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Light in the Sea

 

The penetration of sunlight into the sea and its interaction with water and dissolved and suspended materials is arguably the most important physical phenomenon in the ocean. Processes like evaporation and precipitation determine the salinity of the ocean in any given region and both of these processes depend on energy from the sun. Heat from the sun drives the ocean currents and modifies our climate. Sunlight also provides an energy source for the photosynthetic processes of phytoplankton, on which most life in the sea depends.

 

 

The study of light in the sea is a recent subdiscipline of oceanography called optical oceanography. Oceanographers who study light in the sea and its interactions with the flora and fauna are called bio-optical oceanographers. My research in the nature of light, photosynthesis and natural fluorescence fall within the domain of bio-optical oceanography. As you might suspect, this is one of my favorite topics. It will be yours, too, because when you are done studying this topic you will be able to explain why the ocean is blue!

When you look at a blue piece of fabric, a red car, the blue sky, have you ever stopped to consider why you see that particular color? It's very simple, really. Objects of a particular color cannot absorb that color; thus, what we see are reflected or scattered wavelengths of light that are not absorbed. In the case of the fabric, it absorbs all the greens and yellows and oranges and reds, and what's left is blue. Same with the car and the sky. They absorb all the wavelengths of light except the one you see. Gradations in color or combinations of colors work the same way, only a spectrum of colors is not absorbed, or different segments of the visible spectrum are absorbed differentially.

The same is true for the oceans. Water (and seawater) is a very good absorber of all wavelengths of light except blue. Because water makes up a significant percentage of the atmosphere, our skies are also blue, especially in southern California! But what about when the ocean is green or blue green, or even ghastly green (like the summer of 1996 in Santa Monica Bay) or red (like the summer of 1995 in Santa Monica Bay)?

Changes in ocean color are primarily due to changes in the type and concentration of organisms suspended in the water, namely phytoplankton (which include photosynthetic bacteria, such as the cyanobacteria). Areas of river outflow, sewage outfall, or intense land runoff, near the coasts, may contain large amounts of suspended sediments, which give seawater a milky or dirty color. In some areas, such as near pulp mills, discharges of dissolved organic material can cause changes in the color of the ocean. For the most part, however, it is the phytoplankton that cause variability in ocean color.

Before we can appreciate the changes in the color of the ocean, we need to know something about how light in general changes as it penetrates into the ocean. When sunlight hits the ocean surface, some of it is reflected (around 5% on average) and the rest is transmitted through the water where it is eventually absorbed by water and the chemical and particulate components in the water. The zone of penetration of light into the water column is called the euphotic zone.

In general, the euphotic zone is defined as the area between the sea surface and the depth where light diminished to 1% of its surface value. The depth of the euphotic zone depends largely on the concentration of organic and inorganic materials dissolved or suspended in the water column. Thus, with more materials, such as in coastal waters, the depth of the euphotic zone will be shallow, perhaps only a few feet. In waters like the open ocean or tropical waters where terrigenous influences are negligible and concentrations of plankton are sparse, the euphotic zone may be quite deep, perhaps 150 meters (~450 feet) or more. (Quick: what is the depth of the euphotic zone at night?)

The decrease in the intensity of light as it travels through the water column is called light attenuation. Light attenuation is caused by the combined absorption and scattering properties of everything in the water column, including the water itself. In most ocean waters (and for the purposes of this class) light decreases as a function of depth in the water column in a way that can be described mathematically. The mathematical description of light attenuation in the water column is known as Beer's Law.

Beer's Law tells us that light decreases exponentially with depth. An exponential relationship is described by a curved line; human population growth can be described by an exponential curve. What this means to us no-math-heads is that light decreases very rapidly near the surface and decreases more slowly as we go deeper.

Beer's Law as a math equation looks like this:

I z = I 0 * exp -k*z

Hey, that's not too complicated, is it? But what the heck does it mean?

I z (pronounced eye-sub-zee) is a symbol that stands for the Intensity of light at depth z. (Remember that oceanographers use the z-axis for depth? If not, go review Use of Graphs in Section II). I 0 (pronounced eye-sub-zero) is a symbol that stands for the Intensity of light at depth zero (0). Depth zero is the surface. Depth z is any depth below the surface.

Notice that z appears in the right-hand side of the equation as a superscript. Z still stands for depth and what this equation tells us in words is that the light intensity at a particular depth, z, is equal to the light intensity at the surface multiplied by the exponent (which is 2.71828) raised to the minus k times z power. (The * symbol means to multiply.) Okay, that last part is tricky but any good calculator will get you through it without knowing what it means. But for those of you who do care, a negative exponent is the same thing as the inverse of that number, so if we divide 1 by exp k* z we can get the same answer.

The k symbol in this equation is known as the attenuation (or extinction) coefficient. It describes how quickly light attenuates or "goes extinct" in a particular body of water. Just using your gut feelings, what would you expect about light trying to penetrate a water column with high values of k, in other words, a water column with lots of attenuation or extinction? If you guessed that light would diminish rapidly, then you are on the right track. If you didn't guess this answer, then think about it a little bit more.

