Many spring algal blooms on the Central Coast involve large, chain-forming diatoms such as (right to left) Skeletonema costatum
, Chaetoceros debilis
, and Thalassiosira aestivalis
(Sources: NOAA/Seagrant, Plankton Net, UCSC/CIMT)
As the northwest winds of March bring nutrients to the surface, they spark the growth of tiny microscopic algae called diatoms. Like plants on land, diatoms grow by capturing energy from the sun and adsorbing water, nutrients, and carbon dioxide from their surroundings. However, unlike most land plants, diatoms are single-celled organisms. Given plenty of sunlight and nutrients, they don't grow larger, but split in half, essentially cloning themselves.
Under optimum conditions, some diatoms can divide (and thus double their population) every twelve hours. A little math will show you that this can yield a lot of diatoms in a very short time.
The optimum growing conditions for diatoms are similar to those for land plants--they like to have plenty of sunlight, water, and nutrients. Living in the ocean, diatoms have no shortage of water. So the two factors that control the timing and extent of phytoplankton blooms are sunlight and nutrients.
During the winter months, both sunlight and nutrients are in short supply. There isn't much sunlight because the days are short and often cloudy. When the sun does shine, it is at such a low angle that most of the light bounces off the sea surface and does not penetrate into the water below. To make matters worse, winter storm waves and winds from the southeast churn the ocean, mixing water from the surface down to depths of 500 feet or more.
This constant churning often carries diatoms down to depths where sunlight cannot penetrate and they cannot grow. Unless they get lucky and are carried back up into the sunlit waters within a day or two, most diatoms cannot survive this treatment, and will either die for go dormant, forming seed-like cysts, which sink down toward the sea floor.
Given sunlight and a stable water column, however, diatoms typically begin to bloom within a one or two days after northwest winds bring a pulse of nutrients to the surface. Stronger winds bring up deeper water that contains higher concentrations of nutrients. Winds of longer duration bring up a larger volume of deep water and spread it farther across the ocean surface. Thus, the strength and duration of the northwest winds is often directly related to the size and extent of the resulting diatom bloom.
In addition to supplying nutrients, upwelling events may also help trigger diatom blooms by bringing dormant diatom cysts up from the sea floor into the sunlit surface waters, where they may come to life again. In fact, many diatom species that bloom in early spring have cysts that spend the winter on or near the sea floor, just waiting to be carried up to the surface by upwelling events.
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:
Day 1: 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.
Day 2: 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. As described in Chapter 2, 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.
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.
== Insert satellite image showing upwelling plumes along coast from Point Reyes to Point Sur ===
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.
Note: 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.
Day 3: 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.
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)
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.
=====Insert labeled satellite image showing Davenport upwelling plume and oceanic front offshore of Santa Cruz ====
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.
Day 4: 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.
== Insert labeled satellite image showing chlorophyll near Monterey Bay following an upwelling event ==
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 even small animals within the warmer water of the California Current.
============== Sidebar: A brief field guide to 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.
================ End of Oceanic Front sidebar ================
Day 5: 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.”
=================== Insert diagram of a relaxation event =================
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.
Days 6 to 8: 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.
Day 9-12: 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.
Now that we've looked at the progression of a typical spring bloom, let's take a closer look at the star players in these blooms--the diatoms themselves.
On a year-round basis, diatoms are by far the most abundant type of phytoplankton found off Central California. One marine biologist counted over 200 different species of phytoplankton in Monterey Bay, of which almost two thirds were diatoms. Almost all of the diatoms that bloom in spring are cylindrical in shape and are part of a group known as "centric diatoms."
Many spring algal blooms on the Central Coast involve large, chain-forming diatoms such as (right to left) Skeletonema costatum
, Chaetoceros debilis
, and Thalassiosira aestivalis
(Sources: NOAA/Seagrant, Plankton Net, UCSC/CIMT)
Centric diatoms require plenty of sunlight and have a variety of adaptations that help them gather this precious resource. If you zoom in on a centric diatom's “shell” under the microscope, you will see that its surface is etched in beautiful, ornate patterns. Marine biologists have speculated that some of these surface features act like miniature lenses to focus light on the parts of the diatom that convert sunlight into energy.
