Everything we have studied to this point, especially the geological forces that shaped the oceans, the chemical forces that control the composition of seawater, the physical forces that create structure in the water column, and the optical properties that define the depth of the euphotic zone all shape the structure and functioning of our next topic -- oceanic food webs. Through our knowledge of the geological, chemical, physical, and optical processes that occur in the oceans, we can gain an understanding of the interdependent and powerful nature of life in the sea. Oceanic food webs are, perhaps, the most intricate and complex food webs on our planet. The flow of energy and the cycling of elements through these systems govern many of the global processes on our planet, including the carbon cycle, the nitrogen cycle, the sulfur cycle, and, most importantly, the availability of oxygen in our atmosphere. While our examination of these vastly important biological systems will by necessity be brief, you should nonetheless begin to appreciate the way in which these biological systems are interwoven with all the other processes that occur in the oceans and on our planet.
The Structure of Marine Ecosystems
Prior to looking at the groups of organisms that participate in oceanic food webs, it is useful to define a few ecological terms that will help guide us in our study. These definitions provide the foundation of modern day ecology into which we delve as we examine oceanic food webs.
When we speak of any assemblage of organisms and the physical, chemical, geological, and biological factors that govern their numbers, we are speaking about an ecosystem. The community of organisms and the nonliving environment with which they interact is called an ecosystem. A community is defined as all the populations of organisms inhabiting an area. In turn, a population is defined as all the individuals of a single species.
Note that these designations provide a convenient framework for understanding complex and interdependent processes. In reality, we cannot separate our planet into distinct "ecosystems" because, in fact, everything on our planet interacts with everything else. You should recognize this idea as the fundamental premise of the Gaia hypothesis.
Despite the holistic nature of life on our planet, it is, nonetheless, convenient and easier to study biological processes if we look at smaller parts of the planetary system. Thus, we define oceanic ecosystems according to where they occur or the types of organisms that occur in them. For example, we can define the upwelling ecosystem, the coastal ecosystem, the pelagic ecosystem, the benthic ecosystem, the rocky intertidal ecosystem, the estuarine ecosystem, the hydrothermal vent ecosystem, and so on.
We must also be aware that ecosystems function in a larger sense than the populations of organisms that inhabit them. Ecosystems function to convert energy from one form to another, such as converting the radiant energy of sunlight to the chemical energy of sugars and other compounds in phytoplankton or algae. In turn, this chemical energy is consumed by other organisms who are themselves consumed. At each transformation, i.e. at each point where one organism consumes another, there is a loss of energy (usually as heat), because energy transfer is not 100% efficient. As a result, energy flows through ecosystems in a one way manner, from the sun (or chemicals found in a hydrothermal vent) through the organisms and eventually back out into outer space as heat.
The first law of thermodynamics states that energy is neither created nor destroyed. Energy changes from one form to another (i.e. from sunlight to chemical energy to heat), but eventually all the energy inputs (from the sun) must equal all the energy outputs (as heat). Secondly, the second law of thermodynamics states that when energy is converted from one form to another, some of its ability to perform useful work is lost (better known as entropy). Thus, the transfer of energy from phytoplankton to zooplankton can never be 100% efficient. None of the energy "transformations" shown (i.e. one organism eating another) are 100% efficient, due to the second law of thermodynamics.
Ecosystems also function as storage places for various chemical elements. As all organisms are composed of carbon, nitrogen, oxygen, sulfur, and other elements, these chemicals are moved from seawater to an organism (such as phytoplankton, algae, or bacteria), and then from one organism to another as each is consumed in succession. At each step, some compounds are lost through excretion, feces, or death of the organism. In this form, many elements are returned to their nonliving inorganic form as bacteria decompose the compounds in which they were constituted. Once returned to seawater or sediments, these elements become available to organisms, thus starting the cycle of elements all over again. As a result, elements (or matter) cycles through ecosystems and continues to cycle through ecosystems until lost permanently, either by conversion to a permanent form (such as petrified wood or limestone) or by subduction into the interior of the earth.