We can use Beer's Law to answer this question about the effects of k on the depth of the euphotic zone (i.e. whether light penetrates deeply or diminishes rapidly). First, consider the case where depth, z, is zero (0), i.e. at the surface. What is the value of I z?

First, compute k * z. If z equals zero (0), then k * z is zero (0). (See, it's not so hard.)

If k * z is zero, then exp -k*z equals one (1) because any number raised to the zero (0) power is one, by the authority of the High Council on Mathematics (and because all the math books tell us that).

So, if exp -k*z equals one (1) then I z = I 0 and we are done. The intensity of light at the surface equals the intensity of light at the surface.

I'll admit that is not a shocking conclusion and it sure seems like a lot of mental loop-dee-loops to figure out something obvious, but what I want you to get out of this is a willingness to understand math. If you are willing to give this simple math a try, then you will gain confidence in your math abilities and lose the fear of math that pesters us worse than that guy in our dreams with the funny fingernails.

Now, if you are feeling frisky,  give Beer's Law a try with real numbers and see how it operates under different values for k.  Here's a table for you to fill out using Beer's Law. Use a surface light intensity (I 0) of 1500 microEinsteins per square meter per second and compute I z for each depth listed.

Depth
(meters)
K=0.02 K=0.04
0
20
40
60
80
100
120
140
160
180
200

What is the approximate depth of the euphotic zone in each case, i.e. where I z equals 1% of I 0? How do values of k change the depth of the euphotic zone?

In addition to changes in the intensity of light as we go deeper in the water column, light changes in color as we go deeper in the water column. Recall that each color of light has a particular wavelength (or range of wavelengths). Each one of those wavelengths (i.e. colors) will have a particular intensity, right? In other words, there a certain intensity of blue light, a certain intensity of green light, a certain intensity of yellow light and so on. Following me? When we stated above that some materials absorb certain colors and others reflect those colors, all we were saying is that each color of light has its own attenuation coefficient, its own specific value for k.  If that wasn't the case, the entire world would be black and white!

Changes in the color of light are called changes in the spectral distribution of light. Spectral means color and distribution means how something is distributed so the whole thing put together, spectral distribution, means how the intensity of light changes for each color. Read that sentence again a couple times (or ten) if you don't understand it.

To put a one-sentence summary on what you have learned so far: light decreases in intensity (according to Beer's Law) and varies in spectral distribution as it goes deeper in the water column.

Let's examine more closely why the spectral distribution of light changes in the ocean. Though I didn't tell you before, the attenuation coefficient, k,  is a kind of catch-all coefficient: it describes two processes that are happening at once, namely, the scattering and absorption of light. In our discussion of the spectral properties of the ocean, we will only concern ourselves with changes in the absorption properties because it is those properties that exhibit the greatest variability (and the ones that are easiest to understand).

The absorption properties of the ocean can be described by the combined absorption coefficients of five components: 1) water; 2) suspended particles; 3) phytoplankton; 4) detritus (zooplankton poop) and 5) dissolved substances. Each of these separate compartments has a distinct absorption spectrum, a graphical representation of their absorption coefficients as a function of wavelength. Absorption coefficients are simply an expression of the amount of absorbance at a particular wavelength of light.

For definitional purposes, let's expand our description of each of these components of absorption.

Absorption Component Description
Water Pure water free of particles, gases and any dissolved substances
Suspended Particles Inorganic minerals, such as fine clays, suspended in seawater
Phytoplankton Pigmented, photosynthetic microorganisms (usually single-celled), including photosynthetic bacteria, such as the cyanobacteria (blue-green algae)
Detritus Bits of organic matter: photoxidized phytoplankton and the fecal remains of microzooplankton
Dissolved Substances Usually colored organic compounds dissolved in seawater

Note that suspended particles and detritus tend to be very fine, such that they sink slowly out of the water column. Larger fecal pellets, such as those produced by larger zooplankton, shrimp, and fish, tend to sink rapidly out of the euphotic zone and, thus, don't contribute to the optical properties of the water column.

We can describe the absorption properties of the sea using a simple mathematical equation (it's just an addition problem and it's one of the few equations you will ever see in this class, so don't panic):

a total = a water + a particles + a phytoplankton + a detritus + a dissolved stuff

where "a" stands for absorption. Thus, the total absorption of sunlight in the oceans can be described by this simple equation. A similar equation can be written for scattering in the ocean, but since any light scattered within the ocean eventually gets absorbed, we will focus our discussion on the absorption properties of the sea.

Variations in any one of these components can lead to changes in the absorption properties of the water column. If the change is large enough, the color of the water will change. Let's look at the absorption spectra of each of these components to get a better idea how they affect the color of the sea.

