The Diver's Guide to Oceanography, Part One
By Alex Brylske
Although he's most famous for his novel, 2001: A Space Odyssey, Arthur C. Clarke is also an avid ocean explorer and marine conservationist. In fact, my favorite Clarke quote has nothing to do with space. He once remarked, "How odd that we call our home planet Earth when it is so obviously Ocean."
Advocating to divers that the ocean is the primary feature on Earth is, of course, preaching to the choir. It takes very few underwater experiences to come away understanding the true meaning of Clarke's words. But it's also interesting to note that the love affair divers have isn't really with the ocean itself, but the creatures that live within it. After all, the ocean is just water, the most abundant substance on the face of the Earth. While it serves as the medium that houses the things we hold so dear, as a subject itself, how important or interesting can the subject of sea water be? The answer may surprise you. Life on Earth exists largely because of the special properties of water and the ocean it comprises.
Let's begin by dispelling some misunderstandings about the field of oceanography. Most folks, including the vast majority of divers, assume that the terms oceanography and marine biology are synonymous. But that's a mistake. Indeed, the two fields are related, but oceanography is far more than biology. Traditionally, oceanography, literally, the study of the ocean, is divided into four sub-disciplines that explore the biology, chemistry, physics and geology of the sea. Biological oceanography studies marine organisms, their interaction with their environment and the controls on their distribution throughout the ocean. Physical oceanography looks at ocean currents, air-sea interactions, waves, tides and global water circulation. Geological oceanography (which includes geophysical oceanography) studies the shape and structure of the ocean basins, how these basins form and evolve, and how sediments accumulate. Finally, chemical oceanography is the study of sea-water chemistry.
While you certainly don't have to be a scientist to be a diver, an elementary understanding of basic principles of oceanography can make the diving experience far more rewarding. This article is the first of a series in which, over the next year, I'll be exploring the four disciplines of oceanography from the perspective of a diver. As most divers need little incentive to explore the biological aspects of the ocean, I'll start with the often overlooked, and assumed uninteresting, examination of one of the most important and fascinating aspects of the sea, its chemistry.
Looking at Earth from the Sea
If humans had evolved in the sea, our perspective would certainly be very different. The sad fact is that a landlubber's outlook robs us of really understanding the vastness and importance of the ocean that Arthur Clarke so eloquently captured in his commentary. But one fact that's become common knowledge even among those who have never felt a seabreeze is that 71 percent of the Earth's surface is ocean. (That's 139 million square miles/360,000,000 sq. km, by the way.) Yet in reality, the facts are much more compelling than this often-quoted factoid of "more than two-thirds of the Earth is water." The world ocean (I use the singular because all of what we term individual oceans are interconnected and function as a single system) provides over 99 percent of all the habitable space on our planet. That's right, over 99 percent. This is a tough concept to visualize for us land-dwelling critters, but a useful point to consider when considering the importance of the ocean.
While still on the subject of amazing facts, the average depth of the world's ocean is 12,230 feet/3,730 m, and it reaches its deepest point, 35,802 feet/10,883 m, in the northwestern Pacific at a site known as the Challenger Deep region of the Marianas trench. That means that if Mount Everest sat on the bottom of the Challenger Deep, it would still be covered by more than 8,000 feet/2,400 m of water. Incidentally, the pressure at the bottom is an astounding 8 tons per square inch, in other words, like one person trying to support 50 jumbo jets.
In terms of total water quantities, the world's ocean holds an unimaginable volume of over 285 million cubic miles or 1.18 trillion cubic kilometers of the precious liquid. But where did it all
come from? The traditional view is that all of Earth's water was liberated from rocks, or more accurately, from the volcanism of Earth's molten interior as it cooled. More recently, however, some have speculated that the Earth may have more water than this mechanism can account for, and that much, if not most, of Earth's water is extraterrestrial in origin. It may have literally dropped out of the sky in the form of comets (the major constituent of which is ice).
From microscopic algae and plankton to whales, the world's ocean is our planet's largest repository of life and includes representatives from all phyla. For example, just one spoonful of ocean water can contain nearly a million bacteria cells. Furthermore, ocean-dwelling organisms are subject to the properties of the sea water which surrounds them, and water composes the greater bulk of their body mass. Therefore, to understand sea life, it's necessary to understand some of the fundamental chemical reactions taking place both within and outside of their bodies.
The Chemistry of the Sea
The compound we call water is a very simple substance. Its formula, H2O, is probably the most recognized chemical symbol in existence. As the formula describes, each molecule is composed of two hydrogen atoms and one oxygen atom. But it's the way this bonding occurs that makes water the very unique and vital stuff of life. The hydrogen atoms bond to the oxygen atom by what's termed "covalent bonding," another way of saying that it shares its electrons with its oxygen neighbor. In this case, the oxygen atom completes its outer shell by sharing with two single-electron hydrogen neighbors. This stable relationship makes water a very stable molecule. But there's nothing especially unusual about water in this respect; many substances are bound by sharing electrons. There's a much grander story to be told.