Autotrophs, the primary producers, form the base of all oceanic communities. Autotrophs are organisms who manufacture their own food using either radiant or chemical energy. Autotrophic organisms who depend on sunlight are called photoautotrophs. Autotrophic organisms who use chemical energy are called chemoautotrophs. Photoautotrophs include the phytoplankton, the cyanobacteria, and the macroalgae. Chemoautotrophs include the bacteria present in the tissues of vent organisms. All autotrophs are known as primary producers because they form the link between the living world and the nonliving world. Through the actions of primary producers, once nonliving matter and nonliving energy are transformed into the cellular material and chemical bonds of living organisms.
Organisms that feed on autotrophs are known as heterotrophs. These organisms must eat or digest other organisms to obtain energy and materials. All organisms other the autotrophs are heterotrophs. Organisms that feed directly on autotrophs are known as herbivores. Organisms that eat other organisms are known as consumers or carnivores or predators. Depending on how far up the food chain they feed, these heterotrophs may be primary consumers (feeding directly on autotrophs), secondary consumers (feeding on organisms that feed on autotrophs), or tertiary (or higher) consumers (feeding on lower levels of consumers).
To complete our discussion of the components of an oceanic community, we must mention the heterotrophic bacteria, who absorb the organic compounds produced by phytoplankton that become dissolved in seawater as a result of "sloppy" feeding or by leakage from phytoplankton cells. Heterotrophic bacteria that consume dissolved organic matter are known as secondary producers. These secondary producers may be eaten by primary consumers known as bacteriovores. These bacteriovores include many of the jellyplankton, as we discussed previously, and many species of marine protozoans, such as the microflagellates, ciliates, foraminifera, and radiolaria.
Finally, I want to stress two key points. First, all heterotrophic organisms depend on autotrophic organisms, ultimately, to obtain the energy and materials needed for their growth and reproduction. Second, any physical, chemical, geological, or biological factor that affects the growth and reproduction of the autotrophs eventually affects the growth and reproduction of the heterotrophs. Keep this in mind as we discuss the effects of nonliving factors on oceanic food webs.
Trophic Levels and Ecological Efficiency
The position that a particular organisms occupies in a food web (or food chain) is known as its trophic level. The word "trophic" means feeding, so a trophic level describes the "feeding" level of an organism. Trophic levels are important because they distinguish various components of the food web and determine the availability of energy and materials at any level. This flow of energy and materials can further be defined by the trophic pathway, that is, the route that a particular parcel of sunlight or chunk of chemical elements takes on its way up the food web. Finally, at each step in the food web, there is a certain efficiency at which energy and material are transferred from one level to the next. The efficiency at which energy and materials are transferred from one trophic level to the next is known as the ecological efficiency.
Let's take a closer look at what all this means. Typical food webs can be described by the amount of organisms, or biomass, present at each level. Food webs usually place the autotrophs at the bottom and the predators at the top in a depiction called a trophic pyramid. Each level of the pyramid is known as the trophic level. Thus, primary producers occupy the first trophic level, zooplankton herbivores occupy the second trophic level, carnivorous zooplankton occupy a third trophic level, fish that feed on them may occupy a fourth trophic level, and fish or birds of whales that feed on them might occupy a fifth or sixth trophic level.
In a typical coastal ocean ecosystem, the biomass of the primary producers is greater than the biomass of all other organisms. Thus, primary producers form the base of the trophic pyramid. Primary consumers, the herbivorous zooplankton that feed on the phytoplankton, have a biomass that is typically no more than 10% of the phytoplankton biomass. That's because the ecological efficiency for oceanic food webs (and many terrestrial food webs) is about 10%. The end result is that nearly 90% of the energy contained within one level is lost as it is consumed by the succeeding level. Another way to think of this is that is takes 100 pounds of grass to make 10 pounds of cow to make 1 pound of human. Energy is lost at each step. For this reason, the primary consumers make up a smaller chunk of the pyramid. The process of stacking consumer on consumer and depicting their biomass as a chunk of the pyramid continues until all organisms in the food web have been accounted for. A trophic pyramid provides useful information on the distribution of living organic material (i.e. biomass) in an ecosystem at any one time.
