Seasons of the Sea

The interactions between physical, chemical, geological and biological processes in determining the structure and functioning of oceanic ecosystems are the essence of oceanography. The lecture you are about to read is my expression of that theme. This is the one lecture that ties together everything that you have learned over the past few months. It is the most important lecture I give all semester. If you understand this lecture, you understand how the ocean and our planet work. If you can achieve that understanding, you can interpret and explain nearly all of the natural phenomena you may witness in your life. It is a very powerful understanding. If at first you don't understand it, keep trying. It might take you a lifetime, but it will be worth it. Perhaps your dying word won't be "Rosebud," but it might be "phytoplankton."

Upwelling

As we have learned, physical processes impart structure to the ocean. The layering of water masses (due to density differences caused by heating and cooling of water and evaporation and precipitation); the formation of cold-core rings (caused when the meandering Gulf Stream pinches off and surrounds cold Labrador current water); and the seasonal heating and cooling of surface waters (creating and destroying the seasonal thermocline) are just a few of the physical processes that give shape and structure to the ocean. One process that we haven't mentioned is upwelling, a physical process that creates the most biologically productive oceanic regions in the world ocean. We will start with an explanation of upwelling.

During our discussion of ocean currents, I introduced a phenomenon known as the Coriolis effect. Recall that the Coriolis effect is a manifestation of the rotation of the Earth on its axis and it has an effect on the way things move around on our planet. In its most simple sense, the Coriolis effect is the apparent deflection of a moving object to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. In the case of ocean currents, the Coriolis effect gives rise to the major ocean gyres and is an important component of geostrophic flow.

The Coriolis effect also gives rise to a phenomenon known as the Ekman spiral. When the wind blows across the surface of the ocean, the surface waters begin to move. In the Northern Hemisphere, these waters also begin to turn to the right. As the surface waters begin to move and turn right, they drag the layer of water immediately beneath them. As these layers begin to move, they also turn to the right and drag the layer beneath them. The result is a series of water layers moving more and more to the right in a kind of spiral. Each layer moves further to the right and a little bit slower than the layer above it. That's because friction reduces some of the energy available for water movement.

The important aspect of the Ekman spiral is the result that the net movement of water is 90 degrees to the right of the wind in the Northern Hemisphere. Most of the water that is set in motion by the wind moves at a 90-degree angle to the wind. Of course, in the Southern Hemisphere, water moves at a 90-degree angle to the left of the wind.

What does this mean for the movement of water when the wind is blowing? In most regions of the ocean, it doesn't have much of an apparent impact. Yet in coastal waters, movement of surface waters offshore presents an interesting situation. If surface waters at the beach where you swim are moving offshore due to a north wind, then what happens? Do you go to the beach and find that you can walk to Catalina? I don't think so. What we find is that colder water from intermediate depths (or even submarine canyons) moves up to replace the warmer surface water that has moved offshore. This movement of cold water to the surface is called upwelling.

Along the California coast, the presence of well-defined headlands (Palos Verdes, Point Conception, Pt. Lobos, Pt. Arguello, etc, etc, etc) and submarine canyons create geological conditions that are ideal for upwelling when the wind blows from the north. This north wind blows surface waters towards the west (90 degrees to the right) and we experience upwelling. Sometimes, this cold upwelled water moves away from the coast in narrow fast-moving streams called coastal filaments or jets. These well-defined ocean structures create conditions that are ideal for phytoplankton growth, as well as affecting local meteorology (i.e. creating fog) and making it pretty dang cold for surfers and swimmers. If you have ever been to the beach after a couple days of strong north winds, then you probably know what I mean.

