Biological Oceanography: The Living Ocean
By Alex Brylske
Note: This article is the third in a series that examines the four primary disciplines of oceanography. Biological oceanography is the study of marine organisms, how those organisms interact with their environment and what controls their distribution in the ocean.
While most divers are interested in the ocean, few of us learn to dive because we are fascinated by currents, mesmerized by the chemical makeup of the seawater or enthralled by plate tectonics. Like me, the reason you probably got into diving is because of the incredible critters that live under the waves. This same interest in living things is reflected in my own oceanography students. As a college professor, none of my lectures on chemistry, physics or geology captivates their interest as much as the "biology of the sea" or biological oceanography. One of the four subdisciplines of the science of the sea, biological oceanography is, in part, the study of marine organisms. But it also includes how marine organisms interact with each other and their environment, and what controls their distribution within the ocean.
From nudibranchs to whales, much is written about the creatures. And there's even a fair amount about the interaction issue, more formally termed ecology. But rarely, be it in formal courses or popular articles, is there much, if any, discussion on what controls the distribution of organisms in the ocean. I'll grant you with all of the attention given to the big, beautiful or charismatic critters, the idea of discussing controls on distribution doesn't sound very sexy. Yet whether a local ocean region is a lifeless desert or teaming undersea metropolis is determined by these control factors; and it is because of this fundamental importance of understanding how the ocean works that we'll explore these basic concepts.
Where Does It All Start?
With a very few exceptions, all energy used by living organisms on earth is ultimately derived, either directly or indirectly, from the energy of the sun. (One notable and fascinating exception is the communities found in the deep ocean around hydrothermal vents and cold seeps of methane gas.) The question is, how does the sun's energy get into the food chain? First, not much of it does. In fact, only about one part in 2,000 of the sunlight that reaches our planet is captured by organisms we term autotrophs ("self feeders"). Organisms such as certain bacteria, algae and green plants are termed producers, and make up the base of the food chain. Using the wondrous process called photosynthesis, they disassemble carbon dioxide and water to build simple carbohydrates and other organic molecules that we call food. Just how this process is cycled is illustrated in Figure 1.
The producers are, in turn, eaten by the next rung in the food chain, the primary consumers. These are termed hetero-trophs [Heter-o-TROPHS] ("other feeders"), creatures like us who cannot produce our own food. Energy is released when food is used (disassembled) for reproduction, growth, movement or other functions in a process called respiration. Note that respiration has nothing to do with "breathing"; it occurs at the cellular level and is the opposite and complimentary process of photosynthesis. Respiration liberates the energy within the food molecules (mostly glucose), making it available for metabolism. The result of most metabolic activity is waste heat, and carbon dioxide is released as a byproduct. All organisms, including the plants that produce their own food, must carry out respiration to live. Appropriately, the heat produced by living organisms eventually flows away from earth back into space, from whence came the sunlight that started the entire process.
In going about their business, all of the free oxygen in the atmosphere and oceans eventually passes through organisms, in by respiration and out by photosynthesis. In fact, about every 2,000 years, all available oxygen on Earth is recycled this way, while it takes a mere 300 years to cycle all of the carbon dioxide. Water, too, is a component in this process, and as a result all of the water in the ocean is broken down and reformed by photosynthesis and respiration about every 2 million years.
The steps in a food chain can be viewed like the levels of a two-dimensional pyramid. At the base are the primary producers. Next come the primary consumers, who are in turn eaten by the secondary consumers, who become meals for the tertiary consumers and so on. This concept is useful in understanding how energy in matter passes through the food chain of any ecosystem; a concept termed trophic dynamics. And whether we're looking at coral reefs of the Caribbean, kelp forests of California or tidal pools of New England, the process works the same. It all starts with the food manufactured by plants and similar organisms, and thus this first step in the process is termed primary production. The concept of primary productivity is a way of telling how much life a particular area can support by giving a measure of food production by photosynthetic plants. Furthermore, food chains can be viewed as grazing (herbivores directly eating plant matter) or detrital (consumption of nonliving organic matter such as dead tissue or fecal matter).
