To get a better understanding of how spring blooms work, let's look in detail at the day-to-day progression of a typical March upwelling event and phytoplankton bloom in the Monterey Bay area:
The upwelling event begins the day after a late-winter storm blows through the Central Coast. As the storm clouds dissipate and the skies clear, the wind shifts from the southeast to the northwest and increases to a steady 20 knots over the coastal waters between Santa Cruz and San Francisco. Instead of dying down overnight as usual, the wind continues to blow hard from the northwest all night long.
The next day brings clear skies and more cool, breezy weather. The northwest wind continues to blow at 15 to 20 knots, with higher gusts over the waters farther from shore. The wind whips up the ocean surface, forming whitecaps and steep choppy seas, so that the entire ocean seems to be flowing down coast, to the southeast.
Even though the currents right at the sea surface are flowing toward the southeast, the water just below the surface is moving in an entirely different direction. Because of the Coriolis Effect, most of the water within about 150 to 300 feet of the surface (depending on the speed of the wind) is actually moving away from shore. This allows deep, cold water to flow toward shore and eventually rise up toward the surface. The upwelling event has begun.
This diagram shows how cold, upwelled water (blue) flows from Point Ano Nuevo down the coast and across the surface of Monterey Bay. Warmer water (orange and red) lies offshore and in the northeast corner of the bay. Note: the vertical exaggeration in this diagram is about 100x (the real ocean is very thin compared to its depth). (Source: David Fierstein / MBARI)
Some of the strongest winds and most intense upwelling occurs just offshore of prominent points of land such as Point Reyes (north of San Francisco), Point Año Nuevo (north of Monterey Bay), and Point Sur (along the Big Sur coast). Most upwelling on the Central Coast takes place near these "upwelling centers," where huge volumes of cold water “bubble up” toward the surface.
By the afternoon of the second day of upwelling, so much cold water has been brought up from the depths that it begins to spread out across the sea surface, where it is pushed to the southeast by the wind. Cold water from Point Año Nuevo, for example, flows down coast toward Monterey Bay. By the time the "upwelling plume" from Año Nuevo reaches the little town of Davenport, about nine miles northwest of Santa Cruz, it has become a broad river of cold water about three to five miles wide and up to 100 feet thick.
The landward edge of the plume typically lies about a mile offshore. Southeast of Davenport, the coast curves in toward Monterey Bay, but the “Davenport Upwelling Plume” continues more or less in a straight line out across the open waters of the bay.
A similar plume of cold, upwelled water flows southeast from Point Reyes during upwelling events. The Farallon Islands lie directly in the path of this plume, and the abundant marine life around these islands depends on the blooms of phytoplankton and krill that result from interactions between the plume and the islands.
As the northwest winds continue to howl along the coast and upwelling continues at Point Año Nuevo, a second plume of cold water begins to flow out to sea from the point. As this offshore plume flows seaward, it interfingers with the slightly warmer waters of the California Current 10 to 20 miles offshore.
Upwelling centers along the Central California Coast. (Source: Modified from NASA Images)
Meanwhile, the Davenport Upwelling Plume continues to flow southeast, across Monterey Bay, until it reaches the Monterey Peninsula. Sometimes this cold water sweeps to the left, flowing into the southern part of Monterey Bay, where it may be entrained in a slow, counterclockwise current that often circles northward within the bay. Depending on coastal currents, remnants of the Davenport Upwelling Plume may also be carried down the coast toward Big Sur or curve out to sea, forming a clockwise gyre about 30 miles off the bay.
Although the newly upwelled water near Año Nuevo doesn't contain much in the way of phytoplankton, the "older" upwelled water flowing across Monterey Bay and out to sea gradually becomes warmer and richer in phytoplankton as it goes. Although the plume seldom contains extremely high concentrations of phytoplankton, you can be sure that those plankton are very busy sucking up nutrients and reproducing like crazy.
A fried of mine who is a dedicated SCUBA diver once described what it was like to dive when this plume of recently upwelled water reaches the Monterey Peninsula: "If you're out there every diving in the Monterey kelp beds, you can always tell when upwelling starts because all of a sudden the water becomes thick and cold and murky."
The water is cold because it originated perhaps 300 feet below the surface. The water is thick and murky because it is full of diatoms and other plankton that have bloomed as the upwelling plume moved along the coast and across Monterey Bay.
During the evening of the third day of upwelling, the howling wind finally dies down. At dawn on the fourth day, the sea is calm. By afternoon, a northwest sea breeze springs up, but only reaches 15 miles per hour over the coastal waters. Without the constant force of the northwest wind, cold water stops rising to the surface at Año Nuevo, and may even begin to sink back down into the depths.
The Davenport Upwelling Plume begins to slow and dissipate. The offshore-flowing upwelling plume thins out and eventually swirls off into the California Current like smoke rising from a blown-out candle. Sunlight begins to warm the recently upwelled water. Diatoms become abundant around the edges and "older" portions of the plume. The bloom begins.
Even from shore, you can sometimes tell when diatoms are blooming because they give the coastal waters a light-green, almost chartreuse color. In fact, many diatom blooms cause such distinct changes in ocean color that they can be seen from satellites in orbit.
