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. 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, where there isn't enough sunlight for algae to grow.
Note:Some nutrients in upwelled water may also come from the California Undercurrent, which flows northward, beneath the landward edge of the California undercurrent (Francisco Chavez, person communication)
Note:There is a longstanding misconception that the deep water in Monterey Canyon provides nutrients that fuel algal blooms in the bay. It is true that tidal currents and internal waves can occasionally cause this water to "slosh" out of the canyon and cause local blooms of dinoflagellates below the ocean surface. However, deep water from the canyon rarely reaches the ocean surface, and the amount of nutrients carried by this water is very small compared to the nutrients provided by wind-driving upwelling.
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.
As upwelling continues through the spring, the near-bottom turbid layer is swept clear and less iron is brought to the surface, allowing different types of algae to bloom. By late summer or fall, very little iron is available, and blooms of centric diatoms become much less frequent.
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.
Runoff from streams and rivers also supplies nitrate and other nutrients to Central Coast waters. This is especially true in the winter months, when large plumes of sediment and nutrients (especially nitrate and phosphate) spread widely along the coast and sometimes offshore. In the summer months, nutrients from agriculture and residential runoff may also have local effects near year-round streams and rivers.
Municipal wastewater (processed sewage and runoff) can also add nutrients near the outfalls for wastewater treatment plants.