Given the success of life on earth, it may come as a surprise that the transfer of energy from one trophic level to the next is not an efficient process. As described, most of the energy is lost as heat. Therefore, the biomass (amount of living material in a given area or volume) decreases at each point. Hence, the pyramid concept. For example, when plants are grazed by herbivores ("plant eaters") most of the energy is released as heat, motion or used in manufacturing non-nutritive tissue like shell, scales or bone. It's only the remaining fraction of energy from food that's used to increase mass. This same relationship exists at all levels of all food chains. Transfer efficiency varies between 3 and 23 percent, but averages 10 percent.
Using this 10 percent figure, let's look at a simplified food chain, as illustrated in Figure 2. At the base of our pyramid are 22,000 pounds (10,000 kilograms) of primary producers such as diatoms (a common form of phytoplankton). It takes this number of primary producers to support the first level of primary consumers, 2,200 lbs (1,000 kg) of copepods (the most common zooplankton). This number of copepods is required to support a mere 220 lbs (100 kg) of small fish. These small fish, in turn, provide the energy requirements for 22 lbs (10 kg) of larger fish. Finally, the 22 lbs (10 kg) of larger fish are needed to support the growth of just 1 kilogram of tuna or shark, the apex predator. And by the way, if we get involved in the food chain, that one kilogram of tuna will only sustain 3.5 ounces (one-tenth of a kilogram) of us. (Remember that it took 22,000 lbs [10,000 kg] of primary producers to get us here.) Of course, if the food chain is even longer, considerably greater amounts of primary production are needed to support those organisms at higher levels.
Note how the dynamics would change if we humans ate copepods rather than tuna. In this case, with only three levels to the pyramid, 22,000 lbs (10,000 kg) of diatoms would support 2,200 lbs (1,000 kg) of copepods and, in turn, 2,200 lbs (1,000 kg) of human. This is why some of the largest organisms on earth, the baleen whales, eat small organisms such as zooplankton. As their food chain contains only three levels, primary producers (phytoplankton), primary consumers (shrimp-like krill) and secondary consumers (the whales), the energy transfer is much more efficient than in a chain having several more levels.
Quantifying the Base
Understanding primary productivity is fundamental to understanding biological oceanography. As we've seen, primary productivity refers to the production of organic material (food) from inorganic compounds (carbon dioxide and water) using the energy of the sun (photosynthesis). But, in addition to what it is, we must also have a way to measure primary production, and a way to standardize the measurement across various ecosystems. This measurement is expressed in grams of carbon bound or "fixed" into organic matter per square meter of ocean's surface area per year. It's abbreviated simply as gC/m2/yr.
As land-dwellers, it's easy to assume that seaweed, which often festoons the shoreline, accounts for the greatest mass of primary productivity in the ocean. But this is a serious mistake. Fully 90 to 96 percent of oceanic carbohydrate production comes from the tiniest autotrophs, phytoplankton. These wandering single-cell "plants of the sea" account for far more productivity than the more obvious seaweeds. In fact, seaweeds account for a mere 2-5 percent of oceanic productivity. While it's difficult to determine the primary productivity of the world ocean as a whole, estimates range between 75 to 100gC/m2/year. But how does this compare with productivity on land?
Primary productivity on land and sea are about even. Marine ecosystems account for 35-50 billion metric tons of carbon bound into carbohydrates per year, while terrestrial ecosystems incorporate 52-70 billion metric tons. However, there's a big difference between total plant biomass. On land, 600-1,000 billion metric tons of living biomass is produced, while in the ocean this volume is a paltry 2 billion metric tons.
But how can the primary production be nearly equal if the land has so much more biomass (50 billion vs. 1,000 billion metric tons)? The answer is what's termed "turnover time," or how quickly the ecosystem goes through the photosynthesis-respiration cycle. On land, this recycling time is quite slow, averaging between nine and 20 years. But in the sea, the turnover time is much faster, and can be measured in months or sometimes only days. One reason for this startlingly fast turnover is that marine plants (phytoplankton and algae) do not have to invest enormous resources in stiffening structures such as woody limbs; they're supported by the very water in which they live. Imagine, for example, how sturdy a structure of a massive stalk of kelp would have to be if it were a land plant. While primary production may be a simple concept to comprehend in the context of a pasture, forest or wheat field, it's not so obvious when viewed in the context of the marine environment. This is because the most obvious organisms of the sea are animals, such as fishes, while the creatures primarily responsible for the base of the oceanic food chain are the inconspicuous, almost invisible phytoplankton (primarily diatoms and dinoflagellates).
