Nitrogen uptake by natural populations of phytoplankton and primary production in the Pacific Ocean: Regional variability of uptake capacity†

Uptake of nitrogenous nutrients (ammonium, nitrate, and urea) and inorganic carbon by natural populations of phytoplankton in surface waters of the Pacific Ocean was measured by a 13C-15N dual tracer technique. Regional variability of uptake capacity and its correlation with environmental parameters were examined. The uptakes proceeded linearly with time for at least 3 h. Half-saturation constants for uptake of ammonium, nitrate, and urea were generally low (about 0.1 pg-atom N liter-‘). The maximum uptake rate of nitrate was well correlated with the uptake of inorganic carbon; these two rates expressed on a chlorophyll a basis were relatively lower at higher latitude stations. Maximum uptake rates of ammonium and urea clearly covaried with the ambient water temperature. The estimated turnover times of ammonium at subtropical and tropical stations ranged from 1 to 3 days. The contribution of uptake in the dark was larger at low latitude stations. Regional variation of uptake capacity is correlated with temperature and possibly light, but not with nutrient supply.

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