Diatoms in Monterey Bay seem to grow best about 25 to 50 feet below the sea surface. If they stay right at the surface, they may get "sunburned." If they sink too deep, they may not get enough sunlight to survive. Larger diatoms, which sink relatively quickly, are especially favored by strong upwelling events, which enhance vertical mixing and help keep them near the surface.
Some diatoms have long spines that create drag, so that they don't sink out of the sunlit surface water too quickly. Many centric diatoms link up in chains, which sink even more slowly than individual diatoms (and are also harder for small animals to eat).
Healthy, well-nourished diatoms can also increase their buoyancy by storing oil within their bodies (just as we store our excess food as fat). However, if they run out of nutrients, these diatoms must tap into their oil reserves and start to sink. This may explain why entire populations of diatoms sometimes sink down from the surface waters after using all up of the available nutrients.
Small populations of centric diatoms live in Central Coast waters all year long, waiting for just the right conditions that will allow them to bloom. Each upwelling event and subsequent phytoplankton bloom tends to be dominated by a single species of diatom. Like spring wildflowers, different species of diatoms often bloom in succession at specific times of year.
A chain of Skeletonema costatum
diatoms under a microscope. (Source: NOAA/Seagrant)
The diatom most characteristic of the beginning of upwelling is Skeletonema costatum, which typically blooms between February and April. Like another early blooming diatom, Thalassiosira aestivalis, these diatoms spend the winter as dormant cysts on the sea floor. These cysts are carried up into the water column by the first strong upwelling event of the year. They also have an affinity for iron and may respond rapidly to the pulse of iron that enters surface waters during the first major upwelling event of the year.
Note: 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.
The third group of diatoms to bloom in the spring succession are members of the genus Chaetoceros. Different species of Chaetoceros bloom during different months. For example Chaetoceros debilis is most likely to bloom in March or April, while Chaetoceros venheurckii and Chaetoceros radicans are more common in May and June, respectively (and will be discussed in later chapters).
is one of the main diatoms that bloom in spring along the Central Coast> (Source: Plankton Net)
Chaetoceros diatoms account for the vast majority of the phytoplankton that bloom during spring on the Central Coast. They are also common at other times of year. In fact, they are probably the most abundant type of diatom in the entire Eastern Pacific Ocean. Eighteen different species of Chaetoceros have been found in Monterey Bay alone.
What most centric diatoms do well, Chaetoceros diatoms do best--that is, sucking up nutrients and reproducing very quickly when nutrients are plentiful, then forming large stocks of resting spores that sink down from the surface to blanket the sea floor. Chaetoceros diatoms are so good at sucking up nutrients that scientists have seen both nitrate and carbon dioxide virtually disappear from surface waters within one to two days of a Chaetoceros bloom.
In looking at the spring succession of diatoms, it is interesting to note that smaller diatoms such as Skeletonema are more likely to appear early in the season, while larger species such as Chaetoceros tend to bloom later in spring. This seasonal size shift may occur because small diatoms are better at staying near the surface, collecting sunlight, or taking advantage of the less frequent upwelling events of early spring. Conversely, the frequent upwelling and active wind-driven mixing that occurs later in spring may favor large diatoms.
Blooming diatoms provide essential food for tiny marine animals such as copepods, for larger swimming animals such as anchovies and krill, for gelatinous filter feeders such as salps, and for bottom-dwelling filter-feeders such as mussels, oysters, scallops, and clams. Many of these animals rely on the abundant and more or less predictable food supplied by spring phytoplankton blooms. In fact, as we shall see, the life cycles of many marine animals are timed to coincide with the food provided by different types of diatoms that bloom throughout the spring.
In the Introduction to this month, I describe some of the tiny animals (zooplankton) that drift and swim in the surface waters of Monterey Bay during March. This section focuses on the zooplankton that look like science-fiction aliens--the tiny larvae of fish, crabs, sea stars, barnacles, and other animals that live (as adults) in the tide pools and kelp beds. Many of these near-shore animals release eggs or larvae that spend anywhere from several days to several months drifting around in the open ocean. Such eggs and larvae are especially abundant in Monterey Bay during February and March.
During their time in the plankton, larval fish and invertebrates feast on spring-bloom diatoms, as well as on each other. If they survive, the larvae eventually develop into juveniles and (in some cases) begin to look more like their parents. At this point, the young animals must somehow find their way back from the open ocean to the kelp beds, intertidal areas, or the sea floor. If they are unable to return to the coast or to unable find suitable habitat, these young, rapidly-developing animals will die.
