How does secondary productivity relate to npp

For example, we know how many calories a measure of energy a gram of certain carbon compounds such as fats or carbohydrates contain. Autotrophs verses Heterotrophs As a brief review, we recognize that some organisms are capable of synthesizing organic molecules from inorganic precursors, and of storing biochemical energy in the process.

These are called autotrophs , meaning "self-feeding. Organisms able to manufacture complex organic molecules from simple inorganic compounds water, CO 2 , nutrients include plants, some protists, and some bacteria. The process by which they do this usually is photosynthesis , and as its name implies, photosynthesis requires light see Figure 1. For completeness, we should mention the pathway known as chemosynthesis. Some producer organisms, mostly specialized bacteria, can convert inorganic nutrients to organic compounds without the presence of sunlight.

There are several groups of chemosynthetic bacteria in marine and freshwater environments, particularly those rich in sulfur or hydrogen sulfide gas. Like chlorophyll-bearing plants and other organisms capable of photosynthesis, chemosynthetic organisms are autotrophs see microbes lecture notes for more information. Many organisms can only obtain their energy by feeding on other organisms.

These are called heterotrophs. They include consumers of any organism, in any form: plants, animals, microbes, even dead tissue. Heterotrophs also are called consumers. In this lecture we will begin with a consideration of primary production, and in the next lecture we will examine what happens to this energy as it is conveyed along a food chain. The Process of Primary Production The general term " Production" is the creation of new organic matter.

When a crop of wheat grows, new organic matter is created by the process of photosynthesis, which converts light energy into energy stored in chemical bonds within plant tissue. This energy fuels the metabolic machinery of the plant. New compounds and structures are synthesized, cells divide, and the plant grows in size over time. As was discussed in detail in a previous lecture, the plant requires sunlight, carbon dioxide, water, and nutrients, and through photosynthesis the plant produces reduced carbon compounds and oxygen.

Whether one measures the rate at which photosynthesis occurs, or the rate at which the individual plant increases in mass, one is concerned with primary production definition: the synthesis and storage of organic molecules during the growth and reproduction of photosynthetic organisms.

The core idea is that new chemical compounds and new plant tissue are produced. Over time, primary production results in the addition of new plant biomass to the system. Consumers derive their energy from primary producers, either directly herbivores, some detritivores , or indirectly predators, other detritivores. Is there an Upper Limit to Primary Production?

The short answer is "yes". Let's briefly consider how much energy is in fact captured by autotrophs, and examine how efficient is the process of photosynthesis. Recall that the intensity of solar radiation reaching the earth's surface depends partly on location: the maximum energy intensity is received at the equator, and the intensity decreases as we move toward the poles. As we saw in the lecture on ecosystems, these differences have profound effects on climate, and lead to the observed geographic patterns of biomes.

Furthermore, we know that only a small fraction of the sun's radiation is actually used in the photosynthetic reaction in plants at the Earth's surface. Plants strongly absorb light of blue and red wavelengths hence their green color, the result of reflection of green wavelengths , as well as light in the far infrared region, and they reflect light in the near infrared region. Even if the wavelength is correct, the light energy is not all converted into carbon by photosynthesis.

Some of the light misses the leaf chloroplast, where the photsynthetic reactions occur, and much of the energy from light that is converted by photosynthesis to carbon compounds is used up in keeping the plant biochemical "machinery" operating properly - this loss is generally termed "respiration", although it also includes thermodynamic losses.

Plants do not, then, use all of the light energy theoretically available to them see Figure 2. Figure 2 : Reduction of energy available to plants On average, plant gross primary production on earth is about 5. This is about 0. After the costs of respiration, plant net primary production is reduced to 4.

This relatively low efficiency of conversion of solar energy into energy in carbon compounds sets the overall amount of energy available to heterotrophs at all other trophic levels.

Some Definitions So far we have not been very precise about our definitions of "production", and we need to make the terms associated with production very clear. Respiration can be further divided into components that reflect the source of the CO 2. This will be discussed more in our lectures on climate change and the global carbon cycle.