While we're on the subject, note that a wind from the opposite direction, a south wind, causes a phenomenon known as downwelling. In this case, water moves onshore and puts a lid on any planktonic processes. That's because surface waters tend to be depleted in nutrients. Without upwelling, nutrient-rich waters remain below the euphotic zone and the growth of phytoplankton is nutrient-limited. If phytoplankton growth is limited, then so is the growth of herbivorous zooplankton and the things that eat them and the things that eat the things that eat them and so on. The entire food web, at least in upwelling regions, depends on the wind blowing in a certain direction!

Thought question: which way does the wind need to blow for upwelling to occur on the the western side of Africa? How about the coast of Chile?

The Big Picture

If you could spend a year in the International Space Station and had a pair of special sunglasses, you could witness the seasonal cycle of phytoplankton productivity across the entire world ocean. Or you could just look at the satellite images from the Coastal Zone Color Scanner (CZCS) provided below.

Northern Hemisphere Winter: Southern Hemisphere Summer


CZCS winter composite for all months between January to March, 1978-1987: click to enlarge

Northern Hemisphere Spring: Southern Hemisphere Fall


CZCS spring composite for all months between April to June, 1978-1987: click to enlarge

Northern Hemisphere Summer: Southern Hemisphere Fall


CZCS summer composite for all months between July to September, 1978-1987: click to enlarge

Northern Hemisphere Fall: Southern Hemisphere Spring


CZCS fall composite for all months between October to December, 1978-1987: click to enlarge


Color scale for above images

Look very carefully at these images and compare their differences (see the color scale). Regions with yellow, orange and red colors have the highest concentrations of phytoplankton. Regions with green, blue and violet colors have lower concentrations of phytoplankton. How do phytoplankton concentrations change in different regions of the world at different times of the year? Look at the Gulf of Alaska and observe how it changes seasonally. Look at the North Atlantic Ocean and observe how it changes seasonally. Look at the west coast of Africa and observe how it changes seasonally. Look at the equatorial Pacific and observe how it changes seasonally. Try to relate the changes you see in phytoplankton concentrations to physical and chemical processes that must be occurring in the water column.

Also try taking a look at the regional seasonal composite images. Pick your favorite part of the world and have a look!

Northeastern Pacific
Northwestern Pacific
Southwestern Pacific
North Atlantic Ocean
Eastern Atlantic Ocean
Southeastern Pacific
Indian Ocean
Arctic Ocean
Antarctic Ocean

Now, for six million dollars and a brand new Chevy Malibu, who can tell me why these satellite images don't tell the whole story?

Ecological Succession in the Sea

Okay, now that you have grasped the most fundamental and beautiful processes in the ocean, let's take a brief look at the effects of upwelling and cold-core rings on plankton processes.

When upwelling occurs, cold, nutrient-rich water moves to the surface. (Why does water beneath the euphotic zone contain lots of nutrients? Figure it out from the above information. It's a test question.) As nutrient-rich water is exposed to high light levels, a bell goes off in your head and you say "phytoplankton bloom." Yup, upwelling creates a bloom of phytoplankton. As the upwelled water is further transported offshore, the same sequence of events as above occurs. Phytoplankton bloom, zooplankton bloom, the things that eat the zooplankton bloom (and so on), surface waters become depleted of nutrients, the bloom shuts down, and the upwelled water eventually warms and mixes and biological productivity ends. Because upwelling brings cold nutrient-rich water to the surface, biological productivity in these regions is very high. In fact, productivity in upwelling regions is the highest of any region in the ocean, with the exception of kelp forests and coral reefs. Nonetheless, upwelling productivity is the highest of all oceanic ecosystems and that is why most major commercial fisheries are centered around upwelling regions. The California sardine industry, the Peruvian anchovy industry and the Benguelan fishery are good examples of fisheries supported by upwelling.


Phytoplankton productivity associated with upwelling off the northwest coast of Africa

One example of the importance of upwelling is illustrated by the effect of El Nino on upwelling off the coast of Peru. It was the Peruvian fishermen who coined the term El Nino, a time during which the fish disappear because warm water from the equatorial Pacific prevents upwelling. Thus, the entire El Nino-thing started because of its impact on upwelling and the availability of fish.