Over the next few pages, we will consider some of the challenges that these larvae face as they drift on the currents during the spring upwelling season. We will also look at how these animals may benefit from this lifestyle. Then we will look at several types of fish whose larvae are particularly common in the plankton during March.
Releasing eggs and young out into the plankton is a huge gamble, with the odds favoring “the house.” Typically only one out of 100,000 planktonic larvae survive to settle back to shore. Most marine animals deal with these horrendous odds by releasing hundreds of thousands or even millions of eggs or larvae into the water column. Apparently the effort required for reproduction on such a massive scale is more than balanced by the advantages of having planktonic young.
Marine biologists are still trying to figure out why so many marine organisms that live in safe, cozy little holes in the sea floor would send their newborns out into the great big ocean, where they face innumerable predators, not to mention the strong possibility of never finding their way “home” to shallow water.
One obvious benefit of releasing planktonic larvae is that it allows attached or slow-moving animals to colonize new areas and expand their range. This can help a species survive if all individuals in one area are killed, for example, by a severe storm or an oil spill. Dispersing larvae over a wide area also allows the young of one individual to interbreed with young from other individuals that may be dozens or even hundreds of miles away. This increases the overall genetic diversity of the species and improves its ability to survive changing environmental conditions or disease.
Another possible reason for sending ones young out to sea is that they may find more food than they would near shore. In this, as in many aspects of the larval lottery, timing is of the essence. For example, some barnacles spawn in February and send their larvae out into the plankton just in time to feed on the small Skeletonema diatoms that bloom at this time of year. Three or four weeks later, when these same barnacle larvae settle onto intertidal rocks, they will have grown large enough to eat the larger Chaetoceros diatoms that begin to bloom in March.
If have to send your young out into the open ocean, early spring is a pretty good time of year to do so. Some zooplankton predators, such arrowworms and jellies, are less common at this time of year. Other animals, such as anchovies and krill, are still in their diatom-eating larval stages in March, but may add zooplankton to their diet when they mature in late spring or summer.
Although the advantages of having larval young are still poorly understood, many of the disadvantages are pretty obvious. For example, an octopus that actively broods and guards its eggs can usually ensure that most of these eggs will survive. On the other hand, an anchovy, which releases its eggs to drift around on their own for several months, must rely on chance for the survival of its offspring. On the other hand, the anchovy might release 100 times as many eggs as the octopus, so the total number of eggs that survive to adulthood could be similar in each case.
The main reason that only a small proportion of planktonic larvae survive is that most of them get eaten. Some larvae (those without yolks) also face the prospect of starvation if they can’t find enough to eat. Another major risk is that the larvae will be swept far away from the habitat they require as adults. [What happens to larvae that have enough to eat but stay at sea too long? Can they delay settlement or do they metamorphose and settle in water that is too deep? Perhaps settling into deeper water will help them be carried to shore!]
Strong, persistent upwelling events often generate surface currents that flow away from the coast. Since many eggs and larvae live at or near the sea surface, such surface currents can sweep them far offshore. Some planktonic larvae can swim, but few can swim fast enough to resist these currents. Thus, marine biologists often assume that the more time animals spend as drifting larvae, the more likely they are to be swept offshore and the less likely they are to find their way back.
Some marine animals reduce the chances that their larvae will be caught in upwelling currents by having young that spend only a few days in the plankton. Other animals achieve the same effect by releasing larvae before or after the upwelling season. Still others, such as rockfish, have larvae that spend six months or more in the plankton right in the middle of the upwelling season, with all the risks (and presumably benefits) that this extended stay entails.
So what happens to those larvae that are swept dozens of miles away from shore by upwelling currents? How do any of them get back to shallow water areas where they can become adults and settle onto the sea floor? Scientists don’t know for sure. But they do know that young fish, crabs, barnacles, and even worms often appear suddenly along the coast in “waves” or “pulses” lasting just a few hours to a few days.
These pulses of young animals typically coincide with dramatic physical processes that may or may not be visible at the sea surface. Such processes include internal waves or relaxation fronts that move toward the coast and changes in ocean currents that bring offshore water toward the shoreline. The following paragraphs discuss each of the three transport methods in turn.
