The Seasonal Thermocline and Its Effect on Phytoplankton

As discussed in Water, Water Everywhere..., density is defined as the mass per unit volume of a substance. Remember the box of cereal?

Temperature affects density by changing the spacing between water molecules in a given volume of water. As temperature increases, water molecules get more energetic (they dance harder) and they tend to space out (i.e. have more space between them, not get dizzy). Thus, increases in temperature decrease the density of seawater. Conversely, as we learned about the formation of ice, decreases in temperature increase the density of seawater. Seawater density and temperature are said to have an inverse relationship; that is, as one goes up, the other goes down. Sometimes it is easier to remember this way.

Salinity, the amount of salts dissolved in seawater, affects density by adding mass to a given volume of seawater. As more and more salt is dissolved in a particular volume of water (as water evaporates or ice forms leaving the salts behind), the density of seawater increases. Thus, increases in salinity cause increases in density. Similarly, decreases in salinity, caused by the addition of freshwater (i.e. rain, snow, ice melt, river outflow) or seawater with a lower salinity, causes decreases in the density of seawater. Salinity and density are said to have a positive relationship; as one goes up the other goes up.

Both temperature and salinity affect the density of seawater and both may change at the same time. Any given combination of temperature and salinity may produce the same density. Typically, temperatures we may encounter in the ocean. range from -2 to 28 degrees C (except near hydrothermal vents or in inland seas, where temperatures may be higher). Salinity typically ranges between 28 to 41 parts per thousand (ppt), although at the mouths of rivers and in surface waters of the polar oceans in spring, salinity will be much lower. Recall that the highest salinity given here occurs in the uppermost extension of the Red Sea. The average salinity usually quoted by oceanographers is 35 ppt.

While we didn't mention it previously, the density of water is 1 gram per cubic centimeter (g/cm3) at 4 degrees C, its maximum density. At a salinity of 35 ppt, the density of seawater at 4 degrees C is 1.0278 g/cm3. While this may not seem like a big difference, its enough to move considerable volumes of water. Density is usually symbolized by the greek letter ro. Average density values for seawater range from 1.024 to 1.028 g/cm3. Again, while these differences may not seem large, these minute differences mean a lot when you're talking about tons of water.

Now that we have defined a whole bunch of terms and pretty much beat this ocean chemistry horse into shape, let's take a look at what all this means for the world oceans. Recall that temperature and salinity affect the density of seawater. Remember also that evaporation and precipitation, which differ in various regions of the ocean, are the principal factors that influence salinity. Obviously, heat exchange, driven by the seasonal cycle of the Earth's movement about the sun, also influences the density of seawater. As a result of these factors, a world ocean develops with a layered structure made up of water masses of different density.

Consider how the layered structure of the ocean might appear. Lighter, less dense lenses of water float on heavier, denser slabs of water. The end result is an ocean that varies in density both on horizontal spatial scales (across the surface of the ocean or at a particular depth in the ocean, i.e. along the X and Y axis) and on vertical spatial scales (from the surface to the bottom of the sea, i.e. along the Z-axis).

Rapid changes in seawater density across horizontal scales form what are known as fronts. Fronts exist at the interface between any two water masses with distinct properties (that give it a unique density). The most common fronts occur where deep water masses are formed, but fronts may be present as a result of wind-driven upwelling and downwelling of ocean water, or where eddies form. We will talk more about upwelling, downwelling, and eddies when we talk about wind-driven processes in the sea.

On the vertical scale, as we examine density as a function of depth in the water column, we notice regions where the changes in density (or temperature or salinity) are quite rapid. These areas of rapid change in density, temperature, and salinity are quite distinct. And now I must introduce you to the family of the clines. A cline is a region of rapid change. Areas of rapid change in density are called a pycnocline; areas of a rapid change in temperature is called a thermocline, and areas where salinity changes rapidly are known as the halocline.

It is quite likely that all of you are familiar with thermoclines. If you have ever wandered far from shore on a lake, you have probably noticed the cold water at your feet not so distant from the beach. What you experienced was a thermocline, a rapid change in temperature. This thermocline was formed as a result of a lens of warm surface water, heated by the summer sun, lying on top of a pool of deeper, colder water, a permanent year-round "resident" of large lakes and oceans.

This familiar example introduces us to the concept of the seasonal cycle of changes in the vertical structure of the water column. Seasonal changes in heating and cooling, particularly in temperate and polar climates, drive changes in the temperature and, hence, density structure of the water column. As surface waters heat, the thermocline becomes shallower and more distinct, as warm, less dense surface water sits on top of colder water. Alternatively, as surface waters cool, the resulting density change in the surface water causes it to sink and "mix" the water column. This mixing tends to deepen the thermocline or remove it entirely. This layer of water above the thermocline where mixing occurs, usually from the actions of the winds, is known as the mixed layer, and it is one of the most important concepts in determining the productivity of phytoplankton.