One other interesting feature of upwelling concerns the spatial distribution of the processes described above. If we imagine a series of boxes from nearest the coast to the offshore extent of the upwelled water, we can see that the first box (nearest the shore) is the region of greatest phytoplankton productivity, the next box is the region of nutrient depletion, the next box has the bloom of zooplankton, the next box has the highest concentrations of organisms that eat the zooplankton, and so on. Thus, the "seasonal" cycle (which really isn't seasonal in the case of upwelling) is distributed all along the region of upwelling from the shore to its outward extent. As long as water continues to be upwelled, the phytoplankton will continue to bloom and the entire upwelling system will be productive. Once the winds stop, however, the upwelling system will lose its source of nutrient-rich waters and the system will "evolve" in  a "summer-fall-winter" fashion (although the intermediate stage will not see a bloom.)

We can apply these same principles to cold-core rings. Because of satellite images, we know that the Gulf Stream meanders much like a river as it meets head on with the Labrador current. As the Gulf Stream flows north, it encounters the Labrador flowing south along the banks of Cape Hatteras. As these two currents meet, they begin to meander (they wind back and forth like a snake). Eventually, these meanders "pinch off" from the main flow and become independently rotating structures, known as rings.

One set of rings, which contains the cold water of the Labrador Current, are known as cold core rings. These cold core rings spin off from the Gulf Stream and are propelled eastward into the North Atlantic Ocean. Their movements may take them quite far from the Gulf Stream and, depending on their size, they may retain their characteristics for months. As they spin off from the Gulf Stream, they have a certain rotational velocity, a net direction, and a characteristic temperature structure. Because their centers contain cold nutrient-rich water, a plankton bloom develops in the middle of these rings. The development of the plankton bloom, the development of organisms that feed on the plankton, and the eventual "death" of the ring as it mixes with the surrounding water is a fascinating study in the ecological succession of plankton communities. In recent years, entire oceanographic studies have been devoted to understanding the nature of the formation and evolution of these rings, and the biological communities that develop as a result of these rings.

On the other side of the Gulf Stream, warm water pinches off into structures called warm core rings. These rings typically spin off west and north of the Gulf Stream, and travel against the flow of the Labrador Current. Because their centers are composed of warm, nutrient-poor water, conditions are not ripe for plankton blooms. As such, these rings typically don't develop the kinds of biological communities we observe in cold-core rings.

The satellite image of sea surface temperature reveals quite nicely the differences between cold-core and warm-core rings. In the image shown, reds, oranges, and yellows are warm water, and greens and blues are cold water. Take a look at the center of the picture. You should be able to make out swirling masses of water associated with the northernmost part of the Gulf Stream. Can you see two dots of green in the middle of the red and yellow water? These are cold core rings. Just above them, on the other side of the Gulf Stream, is a large swirl of yellow water floating in the middle of green water. This is a warm-core ring. Note also the meanders of the Gulf Stream as it bends towards the east. It is these meanders that give rise to these rings.

In the process, "pieces" of the river break off as separate eddies, whirlpools of water. As cold, nutrient-rich water is surrounded by warm water, a type of stratification occurs. In the core of the ring are lots of nutrients and lots of light. Ding! A phytoplankton bloom occurs and the rest, as they say, is history.

From these three examples, we can see how a physical event sets off an entire sequence of biological and chemical events. In reality, it doesn't matter whether it's winter, spring, summer or fall; if a physical process promotes the growth of phytoplankton, then the entire food web gets going as energy and matter are pumped into it.

While we don't have time to cover all of the dynamics of these events, your general understanding should provide you with clues as to other changes occurring in the food web. One of these is a change in the kinds of species we observe from winter to spring to summer to fall. As we know, each species of organisms is uniquely adapted to specific environmental conditions. As conditions change, so do the species. The change is species composition that occurs over time is called ecological succession.

