The nutrients in upwelled water come primarily from the deeper layers of the California Current, which flows from north to south along the California Coast. [Francisco Chavez says more nutrients come from California Undercurrent - CHECK THIS?] This water has spent months or even years traveling in a slow clockwise arc around the margins of the North Pacific, about half a mile below the surface. During its long journey, this water receives a slow but continual drizzle of “marine snow--bits of dead plants, animals, and feces that sink down from the sunlit surface waters.
Marine bacteria gradually break down this sinking organic matter into simple compounds such as nitrate and phosphate. Such dissolved nutrients would be quickly consumed by phytoplankton in shallower water. However, no phytoplankton can live at these depths, where sunlight never penetrates. Thus, the nitrate and phosphate gradually build up over time, as long as the water remains at depth.
The deep water of the California Current would be a perfect fertilizer for diatoms except that it doesn't contain much iron. Fortunately for spring-bloom diatoms, the deep water picks up an iron "supplement" from the muddy seafloor just before it rises to the ocean surface during spring upwelling.
Here's where the iron comes from: During winter storms, rivers and streams along the Central Coast often carry millions of tons of iron-rich dirt out into the ocean. This dirt typically contains lots of mud, which drifts around for weeks before settling down to the continental shelf.
This iron-rich mud is so fine that even by spring it has not quite settled onto the sea floor, but remains suspended in a murky layer five or ten feet thick, just above the bottom. During early spring upwelling events, as deep water moves toward shore, it flows across the continental shelf, mixing with the iron-rich bottom water and carrying the dissolved iron up toward the sea surface.
Recent studies suggest that where iron primarily comes to the surface where the continental shelf is relatively wide, for example, along the coast from Monterey Bay to Point Reyes. Off the Big Sur coast, where the continental shelf is narrow, there is much less iron, and diatoms do not bloom as often or as intensely. This may be one reason that the waters off Big Sur are often very clear and blue even during upwelling events--they don't usually contain enough iron to support large phytoplankton blooms.