Topics Covered in this Lecture:
- The Composition of Light
- Sunlight On the Ocean
- Why is the Ocean Blue?
In this lecture, we stop to examine the nature of sunlight and its interaction with 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. As we saw last lecture, the process of evaporation and precipitation determines the salinity of the ocean at any given spot. Both of these processes depend on energy from the sun. We have also mentioned that 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. Thus, it behooves us to know a little about sunlight and a subdiscipline of oceanography called optical oceanography. Oh, and I'll warn you, this is one of my favorite topics to talk about!
The Composition of Light
How many of us have stopped to stare at a rainbow and wished for a pot of gold? Nearly everyone finds some symbolic significance to a rainbow. Rainbows have inspired hopes and dreams since the earliest man. They are certainly one of the more stirring natural phenomena to occur on our planet.
While an oceanography class may not seem the place to be chasing rainbows, there is a lesson in rainbows that has relevance to our studies of the sea. Rainbows remind us that sunlight is composed of colors. What may seem like a pure stream of white light is actually a mixture of many different colors. These colors are part of what is known as the electromagnetic spectrum, and the part of this spectrum that we see is known as visible light.
Light, or more properly, electromagnetic radiation, is composed of indivisible units called photons. You might think of photons as the "atoms" of light. These discrete packets of energy impinge on objects just like raindrops, only they are much faster and much smaller. Light through a vacuum travles at 300,000,000 meters per second. In other media, light travels more slowly; about 225,000,000 meters per second in the case of water. On a typical sunny day at noon, as many as 1,000,000,000,000,000,000,000 photons of light hit one square meter of pavement (or any other surface) every second. How fortunate that we cannot hear them! (The noise would be deafening and we would have to teach this class in the dark.)
Light also behaves like a wave, and, thus, every photon has a wavlength. It is these different wavelengths of light that appear as color to our eyes. We won't convern ourselves here too much with the quantum electrodynamics of light, but if you're really interested in this topic, check out a book called QED by the late Richard Feynman. It will blow your mind (and distort your perception of the world forever.)
The important thing to remember is that the electromagnetic spectrum is characterized by radiation of different wavelengths. Visible light is only a small part of the electromagnetic spectrum. Visible light is defined as light with wavelengths between 390 and 750 nanometers (nm = 10-9 meters). Each color that we see in a rainbow represents one particular wavelength of light, such as blue light which has a wavelength of approximately 480 nm. Shorter wavelengths, such as gamma rays, x-rays, and ultraviolet radiation, are less than 400 nm long, while long wavelengths, such as infrared radiation (heat), microwaves, radar, TV signals, and long-wave radio signals are greater than 700 nm and may be longer than a kilometer. FM radio and TV signals have a wavelength about the height of an average person. Maybe that's why we spend so much time paying attention to these signals.
For purposes of our discussion of the optical properties of the sea, we will concentrate only on the visible wavelengths of light. But you should be aware that a vast spectrum of electromagnetic radiation hits our planet, of which we are privy to only a very small part of it because our eyes are sensitive only to visible light. If we had eyes like the hydrothermal vent shrimp, we would be able to see the heat patterns radiating from our fellow students and the food down at the cafeteria.
Returning to rainbows, we notice that there is a pattern to the colors which occur. This color pattern is the same color pattern you would see if you were to transmit light through a prism (which is how Newton discovered that light was composed of colors). The long wavelengths of visible light are red and orange. The short wavelengths are blue and violet. In order of decreasing wavelength, here are the colors of the visible spectrum: Red, Orange, Yellow, Green, Blue, Violet. To help you, try this: ROY Gives Blue Violets. You can hardly miss. If you don't like this mnemonic, make one up of your own! (Try making one up for VBGYOR.)
Each of these colors are generally associated with specific wavelengths of light and you should familiarize yourself with them. Study the table below to get a feeling for the different wavelengths and their colors.
|Red||650 - 700 nm|
|Orange||600 - 649 nm|
|Yellow||550 - 599 nm|
|Green||500 - 549 nm|
|Blue||450 - 499 nm|
|Violet||400 - 450 nm|
Visible light having wavelengths greater than 700 nm is called far red or infrared, and with a proper pair of glasses, you can see light of this wavelength. Light at wavelengths below 400 nm is called ultraviolet light, a kind of light that has a tendency to zap molecules of DNA and thus create mutations that may lead to cancer. Ultraviolet light is what most good sunglasses try to remove to protect your eyes.
Sunlight On the Ocean
The path of a photon from the sun to the surface of the sea is not an easy one. Besides traveling through the vacuum of space to the Earth's outer atmosphere (a journey that takes about eight minutes), a photon must contend with a whole host of "road blocks" to even reach the Earth's surface. These road blocks fall into two general categories: things that absorb and things that scatter.
