We continue our discussion of primary productivity in the sea, only from a global perspective. Now that we are familiar with the major pathways of photosynthesis, we can better appreciate the physical and chemical factors that affect phytoplankton productivity in the sea. Finally, we take a look at the bird's-eye view that satellites give to oceanographers.
What factors affect the growth of phytoplankton?
Phytoplankton, like all plankton, drift in the sea. In fact, they are present in all the waters of the worlds, inhabiting rivers, lakes, ponds, and even dog dishes at times. Their growth and reproduction is sustained by radiant energy from the sun. Their relationship with the sun, as drifters subjects to the whims and whorls of the liquid medium in which they live, is one of the most precarious elements of their existence -- at any moment they may be plummeted to the unlit waters of the depths or thrust to the blinding waters at the surface. These movements of phytoplankton up and down in the water column as a result of mixing means that phytoplankton are subject to an ever-changing light field over time scales that may vary from seconds to months. Thus, the photosynthetic machinery of phytoplankton, which operates to maximize its use of radiant energy, must be adaptable in its response to the light field to maintain optimal efficiency of energy utilization. These adaptations to changes in the intensity and spectral distribution (color) of the underwater light field are called photoadaptation.
Photoadaptation of phytoplankton can occur in two principal ways. Because the availability of photons decreases with depth in the water column, the probability of capturing these photons is reduced. To compensate for changes in the intensity of light, nearly all phytoplankton manufacture more chlorophyll, the major light harvesting pigment of all plants. Thus, the amount of chlorophyll per cell can be increased. Although the efficiency of capturing light is decreased, the amount of light that can be captured is substantially increased. You might think of photoadaptation as a process whereby phytoplankton expand their umbrella to catch more of the sun's rays.
Another adaptation to light involves expanding the color range over which light can be captured. As we learned earlier, chlorophyll is very good at absorbing blue light (at about 435 nanometers). However, as phytoplankton bloom, the amount of chlorophyll in the water column increases and therefore, the amount of blue light available decreases. In fact, the green color typically associated with coastal waters and bays is due to the high concentrations of chlorophyll found in these waters. To compensate for changes in the color, or spectral distribution, of seawater, phytoplankton manufacture accessory pigments, which are better able to absorb green light.
This ability to adapt to changes in the intensity or spectral distribution of light in the water column means that some organisms can exploit light regimes better than others. This type of photoadaptation also leads to a "layering" of different phytoplankton species over the depths, according to the particular depth to which they are best adapted. Green algae, those little green blobs with tails, tend to lack accessory pigments. Thus, they tend to occur in the near-surface waters. Diatoms and dinoflagellates, on the other hand, can live quite comfortably at the bottom of the euphotic zone, because they have the accessory pigments to capture the light that is available at these depths. Unfortunately, we don't have time to discuss all the varieties of photoadaptation exhibited by phytoplankton. Suffice it to say that this is a very active area of research in the oceans because it affects not only the biology of the seas, but also because it affects the color of the seas. Satellite oceanographers and Navy scientists interested in communicating through the seas using lasers have very good reasons for wanting to understand how phytoplankton absorb light!
Another type of photoadaptation that we should mention is coping with too bright of light. During cloudless days, the intensity of solar radiation may exceed what is tolerable for the phytoplankton. Much like you and me, phytoplankton have found ways to prevent intense sunlight from causing them harm. When too much sunlight is pounding on the photosynthetic apparatus of phytoplankton, they may experience photoinhibition. Photoinhibition is the phenomenon whereby that rate of photosynthesis is decreased as a result of too much light. Perhaps some of you have put a plant out in the sun only to watch it turn yellow and burn up. This can happen to phytoplankton too. Too much sun bleaches the chlorophyll of phytoplankton; the incessant pounding of photons breaks the photosynthetic machinery into bits. To prevent these negative effects of photoinhibition, phytoplankton manufacture a set of pigments called protective pigments. Protective pigments, namely carotenoids (like the orange color of a carrot) have wide absorption bands that capture light energy and turn it into heat. In this way, the photosynthetic apparatus is protected.
No less important are the nutrients that provide them with the chemical ingredients to build their cellular materials, to build the cell walls and cell membranes that help them stay afloat, to construct the internal cellular machinery that allows them to adapt to changes in the intensity and spectral distribution of light. Phytoplankton, like all plants, require nitrogen and phosphorus and other major elements, but many of them also need more exotic compounds, such as silica and other trace elements. Any of these compounds, if not present in sufficient concentrations, can become limiting to the growth of phytoplankton. Nutrient limitation is often the primary factor that determines the abundance of the phytoplankton crop in any one region of the world ocean. As such, physical processes that supply an abundance of nutrients, such as upwelling zones, cause the phytoplankton crop to flourish.