Perhaps the fastest way for drifting plankton to reach the coast is by "surfing" on internal waves that move toward the coast. Internal waves are like the waves that slosh back and forth inside an “ocean in a bottle”--they form along boundaries between different ocean layers. The most obvious of these boundaries is the thermocline.
Although they may be virtually invisible at the sea surface, internal waves may cause the thermocline to rise and fall by hundreds of feet. Just as a river flowing over a shallow rock ledge sometimes forms a “standing wave,” so currents passing along the edge of a submarine canyon or the continental shelf can create standing internal waves.
Although virtually invisible at the sea surface, internal waves are common along some parts of the coast, and form almost every day in Monterey Bay. Internal waves often form long, linear slicks, where organic oils and drifting plankton are concentrated.
During incoming tides, internal waves sometimes move from the edge of the canyon or continental shelf in toward shore. In this case, the surface slick also moves toward shore, carrying its load of accumulated drifters, including larval crabs and other young animals.
During the particularly extreme tides that coincide with new moon and full moon, internal waves not only move toward shore, but may become unstable and break, forming churning masses of mixed water that move rapidly toward shore (but are still virtually invisible at the surface). Plankton at various depths may be carried rapidly toward shore by such breaking waves, eventually settling down after the wave dissipates.
Because internal waves are most common over the continental shelf, plankton that have been carried out beyond the continental shelf must have some other means of hitching a ride back to shore.
A second means by which larvae can get back to the shore is through relaxation fronts. As mentioned previously, relaxation events occur when northwest winds die down and upwelled water surges back toward the coast. As a relaxation front moves toward shore, it sweeps up and concentrates both zooplankton and phytoplankton at its landward (leading) edge, carrying these critters back toward shore. One study of relaxation fronts found that plankton populations were up to ten times higher on the landward sides of these fronts than on their seaward sides.
Sometimes relaxation fronts move all the way to the coast, bringing both phytoplankton and drifting animals into kelp beds and intertidal areas. Such events provide sudden feasts for filter feeders and small predators in these areas. Those larvae that survive the feast settle down by the thousands on rocks or other available surfaces.
Several studies have found that barnacles are much more likely to settle on rocks within Monterey Bay when winds are light than when the strong northwest winds are blowing. One study showed that the greatest number of barnacles colonized rocks within the bay about two days after the northwest winds had abated.
In addition to causing surface waters to flow back toward shore, relaxation events can also cause currents to flow northward along the coast instead of southward as they usually do during upwelling events. Thus, relaxation currents can sometimes transport water (and larvae) northward from within the lees of headlands. This brings pulses of larvae to coastal areas north of Point Sur, Point Ano Nuevo, or Point Reyes, which are usually exposed to strong upwelling currents, and are infrequently colonized by drifting larvae. In fact, relaxation events may be the only times when larvae can colonize these areas.
Perhaps the most obvious way for larvae or juvenile animals to get back toward the coast from far offshore is by riding on the shifting ocean currents. As described previously, upwelling plumes that move offshore often interfinger with shoreward-moving streamers of water from the California Current. As long as upwelling remains strong, these streamers seldom reach the coast. However, when upwelling dies down (particularly in Fall), California Current water often flows all the way to the inner parts of Monterey Bay.
When upwelling is weak or nonexistent, tongues of water from the California Current sometimes flow into Monterey Bay at its southern end, near the Monterey Peninsula. This offshore water often carries planktonic barnacle larvae that settle out wherever the water meets the shoreline (and where there are hard surfaces onto which they can settle). Barnacle larvae in such plumes might settle out first near Point Pinos (at the southern edge of the bay). A day or two later they might settle out farther east, near Monterey Wharf. Sometimes these tongues of water and larvae continue around the perimeter of the bay, bringing larvae to Capitola or Santa Cruz.
Another possibility, not yet documented in the scientific literature (to my knowledge) is that some larvae, when they are ready to return to shore, sink toward the bottom while still offshore, but are carried toward shore by shoreward-flowing bottom currents (if such things exist). This would work particularly well during the peak of the upwelling season, when bottom currents would presumably be strongest and affect areas farther from shore. It might also apply to larvae that perform diurnal vertical migrations with the deep scattering layers.
Some parts of the Central Coast, such as the inner Gulf of the Farallones and the northeast corner of Monterey Bay, are relatively unaffected by upwelling currents. These areas, known as “upwelling shadows,” are described in detail in Chapter 2. Eddies often form within sheltered upwelling shadows, just as they do behind large boulders in a stream. In the ocean, such eddies can retain sun-warmed water and large populations of both phytoplankton and zooplankton (including lots of larvae).