Let's take a closer look at the seasonal cycle of temperature changes. In summer, shallow, warm water forms on top of deeper cold water. In fact, this figure indicates the presence of two thermoclines: the upper shallow thermocline, also called the mixed layer, and the lower deep thermocline. This shallow thermocline is often called the seasonal thermocline while the deeper thermocline is referred to as the permanent thermocline. In the fall, the surface waters begin to cool, causing the shallow thermocline to deepen. The mixed layer, the layer above the shallow thermocline influenced by the actions of the wind, also deepens. In the winter, the mixed layer may deepen as far as the permanent thermocline, removing the seasonal thermocline altogether. In areas where deep water forms, i.e. regions that form the deep waters of the world ocean, even the permanent thermocline may be absent. Finally, in the spring, as the surface waters warm again, the seasonal thermocline appears, and the mixed layer becomes shallow again.

Seasonal variations in planktonic ecosystems are among the first oceanic ecosystems studied by oceanographers. By examining the biomass of phytoplankton in coastal and oceanic waters at temperate latitudes, many of our ideas about the structure and functioning of ocean ecosystems was derived. To explain these seasonal variations, early oceanographers concentrated on the role of light, nutrients and vertical mixing. These fundamental physical and chemical processes and their effect on biological processes still consume the thoughts and actions of oceanographers today.

To help you understand seasonal variations in oceanic planktonic ecosystems, I have prepared four illustrations that you should download, print and tape to your forehead. You may find them here: winter, spring, summer, fall. Please refer to these illustrations throughout the following discussion.

In the discussion that follows, note that we will be talking about the formation of the seasonal thermocline, especially as it impacts processes within the euphotic zone. We will not discuss the permanent (or deep) thermocline that is formed as a result of water mass formation and thermohaline circulation.

In the winter, cooling of surface waters increases their density and they sink. Continued cooling creates an isothermal water column throughout the euphotic zone. This water column is well mixed and any suspended particles, such as phytoplankton, will be mixed along with the water. Thus, rates of vertical mixing (the up and down movements of the water) will be high. Any phytoplankton near the surface will be transported downwards to the bottom of the euphotic zone. Other kinds of mixing processes may bring them near the surface again, but for the most part, phytoplankton are driven away from the surface where the highest intensities of solar irradiance exist. Coupled with the deep mixing are low sun angles and short days, which further reduce the amount of sunlight available for photosynthesis. Though vertical mixing brings nutrient rich water into the euphotic zone, the lack of light really limits how fast phytoplankton can grow. Thus, phytoplankton biomass is low, rates of primary productivity are low and rates of growth of any organisms that depend on phytoplankton are low. In this situation, primary productivity is said to be light-limited.

In the spring, everything changes. Increased sun angles and longer days warm the surface of the ocean. As the ocean surface warms, the surface waters become less dense, creating warmer layers of water on top of colder layers. Lo and behold, a thermocline appears. The formation of this thermocline signals the onset of vertical stratification. Now, rates of vertical mixing are substantially reduced. Phytoplankton in the surface waters remain near the surface. Having lots of light and plenty of nutrients, the phytoplankton begin to divide rapidly. Rates of photosynthesis (gross primary productivity) are high. The abundance of light and nutrients leads to a "bloom" of phytoplankton. Once blue waters now turn green as chlorophyll biomass increases. As phytoplankton biomass increases, light penetration decreases and the euphotic zone becomes more shallow.

Yet, the abundance of phytoplankton provides a rich source of food for herbivorous zooplankton. The zooplankton biomass begins to grow. And as these zooplankton grow, the organisms that eat them begin to grow and the organisms that eat the organisms that eat the zooplankton begin to grow and so on and so forth. Everything in the food web flourishes, as energy from the sun and matter from dissolved inorganic nutrients are turned into living matter by the phytoplankton.

But the good times don't last forever. As the phytoplankton photosynthesize, they absorb dissolved inorganic nutrients and remove them from the surface waters. Because the water column is stratified and because rates of diffusion are low, the surface waters become depleted. Nutrients run out. Photosynthesis stops (or at least slows down a lot). The biomass of phytoplankton, no longer increasing, gets rapidly chomped down by all the hungry zooplankton and so, the biomass of phytoplankton decreases. This is the situation at the start of summer.

In summer, intense surface heating stratifies the water column even more. Layers upon layers of successively warmer water are created at the surface of the ocean. Vertical mixing practically stops. Phytoplankton are confined to the layer in which they live unless they sink to a lower layer. As nutrients are further depleted, waters near the surface are not a kind place for phytoplankton. In summer, phytoplankton growth is said to be nutrient-limited.