Limits to Abundance
Given the ease with which primary productivity can occur in the ocean, and the vastness of the seas, one may wonder why oceanic productivity doesn't far exceed that on land. The reason this doesn't happen is because the conditions that optimize productivity aren't always present. Photosynthetic organisms require four ingredients to produce food. These include water, carbon dioxide, certain inorganic nutrients (especially nitrogen and phosphorus compounds) and sunlight. Water is obviously in abundance in the ocean, and the sea has more than 300 times more carbon dioxide than the atmosphere. The absence of a physical or biological necessity in an ecosystem is termed a "limiting factor." Water and carbon dioxide are never limited in the ocean but the other two factors, inorganic nutrients and sunlight, can be.
Phytoplankton require inorganic nutrients not only to construct the large organic molecules we call food, but also to construct protective shells or skeletons. Many inorganic nutrients vary in their concentration throughout the ocean, and are termed "nonconservative nutrients." These include substances such as nitrate, phosphate, iron and silicate. These nutrients, when consumed, become part of those organisms that eat them. When nonconservative nutrients are abundant, a period of rapid planktonic growth occurs. It's called a "plankton bloom," after which the surface waters are depleted of these vital nutrients.
Every diver understands how sunlight can be a limiting factor in the sea. One of the first lessons learned in an entry-level scuba course is how water can absorb light in a highly predictable way, beginning at the red end of the spectrum. As divers, we rarely concern ourselves with conditions below 130 feet (40 meters), but in most cases, sunlight sufficient to sustain photosynthesis reaches far beyond this depth. This lighted layer of water is called the "photic zone." In tropical oceans, this zone can extend to a depth of 660 feet (200 meters) or more. In midlatitudes, due to more plankton and other small scattering particles, the zone typically reaches no more than 330 ft (100 m) The upper part of the photic zone is where light is sufficient for photosynthesis to exceed loss of carbohydrates through respiration (more is produced than consumed). This is called the euphotic zone ("eu" means good). Amazingly, this vital zone constitutes a mere 2 percent of the ocean's volume. In the deeper part of the zone, sufficient light may be available for organisms to see, but insufficient for photosynthesis. This is termed the disphotic zone ("dis" equals difficult). Below the disphotic zone lies the vast majority of the open ocean, a perpetually dark region where no light penetrates. This is called the aphotic zone ("a" means without). So, unlike land where access to sunlight is rarely a problem for most primary producers, in the sea sunlight is confined to the relatively shallow 2 percent of the water column near the surface.
Yet problems don't end with the scarcity of light. Once the phytoplankton and the zooplankton that feeds upon it die, their remains begin to sink and fall below the euphotic zone. And as dead plankton and other small organisms begin to sink, an array of shredders and decomposers begin to turn their tissue into inorganic nutrients. If this inorganic matter sinks below the euphotic zone, it will no longer be available to the ecosystem. This doesn't present a problem near the coast because in these shallow waters there's no aphotic zone, sunlight can normally penetrate all the way to the bottom. But in open ocean, whether a region is highly productive or an oceanic wasteland depends on whether these vital nutrients can be returned to the euphotic zone through physical oceanographic processes. (The primary process is through upwelling, a phenomenon discussed in last month's article on physical oceanography.) And exactly where the ocean is most productive may surprise you.
Understanding oceanic productivity is one of the most important questions in biological oceanography. As photosynthesis by phytoplankton is the base of most marine food webs, such productivity determines the biological character of entire regions of the ocean. Because of the availability of nutrients from runoff and river outfalls along the coast, productivity is highest near shore. In fact, in some regions it can reach an exceptional rate of more than 400 gC/m2/yr. But the situation is far different in the open sea. Here's where the phenomenon is not what you might expect.
The most productive ecosystems on earth, which is no surprise to most divers, are coral reefs. So one might logically conclude that to support such prolific communities, tropical ocean productivity must be sea's highest. But in reality, tropical oceans are so devoid of productivity that they're often termed "oceanic deserts" by biological oceanographers. It's for this very reason we divers are so attracted to these regions, the amazing clarity of the water. Just like the water from your tap, tropical oceans are clear because they contain very little else but water. By comparison, tropical oceans rarely exceed a productivity of 30 gC/m2/yr. Why is this?