Note that in these definitions we are concerned only with "primary" and not "secondary" production. Secondary production is the gain in biomass or reproduction of heterotrophs and decomposers.

The rates of secondary production, as we will see in a coming lecture, are very much lower than the rates of primary production. To better understand the relationship between respiration R , and gross and net primary production GPP and NPP , consider the following example. This is your "gross production" of money, and it is analogous to the gross production of carbon fixed into sugars during photosynthesis.

That is the "cost" you pay to keep operating, and it is analogous to the respiration cost that a plant has when their cells use some of the energy fixed in photosynthesis to build new enzymes or chlorophyll to capture light or to get rid of waste products in the cell. Measuring Primary Production You may already have some idea of how one measures primary production.

There are two general approaches: one can measure either a the rate of photosynthesis , or b the rate of increase in plant biomass. Will they give the same answer? The method used in studies of aquatic primary production illustrates this method well.

In the surface waters of lakes and oceans, plants are mainly unicellular algae, and most consumers are microscopic crustaceans and protozoans. Both the producers and consumers are very small, and they are easily contained in a liter of water. If you put these organisms in a bottle and turn on the lights, you get photosynthesis. If you turn off the lights, you turn off the primary production.

However, darkness has no effect on respiration. Remember that cellular respiration is the reverse process from photosynthesis, as follows. When calculating the amount of energy that a plant stores as biomass, which is then available to heterotrophs, we must subtract plant respiration costs from the total primary production. The general procedure is so simple that primary production of the world's oceans has been mapped in considerable detail, and many of the world's freshwater lakes have also been investigated Figure 3.

One takes a series of small glass bottles with stoppers, and half of them are wrapped with some material such as tin foil so that no light penetrates. These are called the "light" and "dark" bottles, respectively. Figure 3. The bottles are filled with water taken from a particular place and depth; this water contains the tiny plants and animals of the aquatic ecosystem.

The bottles are closed with stoppers to prevent any exchange of gases or organisms with the surrounding water, and then they are suspended for a few hours at the same depth from which the water was originally taken. Inside the bottles CO 2 is being consumed, and O 2 is being produced, and we can measure the change over time in either one of these gases.

For example, the amount of oxygen dissolved in water can be measured easily by chemical titration. Then, the final value is measured in both the light and dark bottles after a timed duration of incubation. What processes are taking place in each bottle that might alter the original O 2 or CO 2 concentrations? The equations below describe them. In this example we may also have some consumer respiration in both bottles, unless we used a net to sieve out tiny heterotrophs.

Now consider the following simple example. It illustrates how we account for changes from the initial oxygen concentrations in the water that occurred during the incubation. We will assume that our incubation period was 1 hour.

The oxygen technique is limited in situations where the primary production is very low. In these situations, the radioactive form of carbon, C 14 14 CO 2 , can be used to monitor carbon uptake and fixation.

You can also convert the results between the oxygen and carbon methods by multiplying the oxygen values by 0. Consider the following example. Suppose we wish to know the primary production of a corn crop. We plant some seeds, and at the end of one year we harvest samples of the entire plants including the roots that were contained in one square meter of area. We dry these to remove any variation in water content, and then weigh them to get the "dry weight". Thus our measure of primary production would be grams m -2 yr -1 of stems, leaves, roots, flowers and fruits, minus the mass of the seeds that may have blown away.

What have we measured? It isn't GPP, because some of the energy produced by photosynthesis went to meet the metabolic needs of the corn plants themselves. Is it NPP? Well, if we excluded all the consumers such as insects of the corn plant, we would have a measure of NPP. But we assume that some insects and soil arthropods took a share of the plant biomass, and since we did not measure that share, we actually have measured something less than NPP. Note that this is exactly the same situation in the bottle method we described above if small heterotrophs that grazed on algae were included in the bottle, in which case the two methods would measure the same thing.

