Seasons in the Sea - A month-by-month guide to Central California sea life

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Mackerel school: Image credit-Kip Evans

The open ocean

in June

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Diatom populations max out

With upwelling reaching its yearly peak in late May or early June, almost continuous pulses of nutrients allow centric diatoms to reproduce at their fastest rate of the year. Late May sees the densest populations of diatoms in the outer portions of Monterey Bay.

By early June, the diatoms will be reproducing as fast as they can, resulting in the highest "primary productivity" of the year. However, the total number of diatoms is already beginning to decline. This decline may be occur because strong upwelling currents are carrying a lot of the diatoms away from the shore and dispersing them, or it may occur because copepods and other tiny animals are devouring the diatoms as fast as they can reproduce.

In Monterey Bay, diatom blooms during much of the spring upwelling period consist of large, chain-forming centric diatomc of the species Chaetoceros debilis. At the height of the season, in late May, this diatom may share the stage with a northern species of diatom known as Chaetoceros venheurckii or Chaetoceros constructus. [Check current species name]

In addition to grazing and possible iron-depletion another factor that may keep diatom blooms in check is self-shading--that is, when diatoms begin blooming so densely right at the sea surface that they block the sunlight, and diatoms in deeper water can't get enough sunlight to survive. This is a common phenomenon in late May and June along the central coast, when diatom blooms can turn the water to a greenish pea-soup.

As the diatoms reproduce like crazy, they also suck up nutrients. Thus, if even a week goes by without an upwelling event, the blooming diatoms may use up all the nitrate in the sunniest waters within about 30 feet of the sea surface. At this point, the diatoms may start to sink or stop reproducing. Such nutrient-depletin events will become increasingly common in July and August.

As the diatoms begin to use up the iron and nitrate in the water, other types of microscopic algae take advantage of the situation, and begin to reproduce. These so-called "picoplankton" are only one tenth the size of the diatoms, and may be more efficient at using whatever nutrients (such as iron) are left after the diatoms are done blooming [what about ammonium and "regenerated" nutrients" ??]

Populations of these picoplankton remain very low (at winter-time levels) throughout the spring upwelling period. But starting in June, they gradually begin to increase, and reach maximum numbers in August, when nutrients are often very low in the coastal waters.

Zooplankton max out

By late June, the copepods and other microscopic animals have been gorging on diatoms and on eachother for a month or two. This abundance of food has allowed some of the copepods to reproduce rapidly. Thus, the spring diatom blooms on the Central Coast result in secondary blooms of zooplankton that sometimes continue throughout the summer.

Note: The first spring peak in zooplankton along the Central California coast typically occurs in March, and is made up mostly of what are called "meroplankton" - the eggs and young of marine animals that are only temporarily drifting around the open ocean before settling down toward the seafloor or swimming around actively as adults. In June, however, a second peak in zooplankton populations occurs, which is dominated by what marine biologists call "holoplankton" - animals that spend their entire lives drifting or swimming weakly in the open ocean.

Most noticeable among June's hungry hoards are the copeoods. These microscopic animals are related to shrimp, but look like tiny silverfish, about the size and shape of the commas on this page. During June, the large calanoid copepods do quite well, as discussed in Chapter 6, as well as other smaller [check this] copepods in the genuses Corycaeus and Oithona.

Cold-water copepods in the genuses Eucalanus, Pleuromamma, and Paracalanusalso appear along the Central Coast during the summer months. They are apparently swept south to the Central Coast by the California Current (which flows relatively closes to shore at this time of year).

Zooplankton are concentrated in eddies and along fronts

June sees vast aggregations of zooplankton not only at certain depths, but also in areas where currents concentrate the tiny animals. For example, copepods may be relatively abundant in "upwelling shadow" areas--areas such as the northeast corner of Monterey Bay, which are protected from the strong northweset winds. Copepods, krill, and other zooplankton are concentrated along upwelling fronts--places where upwelled water comes in contact with less nutrient-rich waters closer to shore. Both upwelling shadows and upwelling fronts are strongest and most persistent in May and June when upwelling peaks. In addition, as upwelling currents sweep animals away from shore, they sometimes form huge eddies, which can trap and concentrate both copepods and krill.