Organisms that feed on lower trophic levels gain much more in terms of energy. Making an organism at the sixth trophic level requires considerable more energy than making an organism at the second or third trophic level. That's because every transfer of energy means a loss of energy. Thus, organisms that feed on lower trophic levels can exploit far greater resources, in terms or energy. It should also be obvious that in most instances the biomass of lower trophic levels is greater than the biomass of higher trophic levels. Thus, from a materials standpoint, there are advantages to feeding at lower trophic levels.
The Classical and Microbial Food Webs
The food web that most of us are familiar with and that gets the most press in oceanography books is the classical food loop, the phytoplankton-zooplankton-fish pathway. The classical food web of oceanic ecosystems is based on historical plankton research that made us of nets to capture phytoplankton and zooplankton. These nets were often quite "porous", in that particles smaller than about 0.03 to 0.07 millimeters in size, slipped through. Anything caught in the net, typically large diatoms and dinoflagellates, chain-forming phytoplankton, copepods, marine invertebrate larvae, and other larger forms of zooplankton. is known as net plankton. For centuries, our knowledge of oceanic food webs was based on studies of organisms caught in plankton nets.
Improvements in technology radically changed our view of oceanic food webs. The application of electron microscopy, new filtering techniques for removing small particles, and better methods of measuring plankton productivity led to the "discovery" of ultraplankton in the mid-1970s. These plankton may account for 50-70% of all the photosynthetic activity in the world ocean. Without question, a new oceanic world came into focus for oceanographers, much in the same way that astronomers have been afforded a new view of the Universe as a result of the Hubble telescope.
Diatoms and dinoflagellates, the "classical" autotrophs, occupy a small portion of the entire picture. They are eaten by zooplankton who provide food for larger zooplankton, fish, and other top predators (not shown). These organisms comprise the "classical" food web. The microbial loop introduces two different sizes of autotrophs, the nannophytoplankton, small flagellated blobs of chlorophyll, and the ultraplankton, the photosynthetic cyanobacteria. These autotrophs are eaten by heterotrophic flagellates, who capture their prey by engulfing them like an amoebae, and ciliates, who filter their prey from the water by beating their cilia. The flagellates are in a size range similar to nannophytoplankton while the ciliates are larger, on the same scale as diatoms and dinoflagellates. These organisms also feed on heterotrophic bacteria, who consume dissolved and particulate organic matter.
Perhaps the most important part of the microbial loop is how it produces and uses dissolved and particulate organic matter. As a result of "sloppy" feeding by herbivorous zooplankton, and leakage of dissolved organic matter from phytoplankton cells, organic matter is released into the water column. Dissolved organic matter is also released through the excretion of flagellates and ciliates. Heterotrophic bacteria can absorb this material and utilize it to grow. As a result, the bacterioplankton, the free-living bacteria in the water column, help to recycle elements lost from the food web back into the food web through the microbial loop. Thus, the microbial loop functions to improve the efficiency of energy use, by taking advantage of the energy available in organic matter, and to conserve vital elements by cycling them in a highly efficient manner.
The microbial loop is also critical for maintaining primary productivity, especially when the availability of nutrients is low. When flagellates and ciliates excrete, they release ammonia, which contains nitrogen). Because nitrogen is required for plant growth (making it a biologically important nutrient), phytoplankton can sustain photosynthesis. The rate of photosynthesis is determined by the rate of excretion of ammonia. This type of growth, where photosynthesis is controlled by the availability of nutrients, is known as nutrient-limited growth. As we shall see below, the microbial loop plays an important part in the seasonal cycle of oceanic food webs, providing an alternate pathway for energy and material flow when the classical food web is "shut down" by physical and chemical factors.