Because oxygen is a bigger atom than hydrogen, it tends to hog hydrogen's electron, drawing it closer to it than the hydrogen nucleus. Because the electron is negatively charged and it orbits closer to the oxygen than the hydrogen, this tends to give the oxygen side of the molecule more negative energy, or a slightly negative charge. The nucleus of an atom contains positively charged protons, but their effect is more or less balanced by the negatively charged electrons orbiting the nucleus. But remember, the oxygen is hogging hydrogen's lonely electron, so the positively charged nucleus can exercise greater influence on its side of the water molecule. This is depicted in Figure 1.
Because of all this unequal electron sharing, each water molecule has a dual positive and negative charge, somewhat like a battery. Thus, it's referred to as a "polar" molecule. In turn, the positively charged hydrogen end can attract the negatively charged oxygen end of adjacent water molecules, as depicted in Figure 2. These attractive forces create what's termed "hydrogen bonds" between water molecules. But these bonds are weak compared to the electron sharing bonds, only 6 percent as strong, and are easily broken and re-formed.
While this may appear on the surface (no pun intended) to be of little consequence, its simplicity belies its true significance. In fact, there's probably no more important aspect of chemistry on our planet than the polar nature of the water molecule. It is these hydrogen bonds that are responsible for many of the unique characteristics and physical properties of water, giving these infinitesimally weak bonds enormous impact on the organisms that inhabit the Blue Planet.
As evidence of the vital role played by hydrogen bonding, and the polarity of water molecules in physical and chemical processes, consider the following: If water lacked these characteristics, it would be a gas at room temperature. In other words, life as we know it would be impossible.
How "Thick" Is Water?
Hydrogen bonds influence the "thickness," or the viscosity, of water, and that in turn influences how water moves and how organisms move through it. Furthermore, temperature is a major factor influencing water's viscosity, because more hydrogen bonds can form in cold water (less energy to separate them). For example, a 68?F/20?C decrease in temperature doubles water's viscosity. But why is this so important? Well, for example, in cold (highly viscous) water, drifting plankton need expend little energy to prevent sinking, while swimming organisms must expend much more energy to move through cold versus warm water.
Furthermore, on the surface of the ocean, the polar nature of water allows it to form what could be viewed as a sticky "skin" that is strong enough to support objects as large as insects. This phenomenon is known as surface tension, and water has the highest surface tension of all common liquids. The colder the water, the higher its surface tension. This can't be important, you say. But it can. An entire marine community, termed the neuston, lives on and in this skin surface. The neuston includes organisms such as bacteria, protozoa, fish eggs, copepods and even some floating jellyfish.
The air/water boundary layer of the ocean is also under intense investigation in an attempt to better understand how fast the ocean "breathes" (takes up atmospheric carbon dioxide and liberates oxygen). In addition, many researchers are investigating how pollutants affect the neuston, as well as the effects of ultraviolet light on plankton.
How Dense Is Water?
Viscosity is not the same thing as density. Viscosity describes the degree of internal friction of a liquid. Density is a measure of the mass per volume of an object, and in terms of water, ignores hydrogen bonding. Density, however, is of vital importance to marine organisms because, living in water, they don't require extensive supportive structures, such as the massive bones or strong trunks of land animals and plants. As most divers learn in their training, water is 800 times more dense than air, which provides quite a lot of support if you can figure out a way to live in it.
This is why creatures like whales can survive, even swim gracefully at sea, yet could not survive on land because they would be crushed by their own weight. This even applies to plants. Giant kelp (Macrocystis pyrifera), for example, which can reach lengths of 200 feet/60 m, does not have, or need, the extensive root systems and sturdy wooden structure of a tree. It is supported entirely by numerous air-filled sacs.
Water's density is also a function of temperature. As water cools, it forms more hydrogen bonds due to the slowing of molecular movement. Additionally, water molecules move closer together, which increases its density. Pure water actually reaches its maximum density when it cools to 37.8?F/4? C. But amazingly, if it cools further, its density begins to decrease (it expands). As water freezes, the bulkier hydrogen-bonded molecules assume a latticelike structure (crystallization) that pushes the molecules farther apart, increasing volume.
This makes water a very unusual substance, because it's actually less dense in its solid form (ice) than as a liquid. This is also a fortunate phenomenon for life on Earth because it means that ice floats. If water were a more normal substance, where the liquid state is less dense than the solid, ice would sink. This would lock up most of the water on Earth in a permanent mass of ice, which would make ice ages look like nothing more than the dregs in a cooler at the end of a hot day.