[Insert figure showing Monterey Bay upwelling shadow and Davenport upwelling plume]
At the height of the upwelling season, larvae of some crabs and nearshore animals may be up to ten times more abundant within the Monterey Bay upwelling shadow than in open-coast waters. Because of this, larvae settle much more often at the Capitola Wharf, which is in the heart of the Monterey Bay upwelling shadow, than at the Santa Cruz Wharf, which is near the seaward edge of the upwelling shadow.
Upwelling shadows may also help isolate larvae from predators that lurk farther offshore. During a strong spring upwelling event, one researcher found crowds of predatory jellies (ctenophores and siphonophores) concentrated along the seaward side of the front that marks the seaward boundary of the Monterey Bay upwelling shadow. Rockfish larvae, another zooplankton predator, were also much more abundant outside the upwelling shadow than inside.
Animals that enter their young in the planktonic larval lottery may occasionally win big, but they also suffer disastrous losses. Thus, animals with planktonic young (especially those that spend several months in the plankton) are well known for their dramatic population booms and busts. Blue rockfish, for example, are extremely abundant in the kelp beds during some years, but are almost impossible to find during other years.
Recent research has shown that much of this variation can be traced to year-to-year variations in the timing and strength of upwelling events. During years when upwelling is strong (“La Niña” years), blue rockfish and olive rockfish are abundant in the kelp beds, but kelp rockfish and gopher rockfish are scarce. During years when upwelling is weak ("El Niño years), this pattern is reversed.
Blue rockfish and olive rockfish spawn in January or February and their larvae spend up to three months drifting in the plankton. Larvae of kelp rockfish and gopher rockfish, on the other hand, are released in the middle of the upwelling season, but spend only a month or so in the plankton. Thus, strong upwelling apparently favors rockfish that spawn early and whose larvae are adapted to spend more time in the plankton.
Fish eggs and larvae are by far the most common transitory drifters hanging out in Central Coast waters during March (crab larvae are the second most common).
[Add description of some of the crab larvae in spring that result form winter brooding and JAN-FEB hatching of crab eggs from a variety of species, including Dungeness crab, whose eggs hatch, releasing larvae, from DEC-MAR (peak MAR). Usually found floating within 50 feet of surface. Are Dungeness crab larvae and other crab larvae herbivorous? Crab larvae provide food for a surprising number of larger animals, including lingcod larvae, young rockfish, Bonaparte’s gulls, king salmon, and even gray whales (in their Alaskan feeding grounds)]
It is perhaps surprising, considering all the different types of fish in this area, that most of the fish larvae floating around in spring belong to just three groups: anchovies, lampfish, and rockfish (in order of increasing abundance).
Note: Anchovies and juvenile rockfish, along with krill and market squid, are arguably the most important and abundant bite-sized prey animals in Monterey Bay. All four of these animals are in their early stages of development during the spring upwelling season. However, they reach adulthood or peak abundance at different times:
- May - June: Market squid
- June - July: rockfish and anchovies
- July - August: Krill
- September - October: Market squid (again)
- October - November: Anchovies andMarket squid
Note: Schools of lampfish are probably a key source of food for many deep-water animals. However, scientists know so little about midwater food webs that it's hard to tell which animals depend on lampfish.
Anchovy spawn all year long, but those living along the coast from Monterey Bay down to Southern California do most of their spawning between February and April of each year. Sometimes large schools of adults spawn just offshore of estuaries such as Elkhorn Slough. This may allow the incoming tide to carry their eggs into the warm, protected waters of the estuary, where the young will grow more rapidly and perhaps face fewer predators. Other anchovies spawn in the relatively warm, protected waters south of Point Conception.
Female anchovies spawn repeatedly, releasing batches of over 100,000 eggs about every 10 days or so. The eggs hatch within a few days [check this!]. The tiny larvae [how big?] live off their yolk for a week or two [what depths?], and then begin feeding on small diatoms or other microscopic algae called dinoflagellates.
When they first begin feeding, anchovy larvae in the open ocean drift upward from deep water until they reach a depth where there are lots of algae. Once they start feeding, their bodies adjust to the density of the water around them, so that they stay at this one depth or at least within water of this density. Most of the time, this helps them stay where the food is.
