Eastern Tropical North Pacific Ocean Research Articles

VOLTAMMETRIC MEASUREMENT OF ZINC IN THE NORTHEASTERN TROPICAL PACIFIC OCEAN1

Relative zinc concentrations in the upper 1,000 m of the northeastern tropical Pacific Ocean were measured by pH‐controlled differential anodic stripping voltammetry (DASV) aboard ship. The waters above 125 m are significantly lower in zinc content than those between 125 and 1,000 m. A substantial fraction of the measured zinc appears to be adsorbed on particulate matter >0.45 µ. The adsorption can be reversed by lowering the pH to 5.6 with CO2 gas.

Effects of phosphate concentration on cell division rates and yield of a tropical oceanic diatom.

Cell division in batch cultures of the tropical oceanic diatom, Chaetoceros gracilis, was measured in artificial medium containing different initial amounts of phosphate. The rate of growth was limited by phosphate concentrations below approximately 0.22 µg.at./l. and appeared to be a linear function of concentration rather than a hyperbolic function. Ks in the hyperbolic relationship equating growth rate and concentration would be about 0.12 µg.-at./l. The results substantiate an earlier suggestion that low phosphate did not limit the rate of phytoplankton growth in the northeastern tropical Pacific Ocean; and they also verify previous use of a linear nutrient relationship in productivity equations given by other authors. Pauses in cell division at low phosphate concentrations are either attributed to the induction of phosphatases which can act on stored intracellular phosphorus or to low extracellular concentrations. Final cell numbers were a linear function of initial phosphate concentration up to about 0.8 µg.-at./l. The amount of phosphorus per cell is 2 x 10-9 µg.-at./cell or 10-11 µg.-at./cubic micron. A requirement for boron by this diatom is also suggested by increased growth upon the addition of this nutrient.

On denitrification in the northeastern tropical Pacific Ocean

The existence of a maximum in nitrite concentration in the oxygen minimum layer below the thermocline in the northeastern tropical Pacific Ocean has been confirmed. At depths corresponding to the nitrite maximum, the nitrate concentration decreased and no ammonia was found. This nitrate decrease was greater than the corresponding increase in nitrite and it is suggested that further denitrification to N2O or N2 had occured.

SURFACE NITROGENOUS NUTRIENTS AND PHYTOPLANKTON IN THE NORTHEASTERN TROPICAL PACIFIC OCEAN 1

Mean surface concentrations of nitrogenous nutrients and phosphate are reported for various areas in the northeastern tropical Pacific Ocean. Except at the Costa Rica Dome, an area of upwelling, nitrate concentrations were very low. NO3− : PO43− ratios were also generally low and phytoplankton might remove all of the nitrate while appreciable concentrations of phosphate were still present. Measurements of nitrate during growth of batch cultures of Gymnodinium simplex showed that concentrations below approximately 5 µg‐at./liter probably limited the rate of growth of this dinoflagellate. Since surface nitrate concentrations were generally lower than this value, the rate of growth of G. simplex would be limited over much of the area, unless sources of nitrogen other than nitrate were available. Ammonia and labile amino nitrogen were measured as one nitrogen fraction in one offshore water mass northwest of the Dome. The ammonia concentration was greater than that of nitrate and the N: P ratio was increased threefold to fourfold if ammonia was included with nitrate. The growth of three phytoplankton cultures was supported by ammonia and by various amino compounds. Ammonia may be a relatively important nitrogen source in the northeastern tropical Pacific Ocean, and should be included with nitrate as a measure of the total nitrogen available to phytoplankton. Ammonia may moderate the extreme nitrogen deficiency which would be expected if nitrate were considered the only source of nitrogen.