Because of the intense and constant heating of the sea in the tropical regions, there is a deep thermocline. Under normal conditions, this warm-water layer extends well down through the euphotic zone. Due to differences in density, the cold nutrient-laden water below the thermocline never mixes with the warm sun-lit layer near the surface. Even the enormous energy of hurricanes is insufficient to mix these two water masses, so unlike in temperate oceans, the thermocline is a permanent feature. This lack of mixing makes high productivity in tropical oceans all but impossible (except in certain unusual upwelling regions such as the tropical eastern Pacific).
If the tropics is not the champion of productivity, then what about the polar seas? Here, too, the answer may surprise you. Because of the extreme cold characteristic of polar regions, there is no thermocline, the temperature throughout the water column is more or less consistent. This enables easy mixing of the deeper nutrient-laden water into the euphotic zone. And, indeed, in regions like those around Antarctica, primary productivity can be exceedingly high, sometimes more than 250 gC/m2/yr. That's more than eight times the rate of tropical seas. But the problem is that this occurs only during the short 24-hour daylight period of summer. Throughout most of the year the sun angle is too low to penetrate the sea surface very far, or the surface is ice-covered, so the availability of light for photosynthesis is highly limited. While plankton blooms are nothing short of massive in the Southern Ocean, they're less prolific in the Arctic because less upwelling occurs.
As tropical seas cannot support high levels of productivity, and polar regions are able to support high productivity only for very short periods, the overall productivity prize must go to the temperate oceans of the world. The reason for this is the ability for cyclical mixing of deep nutrient-laden water into the euphotic zone. The phenomenon by which this mixing occurs may already be familiar to you, you just didn't realize it. If you live in a temperate region you've undoubtedly experienced the summer phenomenon of an abrupt thermocline. As fall approaches, the water begins to cool and the thermocline starts to recede. By winter, the thermocline is gone, and the water column is of a consistent temperature, facilitating mixing. Winter is also a time of severe storm activity which helps to mix the deeper nutrient-laden water into the photic zone. As spring arrives with its longer days, the conditions are now ideal for productivity (lots of light and nutrients), and the classic "plankton bloom" occurs.
As Figure 3 shows, there are normally two peaks of productivity in temperate regions. The first occurs in the spring when conditions are ideal, and a somewhat smaller peak in the fall after the demise of the summertime thermocline. Typical productivity in the temperate zone averages about 120 gC/m2/yr. Note that this is less than one-half of the most productive polar oceans during the height of summer. But, instead of being restricted to a few summer months, conditions in temperate oceans enable modest productivity throughout much of the year. Figure 4 gives an overview of productivity throughout the world ocean. Note how the highest levels of productivity occur, as explained last month, near upwelling regions or around coral reefs. (See the sidebar for how coral reefs contribute to the productivity equation.)
So, as you can see, understanding the biology of the ocean isn't possible unless you also have some insight into its chemistry and physics. And in the next and final installment, we'll explore the contributions that geologists have made in explaining how our Blue Planet functions.
As explained in the main article, tropical oceans are so clear because they're very low in productivity. However, on a coral reef the productivity can range between 30 and 250 times more than the surrounding ocean, often producing as much as 1,500-5,000 gC/m2/yr. This represents some of the highest rates of primary production in any natural ecosystem on Earth. This seeming violation of the laws of thermodynamics, high productivity in a low-productivity environment, is sometimes called "Darwin's Paradox" and puzzled scientists until the 1950s. Although the reason is quite complex and not completely understood, the basic answer lies in the fact that corals and coral communities are extremely efficient at recycling nutrients (nitrate and phosphate). So, the nutrients that do make their way on to the reef tend to be held there very tightly and continually recycled.
Another reason for this paradox is a neat little evolutionary arrangement worked out between some corals and single-celled algae (dinoflagellates). Instead of being swept away by the current, as would happen if they were planktonic, these specialized algae called zooxanthellae (or "zoox" for short) actually live within the tissues of the coral polyps. A few other organisms besides coral also harbor zoox. These algae, in essence, enable a coral colony to function as a combination plant and animal. The zoox produce food via photosynthesis, while the polyp catches plankton from the water column. Moreover, the byproducts of the algae (oxygen and sugars) are consumed by the polyp, whose byproducts (carbon dioxide and nitrogenous waste) are consumed by the zoox. Neat trick, huh?