The Fate of Primary Productivity
As emphasized in our previous lecture, autotrophic organisms form the base of the food web. It is from these organisms that all of the energy available to ocean ecosystems is derived. Without autotrophs converting nonliving substances, such as sunlight and chemical compounds, into living substances, none of us would be here. The autotrophs determine how much energy will be available to a food web and they also determine the kinds of elements that are available. Within this context it is useful to re-examine the energetic requirements of autotrophs, in particular, the phytoplankton.
You will recall that phytoplankton use the radiant energy of the sun to create organic compounds from carbon dioxide in a process known as photosynthesis. All of the photosynthesis that occurs in the world ocean is known as gross primary production, or GPP. This quantity is the total amount of carbon fixed per unit area of ocean. The GPP also represents the total amount of chemical energy produced by phytoplankton.
Because they are living organisms, phytoplankton also require energy to grow and reproduce and perform all the metabolic functions that phytoplankton perform. To accomplish this, they must use some of the energy that they produced. In using this energy, they require oxygen, so that phytoplankton, like you and me and every other living thing on this planet, respire. (The rate at which organisms use oxygen is known as the respiration rate, or R). Fortunately, phytoplankton produce much more oxygen than they use, so there is plenty left over for the rest of us. The amount of fixed carbon left over after taking away the plant's portion (i.e. its respiration) is known as the net primary production. Net primary production (NPP) is the amount of organic materials produced by plants in excess of their own needs, or the GPP minus R.
The amount of phytoplankton in the water column, also known as the phytoplankton crop, at any given time is a function of their growth rate (GPP) and how fast they are eaten. If they are eaten faster than they grow, then the crop is reduced and eventually may be totally depleted. If they grow faster than they are eaten, then the phytoplankton crop will increase in size (or biomass). Finally, if herbivores graze phytoplankton at the exact same rate that they grow, then the phytoplankton crop will remain the same. In this case, the system is said to be in steady-state. Herbivores consume all of the NPP and no more. In any case, the standing stock (biomass) of herbivores will increase as long as there is enough food to supply their metabolic needs. If there is not enough NPP to support the stock of herbivores, then some of them must move somewhere else or die.
The NPP available to herbivores is eaten and used to meet different metabolic requirements. Some of the organic material is released upon feeding, as fragments of phytoplankton are produced as a result of sloppy feeding. (These bits and pieces become dinner for other organisms so all is not lost!) After ingestion and passage through the digestive system of the herbivores, some of this material is assimilated and some of it is expelled as feces. Fecal material represents unassimilated organic matter; digestion and assimilation is never 100% perfect, so some organic material is released into the water column in nice little fecal pellet packets. Some of the energy and materials absorbed by the herbivores are used to meet their metabolic needs and some is wasted as heat. A small fraction, however, may be left over for growth of the organism, or for the production of eggs from which new herbivores may be hatched.
The amount of NPP left over after the herbivores have had their dinner is known as the net community production (NCP). When a planktonic ecosystem produced excess organic matter (i.e., more than the herbivores can consume), then NCP is positive and there is the potential for export of organic matter from this ecosystem. As we discussed earlier, some communities of organisms depend on exports of organic matter from other ecosystems to meet all of their metabolic needs. This is particularly true for abyssal communities, deep-sea benthic communities, and sandy shore communities. None of these communities have their own autotrophs; thus, their "energy" supply must come from elsewhere.
Whether a given ecosystem exports organic matter, achieves a steady-state, or needs to import organic matter depends on factors that affect the productivity of the autotrophs. In the case of oceanic food webs, seasonal factors as discussed above cause fluctuations in the growth rate and size of the phytoplankton crop. Consequently, these factors cause seasonal fluctuations in the growth rates and size of herbivores and every other consumer in the food web.
The key to phytoplankton productivity (and, by default, food web productivity) in the availability of light and nutrients. Let's start with light. Two processes are important here: the solar cycle and wind- or density-induced mixing. The solar cycle should be familiar to us all and I won't review it here. However, mixing may be a bit more difficult to understand.