The expansion of water when it freezes also has some important biological implications. Most notably, organisms inhabiting polar regions must prevent ice crystals from forming within their cells and body fluids. If ice were to form, it would rupture their internal structures, resulting in death. Why doesn't this happen? Again, Mother Nature to the rescue. Many species of fish that live in polar regions have "antifreeze" (a glycoprotein) in their blood, which lowers the freezing point of their internal fluids. Invertebrates deal with the problem in a different but no less effective way. They increase the salt content within their tissues, thus lowering the freezing point.
The Importance of a Stable Home Life
A primary difference between living in the sea versus on land is the sea's highly stable temperature. Compared with land-based environments, where temperature extremes can vary tens of degrees over the course of a single day, sea temperatures are quite stable. This moderate nature is because water has such a high ability to resist rapid temperature changes, what's termed "heat capacity." The reason for this, once again, is those trusty hydrogen bonds between molecules.
Understanding the phenomenon involves something you might have already learned in advanced diving courses. Heat is nothing more than a measure of molecular motion. So if water temperature increases, it just means that the molecules move faster. But for those molecules to move faster, energy is needed to first break hydrogen bonds. Conversely, for water temperature to cool, a great deal of heat must be removed to allow hydrogen bonds to re-form, thereby slowing molecular motion. This phenomenon is also where we get the concept of a "calorie." A calorie, unlike what you might have thought, has nothing to do with food or fat. It's merely a measure of energy. Specifically, it's the amount of energy needed for 1 gram of water to be changed 1 degree Celsius.
Interestingly, and of great consequence to life in the sea, much more heat is required to change ice to liquid water than to change the temperature of liquid water. This amount of energy needed to change water from solid to liquid is known as "latent heat of fusion." For example, converting 1 gram of water at 0?C to ice at 0?C requires that not one but 80 calories be removed. To melt 1 gram of ice at 0?C, 80 calories must be added. Note that the 80 calories are absorbed or released by the water but do not change its temperature.
The practical consequence of this phenomenon has both an environmental and a biological implication. In terms of the sea, because of the latent heat of fusion, the oceans cool gradually during the winter. Biologically, latent heat of fusion lessens the possibility of freezing within the bodies of marine organisms. In essence, water within the tissues of plants and animals acts as a heat reservoir.
Another question that arises when considering the stability of the world's ocean: With such a large quantity of incoming solar radiation, why doesn't ocean temperature continue to rise? The reason is that, while much heat is absorbed, a great deal is also removed through evaporative cooling at the surface. Evaporation releases heat and water vapor into the atmosphere. (Any diver who has ever stood shivering in the wind in wet dive attire knows firsthand how cooling occurs due to evaporation.)
The amount of heat lost during evaporation is termed "latent heat of vaporization" and is much greater than the latent heat of fusion. For example, latent heat of vaporization liberates 595 calories per gram of water evaporating from the ocean at 0?C (less as the water warms because of the decreasing number of hydrogen bonds). Evaporative cooling is also an important process for intertidal marine organisms, which are at times exposed to air.
Finally, heat is transferred within the ocean primarily by convective currents caused by uneven temperatures (which changes the density of the water mass) and even the influence of gravity. But that's a subject for a later article on physical oceanography.
The Universal Solvent
The term universal solvent applies quite well to water, and again this is because of the polar nature of the water molecule. A good example, illustrated in Figure 3, is how salt dissolves in water. In this case, crystals of salt or sodium chloride (NaCl) interact with the charged molecules of water. The water molecules, because of their polar characteristics, act like tiny magnets and pull apart ("dissociate" is the formal term) each crystal. The dissociated salt crystals become charged particles, or ions of sodium and chloride. Substances that do not ionize may still dissolve in water, but by other mechanisms which are beyond the scope of this discussion.
Why Is the Sea So Salty?
The creatures that inhabit the world's ocean swim in a substance that contains almost every known, naturally occurring element (and since the explosion of atomic weapons, some unnaturally occurring elements). But these elements do not occur in equal or even consistent amounts. The most abundant materials, termed "mineral salts," dissolved in sea water are listed in Figure 4. These elements behave in a very interesting and predictable way, they always maintain the same proportion to one another. Thus, if one knows the proportion of one, the others can be easily calculated. This constancy of composition in sea water is sometimes termed "the first law of chemical oceanography."
While this may or may not be of interest to you, it nonetheless has great implications because it affects many different chemical, physical and biological processes in the ocean. But if land-derived materials are continually emptying into the sea, why doesn't the sea's salinity increase continually? The answer is that the input of salts is balanced by the removal of salts by various chemical, physical and biological processes.
OK, even if the salts remain constant, how do they get there in the first place? One way is through river discharge. In this case, rivers flowing into the ocean add salts, as well as other minerals, that are dissolved from the soil when rain percolates through the ground, to sea water.