For example, in June 1993, marine biologists on a research cruise observed dense swarms of zooplankton (an average of 67,000 animals per square foot over the upper 1000 feet of the watr column) trapped in a 30- by 40-mile clockwise eddy about 90 nautical miles west of the upwelling center at Point Ano Nuevo. This eddy was spinning along the eastern boundary of the California Current, between the main stream of the current and the coast. However, based on its direction of spin, the eddy probably originated near shore, as a narrow filament of upwelled water moved away from the coast and then began to eddy around as it slowed down.

The swarms of zooplankton in this eddy were five times higher than anywhere else in the offshore waters. These swarms contained huge number of both small and large copepods, as well as quite a few adult krill, (mostly Euphausia pacifica). Such eddies are prime feeding areas for krill-eating animals such as seabirds, humpback whales, and blue whales.

Zooplankton predators increase

Following the swarms of tiny grazing copepods are a host of predators, which also become numerous or migrate to the Central Coast in June. In addition to larger animals, such as anchovies and krill, many small and/or gelatinous predators become abundant in June or July. Some of these are discussed in subsequent sections of this chapter.

Note: Just as some cold-water species of copepods are carried southward to the Central Coast in June, so are some zooplankton predators that are more commonly found in polar regions. These include krill such as ?? difficilis and Thysanoessa greagaria, as well as arroworms such as Sagitta scrippsae. These species are more likely to appear during years when the California current flows strongly.

The "Deep Scattering Layer" -- a massive, moveable feast

By June, diatoms not only bloom in large numbers but also tend to congregate in thin layers near to the sea surface. This creates a very rich source of food for tiny zooplankton grazers and predators, which spend the daytime hundreds of feet below the surface, but migrate up toward the surface at night to feed. Along with these tiny animals are deep-sea animals such as lampfish, headlight fish, and segestid shrimp. These animals spend most of their lives below 1000 feet, but migrate up to within 30 feet of the sea surface at night.

This daily vertical migration takes place not just along the Central California Coast, but across many of the world's ocean basins. Thus, it is probably the most extensive animal migration on Earth.

U.S. Navy oceanographers first discovered this massive migration during the second world war, when they first tried to use sonar to detect submarines. They soon learned that the "pings" from their sonar would scatter and bounce off of the masses of deep-sea animals as they migrated up and down through the water column, twice a day. Thus, these moving layers of live became known as "deep scattering layers."

Note: During early summer in Monterey Bay, there are such dense aggregations of grazers and predators that the deep scattering layer is not limited to one distinct depth, but forms a continuous band that may fill the entire upper 1,000 feet of the water column with what one author called "a riotous explosion of zooplankton, large and small."

Marine biologists generally presume that animals make this daily treck to get a good meal, as well as to avoid predators such as seabirds, fishes, and squids, which feed by sight (during the daytime).

It's amazing how far some animals take this strategy. For example, some tiny copeods, smaller than the periods on this page, migrate up from almost 1,000 feet below the surface every night. Larger animals, such as krill may spend the daytime as deep as as half a mile below the surface, yet come up to within a hundred feet of the surface to feet at night.

The mass of animals migrating up to the surface to feed creates such as feeding frenzy that, during some diatom blooms, each evening's feeding frenzy results in "storm" of "marine snow" (mostly poop from copepods and krill) that sinks down toward the seafloor, far below.

Note: A few species of copepods have been observed to do a "reverse commute," migrating toward the surface in the daytime and hiding in the depths at night. Others may be able to shift their migration strategies depending on whether predators are more numerous in surface waters during the daytime or at night.

The copepods in the deep scattering layer apparently rise up toward the surface as light levels at the surface decline in the evening, and sink back down at dawn, thus keeping themselves in a perpetual twilight zone.

Typically, both grazers and predators rise toward the surface until they find suitably dense layers of pray /at a particular depth. Then they begin migrating horizontally, feeding at this depth. However, they can't stay at any one depth too long, or they are likely to be tracked and eaten by larger animals hunting (and presumably following chemical trails) in the same crowded layer. Amazingly, this has been documented even for predatory comb jelles (ctenophores), such as Leucothea and Beroe.