Mixing of phytoplankton throughout the euphotic zone causes them to see a light environment that changes continually. As they are mixed deeper, less light is available for photosynthesis and their rates of growth slow down. On the other hand, as they are mixed upwards towards the surface waters, their rates of growth increase as the availability of sunlight increases. Recall that plants respire, also. As such, they require a minimum amount of photosynthesis just to maintain their own cellular metabolism. The depth at which the amount of photosynthesis equals the amount of respiration required by a planet is known as the compensation depth.
At the compensation depth, the amount of light available for photosynthesis is sufficient to produce just enough organic materials for the phytoplankton to meet their daily metabolic requirements. However, phytoplankton rarely stay at the same depth in the water column. As shown below, phytoplankton can be mixed above and below the compensation depth. If they are above the compensation depth, they produce more organic material than is respired. If they are below the compensation depth, then respiration requirements exceed the amount produced. The depth to which phytoplankton can be mixed and still meet their metabolic requirements from photosynthesis is known as the critical depth. The critical depth is deeper than the compensation depth because phytoplankton spend part of their time above the compensation depth and part of their time below the compensation depth. Phytoplankton that spend any time below the critical depth will respire more carbon than they fix, and they will soon die.
Now, think about how the critical depth might change as a function of seasons. During the winter, the critical depth will be shallower because days are shorter and the sun is lower in the sky, which means that the depth of light penetration will be shallower. On the other hand, during the summer, days are long and the depth of light penetration is greater. As a result, the critical depth is deeper.
On top of all this, we must also consider the vertical structure of the water column. During the winter, as we learned earlier, phytoplankton are mixed deep in the water column, far below the critical depth. In this case, they are said to be light-limited. In the spring, as the seasonal thermocline forms, some of the phytoplankton are "trapped" in the surface waters where lots of light is available and they begin to grow rapidly. As a result, we have a spring "bloom" of phytoplankton. However, the spring bloom doesn't last forever. Why?
Phytoplankton require nutrients to grow. As they proliferate rapidly in the spring, the rate at which they use nutrients exceeds the rate at which nutrients are resupplied. In the open ocean, nitrate, the primary nitrogen source of most phytoplankton, especially diatoms and dinoflagellates, is the nutrient that is in least supply. The element or compound in least supply that limits the growth of an organism is called a limiting factor. In the open ocean, nitrate tends to be the most common limiting factor for the growth of diatoms and dinoflagellates.
When nitrate (or any other key nutrient) runs out, the phytoplankton quit growing. However, there is a catch to this. Some phytoplankton are able to use alternative nitrogen sources, such as ammonia, and they continue to grow., Ammonia is supplied through the excretion of zooplankton, especially microzooplankton, the flagellates and ciliates that feed on nanno- and ultraphytoplankton. As it turns out, these smaller phytoplankton are best adapted for using ammonia as their nitrogen source.
The Seasonal Food Web
The seasonal cycle of anything should have a familiar ring to you by now. In earlier lectures, we learned about seasonal changes in incident solar radiation that occurred as the earth, tilted at an angle, orbits the sun. We saw how differential heating of the earth created winds that have a seasonal pattern. We learned about the seasonal formation of the seasonal thermocline and how changes in stratification of the water column lead to blooms of phytoplankton in the spring and fall. Now that these processes are deeply etched in our minds, we can examine in greater detail the seasonal processes that affect oceanic food webs.
Thus, we can now begin to form some idea of the seasonal changes of the oceanic food web. (I bet you were wondering when I was going to get back to the main topic.) In the spring, diatoms and dinoflagellates, the principal autotrophs that make up the "classical" food web, begin to proliferate rapidly. As the numbers of phytoplankton increase, herbivorous zooplankton, primarily copepods, also begin to grow. Increases in food supply (i.e. NPP) make more energy available to the copepods. As copepods grow and increase in number, so too, any organisms that feed on copepods begin to grow. Thus, energy that is pumped into the autotrophs as a result of water column stabilization makes its impact felt on the entire food web, at least, the classical food web.