In recent years it has also become possible to estimate GPP and R in large plants or entire forests using tracers and gas exchange techniques. These measurements now form the basis of our investigations into how primary production affects the carbon dioxide content of our atmosphere. Production, Standing Crop, and Turnover With either of these methods, the primary Production can be expressed as the rate of formation of new material, per unit of earth's surface, per unit of time.

Standing crop , on the other hand, is a measure of the biomass of the system at a single point in time, and is measured as calories or grams per m 2. The difference between production and standing crop is a crucial one, and can be illustrated by the following question.

Should a forester, interested in harvesting the greatest yield from a plot, be more interested in the forest's standing crop or its primary production? Well, the key element to the answer is "TIME". If the forester wants a short term investment i. If instead the forester wants to manage the forest over time sell some trees while growing more each year , then the rate at which the forest produces new biomass is critical.

Thus the stock or standing crop of any material divided by the rate of production gives you a measure of time. Notice how similar really, identical this turnover time is to the residence time that you learned about in earlier lectures.

It is really important to consider this element of "time" whenever you are thinking about almost any aspect of an organism or an ecosystem or a problem in sustainability.

Learning about how much of something is happening and how fast it is changing is a critical aspect of understanding the system well enough to make decisions; for example, the decision of the forester above may be driven by economic concerns or by conservation concerns, but the "best" choice for either of those concerns still depends on an understanding of the production, standing crop, and turnover of the forest.

This highlights the point made in earlier lectures that to make decisions about sustainability you must understand these basic scientific concepts. Patterns and Controls of Primary Production in the World's Ecosystems The world's ecosystems vary tremendously in productivity, as illustrated in the following figures. In terms of NPP per unit area, the most productive systems are estuaries, swamps and marshes, tropical rain forests, and temperate rain forests see Figure 4.

Figure 4. Net Primary Production per unit area of the world's common ecosystems. If we wish to know the total amount of NPP in the world, we must multiply these values by the area that the various ecosystems occupy. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4.

Climate Change 5: Evolution 1. Evolution Evidence 2. Natural Selection 3. Classification 4. Cladistics 6: Human Physiology 1. Digestion 2. The Blood System 3. Disease Defences 4. Gas Exchange 5. Homeostasis Higher Level 7: Nucleic Acids 1. DNA Structure 2. Transcription 3. In the same forest, stems including trunks and branches are the largest fraction of standing biomass, but roots comprise a quarter of the total biomass present in the ecosystem. Figure 2: Meteorological towers like this one located in a temperate forest are distributed across ecosystems in all continents except Antarctica, providing assessments of carbon uptake by forest, grassland, desert, and crop ecosystems.

Scientists use several complementary tools for quantifying terrestrial gross and net primary production at ecosystem to global scales. On-the-ground inventory based methods are commonly used in cropland, grassland, and forested ecosystems to measure NPP.

This approach requires estimates of biomass production through periodic measurements of root, stem, leaf, and fruit growth. The growth over time of all plant tissues within a terrestrial ecosystem is equal to NPP. In this approach, aboveground ears, stalks, leaves and belowground roots corn biomass yield over a single growing season is equal to annual NPP of this crop ecosystem.

Recent technological advances also allow for on-the-ground estimates of terrestrial primary production using meteorological towers that measure the uptake or emissions of CO 2 by ecosystems Figure 2.

Meteorological towers measure net ecosystem CO 2 exchange NEE , which is equal to GPP minus ecosystem respiration or the quantity of CO 2 respired by both autotrophs plants and heterotrophs primarily microbes. Meteorological approaches are employed worldwide in forest, agricultural, grassland, and desert ecosystems to track terrestrial primary production. At the global scale, satellite data combined with mathematical modeling is essential to providing worldwide estimates of terrestrial primary production.

Several approaches have been used, but most notable are products derived from NASA's Moderate-resolution Imaging Spectroradiometer MODIS , a satellite-mounted instrument that collects surface spectral, or color, data useful for tracking changes in the productivity of terrestrial and marine ecosystems.