Moon jellies swarm (and eat copepods)

Near the sea surface in June, you can sometimes see immense swarms of one of the most obvious copepod predators--moon jellies. These are clear, dish-shaped jellies with a white cross in the middle.

All spring, the year's crop of young moon jellies has been eating copepods and other zooplankton, and growing rapidly. By June, you can sometimes see thousands of moon jellies, all about the same size, drifting in swarms at the sea surface. June is the month when moon jellies typically reach maximum size, abundance, and fecundity.

As they swim through the water, the moon jellies bump into and sweep copepods and other crustaceans into their bells, where the prey become trapped in a layer of mucs that coast the bell. Tiny, hair-like cilia then move the prey across the surface of the bell to the moon jelly's mouth, which is in the middle of the animal, underneath the bell). Moon jellies also have small, stinging tentacles that they use to discourage would-be predators, such as blue rockfish, sea turtles, and ocean sunfish (mola molas).

Note: Some research suggests that moon jellies (like some other types of medusae) can survive hungry times by shrinking their bodies from the size of a dinner plate to as small as a dime.

The dense swarms of moon jellies in June make it much easier for the animals to reproduce (June is the peak of their reproductive season). Male moon jellies release sperm into the water, which fertilizes eggs within the female jellies drifting nearby.

The fertilized eggs stick to their mother's bell for a few days, until they hatch, releasing tiny, pear-shaped "planula larvae." At some point, the larvae swim away from their mother and head toward protected waters near shore or in estuaries such as Elkhorn Slough. Once in the shallows, each larva uses it's tiny tentacles to find a hard surface?a bit of rock, wood, or shell?where it can attach itself and spend the winter. If it gets enough copepods and other zooplankton to eat, by the following February the larva will have developed into a tiny, anemone-like "polyp," and the yearly cycle will begin again.

However, the honeymoon is short for the adult moon jellies-by the end of July most of them will be dead. You can sometimes masses of dead moon jellies washed up on sandy beaches. If you look closely at their bells, you can sometimes find a female jelly whose bell still contains tiny, whitish, tube-like structures that form the next generation of jellies.

Arrowworms eat copepods too

Some of the most aggressive predators on zooplankton are arrowwoorms such as Sagitta scrippsae-the half-inch-long barracudas of the zooplankton. They are not actually worms at all, but are in a family called chaetognaths, and known among marine biologists for their insatiable appetites and extensible jaws.

Like miniature barracudas, arrowworms hide in plain sight, floating motionless in the water, then lunging forward when a small animal swims past and seizing the animal in it jaws. When an arrowworm finds a dense patch of copepods or other prey, it goes on a feeding frenzy, and can eat enough in fifteen minutes to survive for an entire day. [Add info on seasonal variations in arroworm populations]

Siphonophores eat copepods (and everything else)

One of the overlooked but most important predators in midwater food webs are the gelatinous animals, and in particular, a group known as the siphonophores. These are colonies of individual animals that are all attached to one another, and function as a single "super-organism." They often take the form of a long chain, that moves through the water like a freight train.

At the "head" of the train are members of the colony that are shaped like bells and pull the animal through the water with jellyfish-like pulsations. Behind, and attached to them are other animals that specialize in reproduction. Still other members of the colony (often the most numerous), trail behind, and have tentacles, stinging cells, and stomachs that capture and digest prey (the nutrition is shared with all other animals in the colony).

The most famous type of siphonophore is the Portuguese Man-of-War, which does not form long chains. We don't have Man-of-war on the Central Coast, but we do have an amazing variety of open-ocean siphonophores, ranging from the size of your fingernail to over 100 feet long. These large siphonophores have immense curtains of stinging tentacles that act like living drift nets, and can capture almost any small animal that swims into them, including salps, arroworms, other jellies, shrimp, small fish and even squid.

In contrast, some of the smallest local siphonophores, Chuniphyes multidentata and Lensia conoidea, are specialists, eating almost nothing but copepods. However, they make up for their small size by being some of the most numerous siphonophores in deep coastal waters, especially in June. They are commonly called "rocket-ship siphonophores" because their swimming bells look like tiny rockets, pulling a chain of stinging and feeding cells along behind. The whole colony, however, is typically only x inches long.

