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 that is just being explored in the scientific literature 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).
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 and Market squid
Lampfish are small, extremely abundant deep-water fish. They have a similar ecological role to anchovies, but in deep water. They are probably a key source of food for many deep-diving and deep-living animals. However, scientists are just beginning to learn about deep-sea food webs, so it's hard to tell all the animals that depend on lampfish.