Absorption is a process whereby light of a particular wavelength interacts with a molecule (or group of molecules) such that it is removed from the light stream. This absorbed light may add a charge or confer vibrational energy to a molecule; it may be transformed into heat and re-radiated; or it may be "trapped" and converted to chemical energy through the process of photosynthesis. I think most of you have an intuitive feel for absorption, so we won't belabor the technical description of the process. While it may seem simple enough, we could spend half a semester just on absorption, if we were learning optics.
Scattering is a process whereby the path of a particular wavelength of light is altered by a molecule (or group of molecules). Light may be scattered forward (forward scattering) or it may be scattered backwards (backscattering). The figure in your handouts gives you some idea of these processes.
I should mention two other phenomena of light that we are all familiar with, but that won't concern us too much here. These are reflection and refraction. Reflection and refraction occur at the interface between two different media, such as air and water. As light enters the air-sea interface, some of it will be reflected, i.e. some of it will bounce off and travel back through the atmosphere. Light also slows down in water; this process of slowing down causes the light to bend downward. When looking in a pond or a glass of water with a coin in it, this has the effect of making the object seem where it is not. Figure 4.8 in your book illustrates this quite nicely.
Light transmitting through the atmosphere and the oceans must contend with the absorption and scattering properties of water, particles, gases, dissolved substances, and microscopic organisms. Each of these components act to alter the course of a photon, or even transform it into something else.
Let's look at what happens to light as it enters the atmosphere first. The figure below (taken from Gross) depicts the heat budget of the Earth, but it suits our purposes for looking at what happens to sunlight as it enters the atmosphere.
First of all, sunlight entering the outermost atmosphere is absorbed by gases, namely ozone. As we have all heard over the past several years, ozone is a very good absorber of ultraviolet light. The presence of the ozone layer protects from harmful ultraviolet rays. Its destruction in the 60s and 70s caused quite an alarm among citizens and scientists. Fortunately, it appears that international controls of ozone-depleting chemicals are starting to change the ozone depletion problem. We'll come back to this issue at the end of the semester.
In addition to atmospheric gases, sunlight is absorbed by water vapor, clouds, and atmospheric dust. Adding up the sources of absorption as presented here, we can see that about 18% of the incoming solar radiation is absorbed before it hits the the Earth's surface.
Backscattering of sunlight also contributes to losses through the atmosphere. (Why isn't forward scattering included here?) Clouds, air, dust, haze and even the surface of the ocean (or land) cause backscatter. About 35% of the incoming solar radiation is reduced by backscattering. Note that clouds alone contribute 24% of the backscattering. Thus, clouds are a very important regulator of solar radiation. We will examine this further when we study the greenhouse effect.
On the whole, only about half (47%) of the sunlight that hits the outer atmosphere makes it to the Earth's surface. On average, 2.59 x 1024 Joules (another unit of energy) of solar radiation hit the Earth's surface each year (a number you do not have to remember).
Once 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. 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 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.
The euphotic zone (the "lighted waters") is probably the most important place on Earth. Because the Earth's surface is two-thirds water, most of the photosynthesis on our planet occurs in the oceans. Oceanic photosynthesis has a profound effect on the global carbon cycle and coastal fisheries. We will study the euphotic zone in more detail when we talk about phytoplankton and global productivity.
One other factor of major importance is how the different colors of light are separated as a result of their transmission through water. Due to the absorption properties of water, longer wavelengths are absorbed first. This means that red light disappears first, often within the upper few meters of the water column. Orange light is next, then yellow, green, and blue. Blue light at approximately 430 nm is absorbed the least by water. In other words, blue light of 430 nm penetrates the deepest into the ocean. This is a very interesting wavelength and I challenge any of you to tell me why! Interestingly, voilet and ultraviolet light are more strongly absorbed than blue light.
The lack of absorption of blue light by water is our next topic of discussion.
Why is the Ocean Blue?
Now we can get down to the real reason why you came to class today. To understand the color of the ocean, we first need to briefly review how we perceive color.
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 past summer in Santa Monica Bay) or red (like the summer before 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.
The absorption properties of the ocean can be described by the combined absorption coefficients of water, suspended particles, phytoplankton, detritus (zooplankton poop) and 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.
|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||Dead, 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):
atotal = awater + aparticles + aphytoplankton + a detritus + adissolved 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 graph of the absorption spectrum of pure water are provided below. (This graph extends beyond the visible wavelengths because the infrared wavelenths 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, these three components absorb maximally at the lowest wavelengths, 350 nm as shown here. 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 coefficents 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. We will lean much more about accessory pigments and photosynthesis in a couple weeks.
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?
Now who can tell me why it is so interesting that water doesn't absorb much light at 430 nm.
A Few Closing Remarks
Our discussion of the optical properties of seawater and the components suspended and living within it has taken us into the coves of physics and quantum dynamics. However, if you have paid attention, I trust you have survived. We want no captives of Calypso here‹we have many more oceans to cross.
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, 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 this last summer (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!
from The Remarkable Ocean World