The principal nutrients that govern the growth and dynamics of the phytoplankton crop are nitrogen compounds, namely nitrate and ammonia, and phosphorus. Other elements, such as silica and iron, may also limit the growth of phytoplankton. Collectively, these elements are known as biologically important nutrients. When an element is not present in sufficient concentrations to sustain the growth of an organisms, it is said to be limiting. A limiting factor, therefore, is the one factor that runs out first that prevents further growth. In the ocean, the most limiting factor appears to be nitrate, although iron limitation has received a great deal of attention lately. Most likely, any element can be limiting at one time or another, depending on the rate at which it is resupplied in the water column.
Nitrogen limitation in the sea occurs as nitrate, supplied by cold, nutrient-rich, deep waters, is used up. At the onset of the spring phytoplankton bloom, triggered by the formation of the seasonal thermocline, as discussed earlier, nitrate is available in great quantities. However, as the phytoplankton crop enlarges, all of the available nitrate is depleted. This leads to the formation of a nitricline, a depth where the amount of nitrate increases rapidly. It should make sense to you that the nitrate in the near-surface waters is used up first, then steadily "chewed down" as phytoplankton photoadapt to less light and use up more nitrogen. At some point, namely, the bottom of the euphotic zone, light becomes the limiting factor.
During the summer, when the water column is well-stratified, most of the chlorophyll (i.e. phytoplankton) occurs at the bottom of the euphotic zone. This results from the depletion of nitrate in the upper euphotic zone and the photoadaptation of phytoplankton in the lower euphotic zone. The seasonal development of the chlorophyll maximum is a well-studied feature of the euphotic zone. It results from the switchover from nitrate-limited phytoplankton growth to light-limited phytoplankton growth.
Finally, as surface water mixes as a result of cooling in the fall, nitrate is made available to the euphotic zone, and another phytoplankton bloom occurs. The fall bloom is usually less intense than the fall bloom and is quickly extinguished as shorter days and deeper mixing of the water column reduce the overall amount of solar radiation available for photosynthesis. Once again, light becomes a limiting factor. Thus, we have a transition from a light-limited regime in the fall and winter to a nutrient-limited regime in the spring and summer.
Light, nutrients, and mixing all have an effect on phytoplankton productivity in the ocean. Mid-latitudes like the Japan Sea experience the classic spring-fall bloom. Tropical environments near the equator experience only minor fluctuations in solar radiation and heating and, thus, exhibit steady-state characteristics. The summer bloom in the tropics is merely a blip on the overall productivity map. In contrast, polar environments experience rapid and dramatic changes in solar radiation, heating, and mixing. Thus, polar latitudes typically exhibit intense phytoplankton blooms in mid-summer, often as a result of the water column stratification induced by ice melt (Why does the melting of ice induce water column stratification?).
One point I want to stress is that these paradigms, i.e. our general ideas about the way things work, are just useful models that help us organize our thoughts. In reality, a bloom of phytoplankton will occur anytime conditions are right. A wind blowing from the north will cause upwelling no matter what time of year. Such episodic events can lead to blooms at uncharacteristic times. However, you should remember that it is the dynamics of the processes that are important; the times of year are merely useful guides.
The same arguments can be made about the formation of cold-core rings. Recall that these structures, spun off the Gulf Stream, have a nutrient-rich center and a stratified water column that are perfect for the growth of phytoplankton. These rings may be formed any time of year, and, while winter conditions are not ideal, a series of days with good conditions can lead to a bloom. Just remember that the ocean is an interconnected set of physical, chemical and biological processes. You get the picture.
Although we will talk more later about food webs in the sea, I should mention here that zooplankton, the animal plankton of the sea, also limit the growth of phytoplankton in the sea. Obviously, if you get eaten, you cannot grow any longer and that's that. Grazing by the zooplankton "chops down" the number of phytoplankton (much like a lawnmower through grass) and shifts the species composition of a phytoplankton bloom. Because some phytoplankton species are preferred to others (much like more kids prefer Trix than Wheaties, or some people prefer pork rinds to broccoli), the preferred species get eaten first.
Typically, large zooplankton tend to eat large phytoplankton, and the phytoplankton populations shift from a nanoplankton-dominated system to an ultraplankton-dominated system. Once all the large phytoplankton are eaten, the populations of large zooplankton begin to decline. At the same time, small zooplankton begin to proliferate, chomping on the smaller nanoplankton and ultraplankton that are available to eat.
How productive are the oceans?
There is considerable debate about the amount of primary productivity in the oceans. This debate is exacerbated, no doubt, by our inability to accurately estimate productivity over the temporal and spatial scales necessary to form an accurate estimate (a problem that could be solved if every one would adopt using natural fluorescence!). Nonetheless, we can form some general ideas about productivity in the oceans and make a few guesstimates about the contribution of the oceans to the global carbon cycle.
