The effects of upwelling were clearly evident in the temperature sections, especially at 110°W (left panel) and 125°W (right panel).

Nitrate concentration was close to twice that measured during the 1992 NOAA Spring Survey. This natural variability provided a unique opportunity to observe the effects of changes in the supply of nutrients on phytoplankton properties in an open ocean tropical setting. The NECC, seen as a trough in the temperature contours centered around 5°N, was much more notable during the 1992 NOAA Fall Survey.

On leg 3, 140°W (left panel) and 125°W (right panel) lines, the chlorophyll maximum averaged 0.36 µg l-1 and was at 80-100 meters at the ends of the transect and rose to 40-60 meters around the equator. An increase in surface chlorophyll concentrations, to a maximum of 0.45 ug l-1 was observed from the equator extending about 7-8 nm to the south. These elevated chlorophyll concentrations were atributed to the presence of Rizosolenia sp. Abundance of this large (625 µm long by 250 µm diameter) centric diatom averaged about 600 cells per liter (over 100 µgC l-1) and these relatively high abundances appeared to be limited to the surface waters. Average surface chlorophyll was 0.157 µg l-1.

On leg 4, 95°W (left panel) and 110°W (right panel) lines, the chlorophyll maximum averaged 0.35 µg l-1, with highest values (0.45-0.50 µg l-1) found around the equator between 15°N and 15°S and lower values (0.20-0.30 µg l-1) toward either end of the 110°W transect. As was the case in the leg 3 with the 140°W and 125°W transects, the chlorophyll maximum occurred at 80-100 meter depths toward either end of the 110°W transect and shallowed to 40-60 meter depths near the equator. The chlorophyll maximum again shallowed to 40 meters at 10°S on the 110°W transect, but at a concentration of only 0.25 µg l-1.Average surface chlorophyll ranged was 0.19 µg l-1.

For leg 5, on the 5°S coastal transect, chlorophyll values were highest in the surface water (3-10 meters) with values exceeding 10 µg l-1 at the nearshore station and surface values over 4 µg l-1throughout. Chlorophyll values in the near surface water were lower along the 12°S transect (about 1-2 µg l-1), although still elevated in comparison to open ocean values. Chlorophyll a values at stations along the transit to and along 95°W were much reduced (<1 µg l-1), with subsurface maxima at 65-80 meters depths towrd the ends of the transect and at 40 meter depth from 4°S to the equator. Average surface chlorophyll was 0.24 µg l-1.

On leg 5 a red tide phenomenon was encountered during the transit between the 5°S and 12°S coastal transects at 5°56.12'S and 82°28.85'W. Several water samples and net tows were obtained in the area and slides for epifluorescence microscopy were prepared. The organisms responsible for the blood red coloration stretching for miles in the seawater appeared to be Mesodinium rubrum.

Autotrophic and protozooplankton biomass were bother higher in the fall (25.0 and 6.2 ug l-1) than the spring (14.5 and 5.1 ug l-1). The relative contributions of the major taxonomic groups was similar between cruises. Flagellates, picoplankton (without prochlorophytes) and diatoms were the major components of autotrophic biomass while dinoflagellates, flagellates and ciliates comprised the heterotrophic assemblages. The heterotropic component contributed 26% of the total biomass. Seven dilution grazing experiements gave average growth rates of 1.09 day -1 for the phytoplankton and microzooplankton grazing rates of 0.46 day -1. Higher autotrophic biomass, without concominant changes in relative abundances of the groups that comprise this assemblage leads to higher carbon to chlorophyll ratios. This may be indicative of a physiological enhancements of intracellular carbon.

The underway nitrate mapping system provided clear records of the equatorial upwelling region and of the so-called long or Legeckis waves. The chlorophyll records were extremely uniform with the exception of maxima around 2°N - 3°N and minima north at 6°N. Areas of lower quantum flux were evident north of the equator, presumably associated with the ITCZ, and were commonly associated with higher values of surface shlorophyll, raising the possibility that rainfall, through the supply of a micro nutrient, is enhancing the production of chlorophyll.