The ability of natural phytoplankton assemblages in the Baja California and northwest Africa upwelling regions to assimilate nitrate, ammonium and silicic acid at simulated in situ light intensity and in the dark, was determined in 6 hr tracer experiments using the stable isotopes 15N and 30Si. Nitrate was almost never taken up in the dark, but dark uptake rates of ammonium and silicic acid ranged from zero to values equal to those measured at saturating light intensity....