The other input is from water circulating through hydrothermal vents (hot springs) of the mid-ocean ridges (which we'll discuss in a later article on the geology of the world's ocean). Here, minerals are added to sea water as it flows through the volcanically heated rocks near the ridges. That may sound like a rather unimportant source until one realizes that all the ocean's water cycles through these hot springs every 8 to 12 million years.
River discharge and hot spring cycling work together to create the salinity of sea water. The hot springs also chemically change sea water by adding some materials while removing others. Thus, over time, the addition of dissolved materials to the ocean is balanced by the removal of dissolved materials, one of the best balancing acts on Earth.
There are many elements within sea water that account for only a tiny portion of its total contents but are critical to life in the sea (and to us). These elements include phosphates, nitrates and silicon. The first two are vital for the life functions of all living creatures, while the latter is required by some plankton to construct their glass skeletons. These elements, however, do not behave like those discussed previously.
Nitrates and phosphates vary in concentration due to biological activity. In some marine communities, where algae and other plants are active in the process of photosynthesis, the nitrates and phosphates can be in short supply. When this happens, the amount of biological activity that can take place is limited. Yet some marine ecosystems, most notably coral reefs, have evolved to exist in low-nutrient conditions, and introduction of high nutrient levels (eutrophication) can spell disaster.
As anyone who has ever been in the ocean off the west coast of North America and then made a dive in some tropical location can attest, the sea is not uniformly salty. But how can that be if the major ions within sea water always maintain essentially the same proportions? The answer is, once again, water itself. Salinity variation results from differences in the local rates of evaporation and precipitation over the ocean, and from the volume of fresh water discharged into a particular ocean basin from rivers. So the addition of fresh water by rivers, or removal of water by evaporation, leaves the remaining salts either more or less concentrated. For example, evaporation rates are high in the sunny tropics, thus tropical waters tend to be much saltier than water in temperate regions.
Such regional variation can be seen in Figure 4. As you see, the salinity of the Red Sea of 40 parts per thousand (ppt), the Mediterranean Sea at 38 ppt and the mid-North Atlantic at 37 ppt is high due to either or both a lack of rainfall and high rate of evaporation. Conversely, the Black Sea at 18 ppt and the Baltic Sea at a mere 8 ppt are regions with large amounts of freshwater inflow and a low rate of evaporation. On average, the salinity of the world's ocean is 34.7 ppt, which usually occurs in the center of the large ocean basins, far from the effects of river discharge. So generally, salinity is more uniform in the open ocean but much more variable in coastal waters.
Salt content of the internal fluids of marine organisms must be in balance or controlled with respect to the external salinity. For example, bony marine fish face the problem of maintaining an internal salinity of about 1.5 percent in water that is about 3.5 percent, and have evolved an efficient system for this purpose (termed osmoregulation). Sharks, by contrast, deal with the problem in an entirely different way. Through production of certain chemicals, they actually balance their internal tissues to the salinity of the surrounding water.
Marine creatures that cannot readily control their internal salt content are subjected to great stress when changes occur in the external salt concentration. It follows that organisms which are best at enduring salinity changes, because they're good at regulating their internal salt concentration, are those that live in estuaries.
Critters that can tolerate large salinity fluctuations are called "euryhaline" organisms; those organisms that cannot tolerate large salinity changes are termed "stenohaline." The way coastal-dwelling stenohaline organisms deal with salinity changes is by continual migration to remain within a water mass of constant salinity. As a general rule, marine organisms in the open ocean, where salinity remains relatively constant, are stenohaline.
Salinity not only varies regionally, but within the same water mass with depth. You can see this in Figure 5, which shows the relationship between water depth and salinity. Due to waves, wind and tides, the surface layer of water is usually well-mixed. This means that within this surface layer, salinity is generally uniform over time. (Surface waters can, of course, show seasonal changes due to the effects of rainfall, evaporation and other weather-related phenomenon.) Beneath the surface water is a zone called the "halocline." This is characterized by a large and rapid change in salinity with depth. Below the halocline, the salinity of deep water is very consistent throughout the year.
Another phenomenon affecting salinity that may not seem obvious is freezing and thawing. Freezing increases salinity, because as ice forms, most dissolved salts are excluded from the growing crystals, remaining in the unfrozen water surrounding the ice. This makes unfrozen water around the sea ice more concentrated (dense). Conversely, when the ice thaws, the fresh water released dilutes the salt concentration of the surrounding sea water. The denser water also sinks and, as we'll see in a later article, has important implications to ocean circulation.
I can't think of a subject in high school that I hated more than chemistry. What turned my dread into fascination was understanding how fundamentally important chemistry is to the sea, and how different life would be, if it could be at all, if it weren't for the wondrous properties of water. I don't know about you, but this topic is making me thirsty.