator, nitrite is also produced by the reduction of nitrate by phytoplankton.