As the amount of nitrate declines and as the number of copepods increases, phytoplankton productivity slows and NCP becomes negative. More herbivores are now consuming phytoplankton whose rate of growth has slowed, so something has got to give; phytoplankton (namely diatoms and dinoflagellates) decrease in numbers. As phytoplankton decrease in numbers, zooplankton (namely copepods) decrease in numbers, and the entire system finds a steady-state where the rate of phytoplankton growth and zooplankton growth is controlled by the rate of nitrate resupply.
However, there is another food web to pick up the slack; that is, the microbial food web. As excretion from copepods increases the ammonium concentrations in the seawater, nanno- and ultraphytoplankton begin to proliferate. Because they are too small to be eaten by copepods, they increase in numbers. As a result, there may be a second "bloom," of phytoplankton, only this time it's the small guys that are "doin the bloomin." Of course, we know that these nanno- and ultraphytoplankton are the favorite food of ciliates and flagellates, such that these zooplankton species begin to increase in numbers.
As summer stratification sets in and surface waters warm, nutrients become less and less available and the oceanic food web switches from a "classical" food web to a microbial food web. Other species, such Oikopleura, a larvacean who builds gelatinous houses with fine-mesh filters, are able to filter these small particles from seawater. Larvaceans, salps, dolioloids, and other forms of gelatinous filter feeders may be able to proliferate in summer months when other organisms find their food supply of large-size phytoplankton scarce. Predators, such as sea turtles, fishes, and carnivorous jellyplankton may also begin to proliferate and so, the flow of energy moves upwards through the food web all the way to the top.
In the fall, density-driven mixing of the water column increases the supply of nitrate and the oceanic food web experiences another phytoplankton bloom, although not as intense as the spring bloom. Shorter days, increased mixing, and higher concentrations of herbivores tend to diminish the size of the fall bloom. Finally, as winter comes, the days shorten and the water column mixes deeper, thus returning the oceanic food web to a light-limited situation once again.
Two important points need to be made here. First, in the examples above, we have only been talking about the mixed layer, the waters between the surface and the seasonal thermocline. As nutrients are used up in the mixed layer, the phytoplankton begin to concentrate at the nitricline (the area of rapid change in nitrate concentration; in this case, a rapid increase in nitrate), which first forms at the boundary of the seasonal thermocline. The high concentrations of phytoplankton at the depth of the nitricline form what is known as the chlorophyll maximum. As the phytoplankton use nitrate, the nitricline gets deeper and, as a result, the chlorophyll maximum gets deeper. The chlorophyll maximum will continue to deepen throughout the summer until it reaches the compensation depth, where low light levels limit any further growth. Thus, organisms making up the "classical" food web may continue to persist, but they are forced to live deeper where light conditions are not ideal to sustain rapid growth. In this way, the "classical" food web is maintained, albeit at a much reduced level.
The other point I need to make here is that this description of the seasonal cycle of the oceanic food web (spring phytoplankton bloom, etc.) only applies to temperate oceans between approximately 30 degrees and 60 degrees latitude. The formation of the seasonal thermocline is much different in tropical and polar oceans. In tropical oceans, a strong thermocline tends to exist all year long. Very little changes in solar irradiation occur in tropical latitudes; thus, seasonal fluctuations in physical and chemical factors are minimal. On the other hand, the thermocline only forms in the summer in polar latitudes. The halocline is far more important in polar waters in producing water column stratification. Melting of sea ice places a lens of fresh water on top of salt water, creating a very stable water column that lends itself to some of the most intense phytoplankton blooms experienced on the planet.
Finally, we can summarize the seasonal cycle of the "classical" food web by graphing the concentrations (biomass) of phytoplankton and zooplankton throughout the year. Blooms of phytoplankton are followed closely by blooms of zooplankton. If we were to include predators, such as herring, we would see a similar pattern, an increase in herring biomass following by a month or two the increase in the zooplankton.
