Six experiments were carried out with natural phytoplankton populations grown under simulated natural conditions, in an attempt to assess the gross effects of major nutrient and micronutrient perturbations upon the growth and species composition of these populations. Water was collected from Yaquina Bay, Oregon, and filtered into twenty-litre growth containers. Some of this water was treated with activated carbon to remove a portion of the trace metals and trace organics. Various combinations of major nutrients, micro-nutrients, and chelators were added to these cultures. Inoculation was with unfiltered bay water, at a ratio of 1:1000. Incubation was in outdoor tanks of water circulating from the bay, under natural sunlight. The major nutrients and micro-nutrients appeared to have fundamentally different effects on the growth and species composition of phytoplankton. The micro-nutrients had marked effects on the species composition of the phytoplankton populations, while major nutrients had minimal effects on species composition. The micro-nutrients also had substantial effects on population growth rates, lag periods, and final yields. The major nutrients had a substantial effect only on final yields.