Any photosynthesis that does occur is determined by the rate at which nutrients are supplied to the nutrient-depleted waters. As discussed elsewhere, inorganic nutrients are formed as organic matter decomposes, a process called remineralization. Bacteria break down the organic matter and produce the nitrate, nitrite, ammonia, phosphate, potassium, silicate, iron, etc. that is required for phytoplankton growth. In this way, matter is recycled in the ocean. Phytoplankton absorb dissolved inorganic nutrients, they get eaten or die, and bacteria remineralize that organic matter back into inorganic nutrients. These biologically important minerals are vital to phytoplankton growth and the rate at which zooplankton fecal pellets and dead phytoplankton are remineralized in surface waters will determine how fast phytoplankton grow in these waters in the summer. Direct excretion of ammonia by zooplankton (much like our excretion of liquid ammonia) will also supply some nutrients to the phytoplankton.

In general, sources of dissolved nutrients for phytoplankton growth include 1) nutrients supplied by diffusion; 2) breaking internal waves that mix deeper nutrient-rich waters with shallower nutrient-poor waters; 3) vertical mixing of deeper, nutrient-rich waters with shallow nutrient-poor waters; 4) remineralization of organic matter by bacteria; 5) zooplankton excretion; and 6) upwelling.

The bottom line is that phytoplankton growth diminishes in summer even though lots of light is available because the rate of supply of dissolved nutrients for phytoplankton growth is slow.

In the fall, the picture changes once again. The sun moves lower on the horizon, daylength gets shorter and cooling of surface waters occurs. The water column is said to "turn over" as cooler water sinks, breaking down vertical stratification and allowing deeper nutrient-rich waters to mix with surface waters. Because more nutrients are now available and because light is still available at sufficient intensities to drive photosynthesis, phytoplankton begin to grow again. The surface waters experience what is known as a "fall bloom." Phytoplankton biomass (and, of course, chlorophyll concentrations) increase and the waters may turn green again.

Two differences characterize the fall phytoplankton bloom from the spring phytoplankton bloom. First, there are generally less nutrients available in the fall than the spring. This is because the water column is well-mixed in the winter preceding the spring bloom, whereas the water column is only partially de-stratified in the fall, preceded by a period of intense stratification in the summer. Second, zooplankton biomass is generally greater in the fall than in the spring. This means that grazing pressure is greater in the fall than the spring, so that any new phytoplankton growth in the fall is almost immediately consumed.

What emerges from the seasonal cycle of phytoplankton growth is the interplay between physical, chemical and biological processes in the ocean. A physical process -- the onset of vertical stratification -- stimulates a biological process -- primary productivity. This biological process -- phytoplankton photosynthesis -- affects a chemical process -- the concentration of inorganic nutrients. As photosynthesis proceeds, the concentration of inorganic nutrients diminishes. In an ironic twist of fate, the chemical processes -- the rate at which new inorganic nutrients are made available -- take over the biological processes -- rates of photosynthesis. In another interplay of processes, we can also see how biological processes -- increases in chlorophyll a, detritus and bacteria, important components of light absorption -- affect a physical process -- the penetration of light into the water column. And though we haven't talked much about it, we can't forget that geological processes -- weathering of rocks -- are the ultimate source of all the nutrients in the sea, and thus, geology affect biology AND we can't forget that biological processes -- sedimentation of organic matter via zooplankton fecal pellets -- creates the tapestry of sediments that we find on the bottom of the ocean. The beauty of this harmony in the way that the ocean works almost brings you to tears!

Don't be fooled by the apparent simplicity of this cycle. Episodic events, such as cold fronts, storms, hurricanes, mesoscale eddies, upwelling, downwelling, coastal jets and filaments, can rapidly change the structure of the water column. Any process which affects a change in the density of the seawater--evaporation, precipitation, heat flux, or wind-driven circulation--will change the depth of the mixed layer. These changes in mixed layer depth, as mentioned above, have a profound effect on the productivity of plankton. Thus, we have a very good example of how physical factors can affect biological factors. In fact, these physical factors also change the chemistry of the seawater and, as phytoplankton grow, they in turn have a profound influence on physical processes, particularly light penetration. The ever-turning wheel of physical, chemical, geological, and biological processes makes an imprint on the water column as well.

Take these notes, go down to the beach, make yourself comfortable, and stare at the sea. Read a few paragraphs, look at the sea; read a few more paragraphs, look again at the sea. You will begin to understand how interconnected are all these processes in the sea. You will begin to appreciate the vast and complex drama that is played out in the oceans everyday. You will begin to feel the call of the sea, your own primordial rhythms urging you back to the birthplace of your ancient ancestors. Once felt, you will never look at the sea in the same way again. But once felt, you will experience a profound and heartfelt awareness that you too, are an important part of this precious cycle, and that you too, can make a difference. Congratulations! You are now beginning to understand the remarkable world of oceanography.