One unusual feature in these graphs is the "peculiar" nature of the temperate North Pacific Ocean. When I was an undergraduate at the University of Washington, I had the opportunity to make a three-week cruise in the Gulf of Alaska. The lack of a spring phytoplankton bloom was quite baffling and a series of cruises were performed to study this phenomenon. To date, many hypotheses have been proposed, but no definitive answer has been found. One contributing factor has to do with the life cycles of the copepods that live in these oceans and differences in environmental factors (the temperate Pacific tends to be foggy in spring and early summer).
Once more, I want to emphasize (read--pound into your head), the importance of physical processes (changes in incident solar radiation, stratification of the water column, mixing of the water column, wind-induced upwelling, etc.) and chemical processes (replenishment of nutrients in winter, formation of the nitricline in summer, switching from a nitrate-driven system to an ammonia-driven system, etc.) in determining biological processes (spring and all phytoplankton blooms, switching from a "classical" food web to a microbial food web, deepening of the chlorophyll maximum, etc.). Remember that all of the energy and matter that fuels oceanic food webs comes from the phytoplankton, who use sunlight to create living matter from inorganic elements.
Another thing: I would be remiss in my discussion of this topic if I did not reiterate the important role that bacterioplankton play in oceanic food webs. Besides utilizing dissolved organic matter and shunting it into the food web, the bacterioplankton function as decomposers, breaking down organic matter and liberating compounds and elements that are important to the growth of phytoplankton. Much like putting cow dung and mulch piles on your garden (which benefit plants through the microbial release of "fertilizers"), the bacterioplankton break down fecal pellets, bits and pieces of phytoplankton, dead organisms, and any other organic matter and, as a result, remineralize them producing nitrate, ammonia, phosphorus, carbon dioxide, and other biologically important nutrients. In addition, a large number of organisms feed on bacterioplankton. Thus, their productivity is important to a number of other consumers in the oceanic food web.
Finally, I want to slip in here another mention of the cyanobacteria, primarily the genus Synechococcus. These photosynthetic bacteria appear to be very important in tropical and temperate waters. Some scientists estimate that they are responsible for more than half of the ocean's primary productivity (NPP). What this means to food webs is difficult to say. Research on organisms that feed on cyanobacteria reveals that many of them slip through the digestive tracts unharmed; they appear to be able to survive being eaten. Some researchers have suggested that they absorb nutrients as the pass through the guts of organisms and thus, they have adapted a very clever strategy for dealing with nutrient limitation. Whatever their ultimate role in the oceans, cyanobacteria are important members of oceanic food webs and deserve mention here.
A Few Closing Remarks
No doubt, a few of you are shaking your heads by now. The sheer number of factors to consider when discussing oceanic food webs is mind-boggling. While it is often very difficult to integrate what you have learned, I am certain that with a couple more readings, you will begin to appreciate the incredible beauty of planktonic ecosystems. The sheer complexity and the tightly integrated nature of these ecosystems has challenged the most brilliant scientists of our times.
I will relate to you a little story. A famous oceanographer told me that biological oceanography is a very difficult field to master. A biological oceanographer must be conversant with ocean physics and chemistry, he/she must understand atmospheric and radiative processes, he/she must know something about microbiology, biochemistry, physiology, zoology, and ecology; he/she must be adept at mathematics and skilled in computer analysis of large data sets; in short, a biological oceanographer must be a master of all trades. For this reason, only the most brilliant minds succeed in biological oceanography. It attracts many mediocre researchers because they are not skilled or knowledgeable enough in their field of endeavor.
The point is that this is a very difficult subject to grasp in all its parts. In a lifetime of studying the sea, I am only now beginning to grasp some of its parts. So, I don't expect you to be able to understand all the concepts presented here. However, I would like you to think about what is presented here and try to develop an appreciation for the fundamental processes. These fundamental processes govern the flow of energy and materials in all ecosystems, including human ecosystems. Understanding the nature of the world around you will give you a keener sense of life, a bolder vision of the world, and a deeper feeling for the true wonder of it all.