During some years, late storms stir up the water column, moving the larvae to depths where there is no food. Since anchovy larvae cannot swim upward or re-adjust their buoyancy, they may starve to death. For this reason, there are huge variations in the number of anchovy larvae that survive from year to year.
Although anchovies and lampfish form huge schools in Central Coast waters, rockfish larvae are the most common type of fish larvae that marine biologists find drifting around Central Coast waters during March. In addition to providing food for a variety of birds and marine mammals, rockfish are also a very popular food item for humans.
Most people have seen rockfish in grocery stories, where they are usually marketed as “red snapper.” In fact, over a dozen different types of fish are sold as "red snapper" in California. Even the experts sometimes have a hard time telling them apart. This is not too surprising considering that perhaps 65 different species of rockfish live along the Central California Coast and hybrids abound.
Most Central California rockfish mate between November and January. Females typically brood their fertilized eggs within their bodies for several months after mating. Some time between January and July (depending on the species) these eggs hatch within the female's body and are released into the water as tiny, swimming larvae, perhaps 1/16 to 1/8 inch long.
Newly hatched rockfish larvae (about 1/16-inch long). (Source: David Csepp, NMFS/NOAA)
Depending on the age and type of rockfish, each female releases between 100,000 and 2,000,000 larvae in a single massive birthing event. These clouds of larvae are often released at twilight or after dark, perhaps to reduce the chances of their being eaten by visual predators. In coastal areas where many rockfish reside, winds and seas may also be calmer at night. [Do newly released rockfish larvae typically float up toward the surface?]
The tiny larval rockfish do not have the yolk sacs that serve as “bag lunches” for some fish larvae. However, rockfish larvae are born with very well-developed eyes and mouths, which “allow immediate feeding on live food” as one researcher delicately put it. Fortunately for us, these hundreds of millions of hungry, well-developed mouths consume only small prey, particularly other larval fish.
Most larval rockfish are visual predators, primarily feeding in the daytime, and staying within about 300 feet of the sea surface. They typically drift or swim in the surface waters for several months before settling down to the sea bottom. During their time in the plankton, they must hunt and eat almost continuously in order to survive. Actually, the vast majority of them don’t survive, which is why so many are released in the first place.
Three to eight weeks after being released into the plankton, the larval rockfish develop into juveniles. At this point, their bodies are still transparent and only an inch or two long. These juvenile rockfish are essential prey for thousands of seabirds that feed and nest along the Central Coast during the summer.
Because different species of rockfish release larvae during different months, several “waves” of rockfish larvae move through the plankton between February and June. In the following paragraphs, we'll look at what's happening to the rockfish in each of these waves during March.
The first wave of rockfish larvae to move through the plankton includes two deep-water species, vermilion and canary rockfish. These fishes release larvae from November through January. Most of these larvae settle down to the sea floor in February or early March, before upwelling really gets going. This reduces the chances that the young rockfish will be carried away from shore by upwelling currents.
A vermilion rockfish
(Source: US EPA)
Vermilion rockfish (Sebastes miniatus) get a particularly early start, with larvae that turn into juveniles and settle down to the sea floor in February, before the upwelling season even begins. Although adult vermilion rockfish live in deep water, their juveniles settle down near shore in water that is less than 100 feet deep.
The young Vermilion rockfish settle down wherever hard surfaces such as rocks, worm colonies, surf-grass beds, or pier pilings stick up through a sandy bottom. At this point, the juvenile vermilion rockfish are mottled brown in color. This provides good camouflage, because they often hide in red the algae that are relatively abundant in early spring.
Like many other deep-water fishes, as the young vermilion rockfish mature, they gradually become orange-red in color and migrate out across the continental shelf to depths of several hundred feet. Deep water rockfish are often orange or red because this color makes them invisible in the deep sea, where red light from the sun never penetrates. Vermilion rockfish are one of the most common species typically sold as “red snapper.”
Another rockfish species in the “first wave” are Canary rockfish (Sebastes pinniger), which settle down to the sea floor soon after the first upwelling event of the spring--slightly later than vermilion rockfish. In fact, their settling appears to be triggered by a drop in water temperature or some other environmental change that coincides with the beginning of upwelling.
Young-of-the-year canary rockfish, about three inches long.