Satellite images show that the densest populations of diatoms typically occur close to shore, especially where upwelling plumes flow near sheltered areas that already have lots of phytoplankton. The sheltered portions of Monterey Bay and the Gulf of the Farallones, for example, usually have more phytoplankton than other parts of the Central Coast, and may continue to harbor phytoplankton blooms long after upwelling ceases.
Other spring blooms occur where diatoms are concentrated along boundaries between different water masses, which oceanographers call “oceanic fronts.” Like the cold fronts and warm fronts that bring changes in the weather, some oceanic fronts move from place to place. Others tend to form consistently in certain locations where different waters converge.
For example, during upwelling events, a very distinct oceanic front often forms just offshore of the town of Santa Cruz, where the Davenport Upwelling Plume comes in contact with the warmer waters of Monterey Bay. Phytoplankton are often concentrated and bloom along the landward side of this front, presumably benefiting from nutrients that mix or diffuse through the front.
Phytoplankton may also be concentrated along fronts formed by the upwelling plume that flows seaward from Point Año Nuevo. Sometimes this plume meanders and breaks off from the coast, forming swirling eddies that trap cold water, phytoplankton, and small animals within the warmer water of the California Current.
About oceanic "fronts"
If you know what to look for, you can sometimes see oceanic fronts on the sea surface. Because fronts mark the boundaries between water masses, sometimes the water on one side of a front will be different from the water on the other side.
The colors of these different water masses can give you an idea of where they originated. For example, along the coast from Monterey Bay northward, nearshore water often has a greenish or brownish tinge and is somewhat murky due to suspended sediment, organic material, or blooming phytoplankton. Recently upwelled water, in contrast, sometimes appears slightly milky (especially early in the spring) or occasionally a dark blue-gray. Offshore water from the California Current is usually a bright, tropical azure blue, and is often very clear.
Oceanic fronts may also show up as long, linear streaks, "wind lines," or "slicks" on the sea surface. These occur when natural oils from diatoms or other organisms accumulate along the front, dampening the wind ripples on the sea surface. Like giant push brooms, moving fronts often sweep up whatever happens to be floating on the sea surface, from sea foam to drift kelp to human trash, concentrating this material in lines that stretch for miles across the ocean surface. During intense diatom blooms, oceanic fronts may be marked by lines of sea foam that are tinted green by all the microscopic blooming algae.
The crews of many fishing boats, particularly those that hunt open-ocean fish, often look for fronts and eddies on satellite images. They know that such fronts often concentrate not only phytoplankton but also bait fish and larger predators, including salmon or tuna.
Some time before dawn on the morning of the fifth day, a damp fog moves up the coast from the south. Throughout the day, the coast lies shrouded in its clammy embrace. Winds over the ocean blow fitfully from the southwest. The upwelling event is over.
Now the upwelling cold-water conveyor belt begins to run in reverse. Without a strong northwest wind to keep it at the surface, the cold, dense, recently upwelled water near shore begins to sink. This, in turn, causes the surface waters to flow back toward shore, a process known as “relaxation.”
Even after upwelling stops and relaxation sets in, diatoms may continue to bloom in the surface waters. In fact, diatom populations near shore often peak after northwest winds and upwelling have abated. Although oceanographers often say that upwelling causes phytoplankton blooms, the relaxation process that follows an upwelling event may be just as important as the upwelling event itself.
When you look at the physical processes involved, it's not too surprising that phytoplankton populations sometimes peak during relaxation events. For one thing, because there is less wind-driven mixing during relaxation events, the surface mixed layer sometimes becomes shallower. This can help keep blooming diatoms close the sea surface, where they can get plenty of sunlight.
Relaxation events also cause "older" upwelled water to surge back toward shore, carrying phytoplankton and other drifting organisms along with it. In fact, these organisms are often caught and concentrated along “relaxation fronts” that form as surface water sweeps back toward shore.
The effects of relaxation fronts are particularly dramatic and biologically important in exposed coastal areas such as the Big Sur coast. These areas are usually bathed in cold, recently-upwelled water, which contains relatively few phytoplankton. When a relaxation front sweeps through such an area, it often brings a wave of phytoplankton and tiny drifting animals that provide welcome food for fish and other kelp-bed and tide-pool creatures.
Within a day or two after the upwelling “fertilizer pump” shuts off, the billions of blooming diatoms are in trouble. They have used up practically all the nitrate that was brought to the surface during the last upwelling event. In some areas, they have turned the surface waters so murky that diatoms a few yards below the surface can’t get enough sunlight to survive.
Starved of nutrients and sunlight, many diatoms start to die or go dormant. The majority are eaten by copepods, crab larvae, and other grazers. The rest sink down toward the sea floor. The bloom is over.
A few days after one group of diatoms meets their demise, the northwest winds may pick up again, starting a new round of upwelling and leading to a new bloom, perhaps with a different species of diatoms. By the end of March, three- to four-day periods of northwest winds often alternate with two- to three-day periods of calm (and sometimes fog). Each new upwelling event brings a new pulse of nutrients into surface waters and a new diatom bloom.