Rocket-ship siphonophores are active predators that eat mostly copepods. A colony will swim through the water until it reaches a swarm of copepods. At that point, it stops swimming and spread out a web of extremely fine, transparent tentacles with stiniging cells that catch and immobilize any copepods that happenn to blunder into them. Each rockets-ship siphonophore "colony" only "lives" about a month, but they can reproduce very rapidly under the right conditions.

Small numbers of rocket-ship siphonophores may be present all year long, but they apparently do not reproduce unless plenty of food is available. These conditions typically do not occur until April or May, when the increase in copepods allows rocket-ship siphonophores ro reproduce rapidly. By May or June, they have become one of the most abundant gelatinous animals in the coastal waters, and are making quite a dent in the copepod populations. They typically reach maximum numbers about 45 days after the peak in diatom reproduction, then gradually become less abundant through summer and fall.

Note: During winter, rocket-ship siphonophores tend to stay within about 300 feet of the sea surface. However, in summer, they may move down to more than a thousand feet below the surface, perhaps following their prey, the copepods that migrate back and forth from the depths each day.

Sardines, anchovies, and hake migrate northward; anchovies congregate in Monterey Bay

As they mature in June and July, sardine, anchovy, and hake that hatched in Southern California in February to April move north toward Monterey Bay by July. They are followed by a whole raft of predators, including CAlifornia sea lions and pelicans.

As the summer progresses, the fish and their predators continue to migrate northward up the coast, eventually reaching as far north as Vancouver.

Market squid eggs hatch

On the sand flats that spread over the continental shelf along the southern part of Monterey Bay, the eggs of the market squid begin to hatch in June. These masses of eggs were laid in orgiastic frenzies during the previous full moon in May. Although the adult market squid provide a bounty for predators when they die after mating, their eggs are apparently much less tasty.

After three to five weeks of sitting on the sandy seafloor, the market squid eggs break open, releasing hundreds of 1/10-inch-long larval squid (complete with tiny tentacles). The larval market squid must start catching their own food within a few days after hatching, or they will starve to death. They soon make their way offshore into deeper water, where they hunt copepods and other zooplankton (which are abundant at this time of year).

Like their zooplankton prey, the young squid migrate vertically, feeding near the surface during the night and spending the daytime about 50 feet below the surface. Presumably this helps them avoid predators, which include sand dabs (near shore) and adults of their own species. If they survive, the young market squid will reach adulthood in just four to eight months.

Blackgill rockfish settle to the seafloor

As spring turns into summer, the last of the winter-spawning rockfish begin to give up swimming near the surface, and migrate down to the muddy seafloor of the continental shelf. Young blackgill rockfish, for example, have been drifting and swimming around the open ocean since February. By June they are about 2/3 of an inch long.

After they settle down to the seafloor, the blackgill rockfish will hang out along underwater cliffs or the edges of submarine canyons. From there, the young rockfish forage up into the water column, hunting first copepods, and eventually krill and salps (open-ocean tunicates). When they near adulthood, the blackgill rockfish will eat small squid, juvenile rockfish, hake, anchovies, and lantern fishes.

Green-striped rockfish give birth

Even as the juveniles of "winter-spawning" rockfish are settling down to the seafloor, the last of the "spring-spawning" rockfish are just giving birth. For example, green-striped rockfish release most of their young in late June or early July. These fish mate in May, and have a relatively short one month gestation period.

The larvae that the green-striped rockfish release in June or July will float around for about two months, turning into juveniles. Eventually they will settle down to the muddy or sandy seafloor near the outer edges of the continental shelf (at depths of 200 to 300 feet) in August or September. This is unlike most of the rockfish juveniles that settle out earlier in the year, which settle out close to shore and only move to deeper water as they get older.

White croaker move to deeper water

White croaker (Genyonemus lineatus) spend summers (June through August), scavenging and hunting a variety of small seafloor animals (e.g. fishes, squids [market squid?], shrimps, octopuses, worms, and crabs at depths down to 100 feet (the inner continental shelf). The rest of the year (September through May) croakers spend spawning and feeding closer to shore, especially in protected areas such as the shallow waters of Monterey Bay.

    All text © Kim Fulton-Bennett                About            Contact            Disclaimer