Like vermilion rockfish, young canary rockfish settle down to the sea floor just offshore of the kelp beds, at depths of about 60 to 80 feet, congregating in rocky areas surrounded by sand. Hiding in the shelter of the rocks during the daytime, the newly settled juveniles venture out at night to hunt over the nearby sand flats.
Canary rockfish, like vermilion rockfish, gradually migrate into deeper water as they get older, eventually ending up on the deeper parts of the continental shelf. They may also migrate parallel to the coast, colonizing areas up to several hundred miles from where they first settled down.
Adult canary rockfish are often found near steep undersea cliffs and pinnacles, where they hunt small fish and krill. Krill congregate in huge swarms in such areas, particularly along the edges of Monterey Submarine Canyon. During late spring and summer, canary rockfish apparently gorge on these swarms of krill, putting on weight that will help them survive the leaner seasons of fall and winter.
The second and largest wave of rockfish larvae are released in between January and March, with a peak in February. The second wave includes many common kelp-bed rockfish, including blue rockfish, olive rockfish, and black rockfish, as well as deeper species such as blackgill, copper, and yellowtail rockfish. Although not in the rockfish family, two other predatory, bottom-dwelling fish--cabezon and lingcod, also release larvae in January or February.
By March, these swarms of second-wave fish larvae are already a month old and are busy devouring their smaller and later-arriving cousins in the plankton.
Larvae in this second wave must be well adapted to upwelling, since many of them return to coastal areas to settle down in April and May, at the height of the upwelling season. In fact, as mentioned previously, some rockfish in this group, including blue rockfish and olive rockfish, are more abundant during years when upwelling is strong.
A school of blue rockfish swim around a kelp stipe. (Source: Chad King, NOAA/SIMON)
Blue rockfish and olive rockfish are common schooling fish of the Central-Coast kelp beds. Blue rockfish (Sebastes mystinus) have a particularly long larval period that may last as long as four to five months (depending on who you talk to). During this time, the young fish may be carried dozens or even hundreds of miles from shore. They survive by eating copepods and krill as well as other planktonic animals. Nonetheless, come April or May, massive swarms of juvenile blue rockfish often appear in the kelp beds. How they make their way back to shore is anyone’s guess.
Note: Another group of rockfish don’t even get around to releasing larvae until the middle of the upwelling season. These spring spawners release a “third wave” of rockfish larvae later in spring and will be discussed in a subsequent chapter.
The practice of releasing larvae before upwelling begins (and thus having well-developed larvae in the plankton just as upwelling starts) must provide some significant advantages for bottom-dwelling predators, because it is not unique to rockfish. At least two other common predatory fish, cabezon and lingcod, have similar habits.
Cabezon on seafloor. (Source: John Ugoretz, California Department of Fish and Game)
Cabezon eggs surrounded by California hydrocoral (Stylaster californicus
) and strawberry anemones(Corynactis californica
). (Source: John Ugoretz, California Department of Fish and Game)
Cabezon (Scorpaenichthys marmoratus) spawn from November through January, laying eggs on the sea floor. These eggs typically hatch in February. The newly hatched cabezon larvae are about one quarter inch long, with a large yolk sac (which acts like a bag lunch for the young fish).
By March, the cabezon larvae have grown to about 3/8-inch long, and have used up their yolk. At this time, they become voracious predators on other plankton, incessantly gobbling copepods, crab larvae, barnacle larvae, fish eggs, and fish larvae, all of which are plentiful in March. Cabezon larvae seem to have a particular fondness for eggs and larvae of their own kind. In fact, cannibalism may account for a significant proportion of the cabezon larvae that die during their larval stage.
Cabezon larvae are frequently found dozens of miles offshore, along with large swarms other fish larvae. Unlike most rockfish larvae, however, cabezon larvae are only observed at the sea surface at night. Presumably they spend the days in deeper waters, and come up to the surface to feed, following their prey, some of which (such as Dungeness crab larvae) also spend their days in deeper water and migrate up toward the surface each night.
Cabezon spend a total of three to four months in the plankton, first as larvae, then as small, silvery juveniles an inch or two long. Like blue rockfish, juvenile cabezon somehow find a way to migrate back to the kelp beds between April and June, during the height of the upwelling season. It may help that by this time, the juvenile cabezon have grown to about 2 1/2 inches long and are very active swimmers. [Since they are vertical migrators, could they be riding deep, subsurface currents toward shore?]