Water, as we mentioned above, absorbs most strongly in the red and absorbs the least at 430 nm. A table and a graph of the absorption spectrum of pure water are provided below. (This graph extends beyond the visible wavelengths because the infrared wavelengths are important to heat transfer into the oceans.) Compare the red and blue regions of the graph (use the chart above to figure out which wavelengths are red and blue). The absorption coefficient at 760 nm (where water absorbs maximally) is 2.55 per meter, whereas at 430 nm (where water absorbs minimally), the absorption coefficient is 0.0144. Absorption of red light is 177 times stronger than absorption of blue light. That's a big difference. It's no wonder the oceans are so good at trapping heat.

The absorption spectrum of particles, detritus, and dissolved substances resemble each other quite closely, close enough that we will treat them as the same type of curve. As shown below, maximal absorption occurs at the lowest wavelengths, 350 nm. Absorption falls off rapidly as wavelength increases, although there is noticeable absorption at blue and green wavelengths. Beyond 500 nm, absorption by these components is negligible, although if present in large enough concentrations, they can have an impact on ocean color.

Finally, we get to the real champs of absorption, the phytoplankton. (These guys make their living by absorbing light so they better be good at it.) Phytoplankton are responsible for most of the changes that we see in ocean color. The concentrations of phytoplankton in the ocean tend to be greater and their absorption coefficients tend to be higher than any of the other four components listed above. Thus, their contribution to ocean color is the most significant and noticeable.

The absorption spectrum shown here comes from a laboratory culture of living phytoplankton. Note the smooth, well-defined shape; the little peaks and bumps that occur throughout the spectrum; the large peaks at 430 nm and 670 nm. This graph represents the classical absorption spectrum of phytoplankton. Who can tell me what gives this classical pattern its shape?

Those of you who answered chlorophyll or photosynthetic pigments (or even pigments) are absolutely right. Chlorophyll absorbs maximally in the blue and red wavelengths and minimally in the green and yellow wavelengths. Now, who can tell me, from looking at this graph, what color were the phytoplankton cells in the lab culture?

Did anyone say green? Chlorophyll, my candidate for the most important molecule on Earth, has maximal absorption coefficients at 430 and 670 nm (although these wavelengths will vary depending on whether the chlorophyll is living or dissolved in some chemical, such as ether, alcohol, or acetone). Chlorophyll is also contained by every single photosynthetic plant and algae that we know. For that reason, most plants and algae and phytoplankton are green.

In addition to chlorophyll, there are many different kinds of photosynthetic pigments that absorb light and, thus, alter the shape of the absorption spectrum. The little bumps you see in this graph are caused by some of these different pigments. These pigments come in two varieties, photoprotective pigments and accessory pigments. Photoprotective pigments act like sunglasses for the phytoplankton cell. Accessory pigments act like ball gloves to catch wavelengths of light that chlorophyll isn't good at catching. These accessory pigments ultimately pass on their photons to chlorophyll and, thus, they help in the process of photosynthesis.

Back to our original question and alternate questions (why is the ocean blue and why sometimes is it not blue), it should now be slightly clear to you why the oceans appear as they do. Recall that phytoplankton cause the biggest change in the absorption properties of the ocean. If that's true, then who causes the biggest change in ocean color? And if phytoplankton are green, then what can we say about their abundance when the water is blue? What can we say about their abundance when the water is green? And what can we say about their abundance when the water is red?

A Little Story About Light in the Sea

Our discussion of the optical properties of seawater and the components suspended and living within it has taken us into deep into the frightening coves of physics and quantum dynamics. However,  I trust you have survived. We want no captives of Calypso here, we have many more oceans to cross. You should feel proud that you made it to the end of this lecture! You should also feel special because very few general education oceanography classes in the world will take you this far into ocean optics.

Understandably, some of the concepts presented in today's lecture are quite advanced. You would probably find that many graduate students in oceanography have no concept of an absorption spectrum. That's because optical oceanography is relatively new and little taught, even at the graduate level. However, if I didn't think you were capable of understanding these phenomena and if I didn't think that this subject was important, then I wouldn't have presented it to you. We will come back to some of these concepts when we study photosynthesis and look at the "green plants" of the sea, on which all ocean life depends (except vent communities).

If you have trouble understanding this lecture, e-mail me or come see me. I have lots of good examples and illustrations that will make these concepts clear to you. As I said, I love talking about this subject, so come and make me talk. The only thing standing in your way is you!

Finally, I want to relate a little story that happened to me while I was standing at the end of the Manhattan Beach Pier on one of my roller blading jaunts. Santa Monica Bay experienced an intense algal bloom the summer of 1996 (some say it was a blue-green algae, but my guess is that it was a green algae containing lots of chlorophyll b). The water and the surf were bright yellow green and highly unattractive but harmless nonetheless. While standing on the pier, a small child, not more than 10 years old, turned to her mother and asked, "Mommy, why is the water yellow?" Her mother answered, "I don't know. It's probably oil." I resisted lecturing the woman about the difference between oil and phytoplankton, but the point that I'm trying to make is that even an innocent child can appreciate the importance of optical oceanography!

 

   
   
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