All of you are familiar with ecological succession even though you may not realize it. Fire in the chaparral opens up new space for invader species and these species modify the soil so that other chaparral organisms can grow, eventually resulting in the creosote-dominated chaparral that we find in southern California. The most famous example of ecological succession occurs when newly erupted lava cools. Lichens colonize the rock, breaking it down into bits of soil that are ideal for small plants. As these plants grow and produce organic matter, they create soil, which is ideal for larger species of plants. Eventually, tree species take root and the forest is created over again. This same process happens in the ocean, in all oceanic environments, all the time.

To wrap up this lecture on seasonal productivity, I want to mention the seasonal cycle in polar and tropical oceans. Polar oceans typically experience only one phytoplankton bloom in the summer because of the low surface irradiance in all seasons except summer. However, polar phytoplankton blooms lead to the highest concentrations of phytoplankton (and chlorophyll) found anywhere in the ocean (with the exception of some bays and estuaries). As ice melts, it creates a lens of fresher water on top of the saltier ocean water. This melting creates stratification. Along with stratification induced by warming of the surface waters, the water column next to an ice edge becomes stratified very quickly. In addition, the melting of sea ice releases large quantities of diatoms and other phytoplankton that overwinter in the ice. The result is fast stratification and "seeding" of the water column, all of which adds up to one massive phytoplankton bloom. So famous are these blooms of phytoplankton in the sea that many whales, especially grey whales and humpback whales, return to polar waters in the summer to feed on the rich organic soup.

In tropical waters, the situation is much different. Because tropical regions experience very small variations in surface irradiance and daylength, they are nearly continuously heated. Thus, tropical waters are a lot like the summer situation in temperate waters. Surface waters are depleted of nutrients and phytoplankton biomass is low. That's why tropical waters are so clear. Thus, tropical waters typically have no pronounced phytoplankton blooms. If a bloom occurs, it typically results from some storm-related event that cools or mixes surface waters with deeper waters.

You should now be getting a better picture of how physical, geological, chemical, and biological processes interact in the ocean. Think about how the physics of the ocean creates rings and eddies, how it creates upwelling centers, how it creates a layered structure that changes over the seasons. Think about how landmasses and the topography of the bottom divert or trap the surface and deep currents; these large-scale geological features create different patterns of flow like the squashed flow of currents in the northern Indian Ocean or the two-gyre expanded system of the South Pacific Ocean. Think about the evolution of chemical characteristics within each of these physical structures; cold-core rings having centers of nutrient-rich water and warm-core rings being like deserts. Think about how these rings and eddies, upwelling centers and layered oceans create habitats for marine organisms; the structure and evolution of these features create plankton blooms, provide conditions well-suited for seaweeds and fish, and are persistent enough over the course of the seasons and years to attract the attention of the smartest animals of the sea, the whales.

You may now appreciate the importance of understanding seawater density and the processes that control it. You may now appreciate better the role of absorbing components in the ocean and their impact on ocean color. You may now appreciate better the role of the wind (created by differential heating of the planet) and Coriolis effect on upwelling. You may now better appreciate the importance of chemical processes in the ocean and the role that nutrients play in primary productivity. You may now better appreciate the phytoplankton as the guardian of all life in the ocean, as the photosynthetic machines that make living matter out of non-living matter, as the conduit for the sun's energy, as the source of half the oxygen in our atmosphere. And hopefully, through this knowledge, you will better understand the role that you play as a living being on this planet, the dynamic interplay between you and the geological, physical, chemical and biological processes that govern our planet.

Useful Links:

Pacific Fisheries Environmental Laboratory Coastal Upwelling Indices
http://www.pfeg.noaa.gov/products/upwell.html

SeaWifs: Sea-Viewing Wide Field of View Sensor for viewing phytoplankton from space
http://seawifs.gsfc.nasa.gov/SEAWIFS.html