Your book provides a good look at the overall patterns of productivity in the world oceans. Take note of those regions with the highest productivity. What do these areas have in common? What processes could possibly contribute to the high productivity in these regions. At the other end of the spectrum, take note of the biological deserts. Where do they tend to occur? Why is their higher productivity at the equator in the Pacific Ocean than to the north or south? How do these patterns compare with the E-P map we saw earlier? Why is the Antarctic Ocean more productive than the Arctic Ocean? Why are coastal regions more productive than open ocean regions? If you can answer all these questions, you are on your way to becoming a top-rate oceanographer! Congratulations. If you are having difficulty grasping these patterns, talk them over with a classmate. Think about the two factors we just discussed, namely, light and nutrients. Come see me if all else fails. I'll be happy to explain.
Note that upwelling centers are the most productive regions of the world ocean (in terms of productivity per square meter), more than twice as productive as coastal regions without upwelling. Despite the fact that upwelling centers make up less than 0.1% of the surface area of the world's oceans, they still make a significant contribution.
On the other hand, even though the per square meter rate of productivity of the open ocean is the smallest, the sheer area of the open ocean (about 85%) more than compensates for its low rates. The open oceans contribute more to overall productivity of the oceans than any other region. In fact, the open oceans contribute more to global productivity than any other region period.
While the issue is not settled, there does seem to be a growing consensus that the lands and the seas contribute about equally to the overall amount of organic carbon produced each year on our planet. Overall, the oceans appear to contribute between 50 to 100 grams of carbon (bound as carbohydrates) per square meter of ocean every year. This translates to a total of 200 to 280 billion tons of material produced by the oceans every year. That's a whole lotta phytoplankton!
How do satellites improve our understanding of primary productivity?
From the very outset, man's view of Earth from space changed our perspective of the planet as a whole. Anyone who has seen pictures from the Apollo missions or recent photos and videos from the Space Shuttle can appreciate the grand scale of global processes on our planet. This view of Earth and the oceans has brought dramatic changes in our understanding of the global processes, and we are just now beginning to uncover the vast and complex links that bind us to our planet.
The first real breakthrough in our understanding of the oceans came with the launch of Nimbus-7 in 1978. For the next eight years, this satellite painted a picture of ocean color far more interesting than had ever been imagined. New oceanic features, such as coastal jets and cold-core eddies, sprung to life when viewed from a satellite. The vast structure of the oceans lay bare before us and, for the first time, we could see how life in the ocean responded to these structures. Upwelling centers and coastal productivity on the global scale could be assessed with better accuracy.
Nimbus-7 provided information on ocean color that could be used to determine the concentrations of chlorophyll at the surface in the oceans. The workhorse of the Nimbus-7 was the Coastal Zone Color Scanner (CZCS). This instrument was designed specifically for the remote sensing of chlorophyll in the oceans. The optical system on board measured four wavelengths of ocean color, centered at 443, 520, 550, and 670 nanometers. In addition, the instrument measured light in the near infrared (700-700 nanometers), and wavelengths in the infrared (1050-1250 nanometers) to obtain data on sea surface temperature. To avoid sun glitter (the reflection of the sun from waves on the sea surface), the sensors could be tilted. At its altitude of 955 kilometers (about 55 miles), the sensors were able to "see" a 825 x 825 meter square (about a half miles by half mile square) of ocean that could be scanned side-to-side over a 1635 kilometer "swath" (about 1000 miles side-to-side, although the resolution of measurements at the edge is typically poor).
From these measurements, oceanographers were able to map the chlorophyll concentrations of different parts of the world ocean. Seasonal changes were visible as the satellite repeated its orbits over the same area. From these data, oceanographers could estimate phytoplankton concentrations throughout the euphotic zone and make inferences as to factors that controlled primary productivity in the sea. The huge volumes of data provided by this instrument continue to be a source of knowledge to this day, as oceanographers scrutinize the data in new ways that lead to a better understanding of productivity in the oceans.
Global ocean productivity maps are widely available at many sites on the world wide web. NASA maintains a site just for satellite measurements of primary productivity. Try a search on "satellite oceanography" and see what you come up with.
While we're on this subject, I want to mention the TOPEX/Poseidon satellite, launched in 1992, which measures the "bumps" in the ocean surface (i.e. sea surface height). Remember the "hill" in the Sargasso Sea, formed as a result of geostrophic flow? The T/P satellite is capable of making measurements of the sea surface with a precision of 4 inches and has returned an extraordinary amount of useful and revolutionary information. One more interesting finding was a bulge of water in the North Pacific Ocean that was created more than 12 years ago during the 1983-1984 El Nino. New information from this satellite will greatly enhance our understanding of ocean currents, particularly thermohaline processes, and may help us to understand better the role of the oceans in global warming.
Clearly, satellite oceanography represents a new frontier for ocean scientists. These synoptic and instantaneous pictures from outer space will vastly improve our ability to understand ocean phenomena. Using satellites, oceanographers can now obtain a more detailed view of the oceans than ever before. New discoveries await us in this exciting new field of oceanography.