An example MODIS product is a "greenness" index of the Earth's surface used to estimate terrestrial primary production. Surface greenness and other remotely sensed data collected from space provide coarser assessments of NPP and GPP than inventory and meteorological tower based methods but have the advantage of providing estimates of terrestrial primary production for large areas where ground-based methods are not feasible.

Annual NPP changes from one year to the next in response to longer-term trends in climate, including shifts in total solar radiation caused by differences in cloud cover from year to year. Decadal patterns of NPP track changes in ecological succession Gough et al. Terrestrial primary production fluctuates over time and is closely coupled with physical i. On scales of seconds to hours, primary production during the growing season responds to environmental drivers of photosynthesis, generally increasing with photosynthetic photon flux density PPFD or the spectrum of solar radiation available to power photosynthesis.

At the seasonal scale, terrestrial primary production of boreal and temperate ecosystems is tied to changes in temperature and photoperiod, or day length, Figure 3 while in tropical regions seasonal precipitation patterns often dictate cycles of high and low primary production. Year-to-year, or interannual, changes in terrestrial primary production are often related to long-term climate variation including prolonged drought and, in some cases, variation from one year to the next in average annual temperature and solar radiation.

Over decades, a period that is meaningful to ecological succession, terrestrial primary production changes in response to shifts in plant competition and disturbance. Consider an abandoned field that undergoes a successional reversion back to forest.

Plant communities will assemble during early succession, with fast-growing plants emerging first and because of low initial plant density there will be little competition for resources. As a result, total plant growth in the ecosystem, or NPP, will proceed at an increasingly higher rate for several years.

NPP generally levels off or declines once plants start crowding one another and begin competing more intensively for limiting light, nutrient, and water resources Figure 3. Terrestrial primary production also may change over time in response to natural disturbances such as insect outbreaks, wind, fire, and pathogens that diminish photosynthesis by reducing leaf biomass and causing plant death.

Long-term increases in atmospheric CO 2 and nitrogen deposition associated primarily with fossil fuel burning generally increase plant growth over long periods of time. Terrestrial primary production varies considerably across the surface of the Earth and among different ecosystem types. Terrestrial primary production, both NPP and GPP, vary from north to south or latitudinally due to gradients in plant community composition, growing season length, precipitation, temperature, and solar radiation.

However, east to west longitudinal differences in terrestrial primary production also exist. For example, there is a precipitous decline in NPP from east to west in middle North America that is largely a function of declining precipitation.

NPP generally declines from tropical regions to the poles because of temperature and light limitations. Tropical forests tend to be much more productive than other terrestrial ecosystems, with temperate forests, tropical savannah, croplands, and boreal forests all exhibiting middle levels of primary production Table 1.

Desert and Tundra Biomes, limited by precipitation and temperature respectively, contain the least productive ecosystems. In addition to climatic regulation of terrestrial primary production, disturbance, management, and land-use change including urbanization play critical roles in determining spatial differences in terrestrial primary production. Tropical ecosystems, because of their high productivity and extensive footprint on the Earth's surface, comprise nearly half of global NPP and GPP Table 1.

Temperate ecosystems and croplands are also a substantial fraction of global terrestrial primary production, accounting for roughly a quarter of global NPP and GPP. Global estimates of terrestrial NPP range from Beer et al. Melillo et al. Haberl et al. Humans exert an additional influence on global NPP through fires. Many ecologists are concerned that the rising global demand for biofuels, together with continued human population growth, will increase this already large human appropriation of global NPP to the detriment of ecological food webs and biodiversity.

Considerable research in ecosystem ecology centers on understanding how climate change is affecting the primary production of terrestrial ecosystems and, conversely, how ecosystems may moderate changes in global climate by absorbing anthropogenic CO 2 emissions. Terrestrial primary production is an important ecosystem service, locking up carbon in biomass that might otherwise exist in the atmosphere as CO 2 , a potent greenhouse gas.

Continued declines in global NPP would not only reduce carbon sequestration by terrestrial ecosystems but also compromise food security and disrupt the foundation of food webs.

Baldocchi, D. FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities. Bulletin of the American Meteorological Society 82 , — Beer, C. Terrestrial gross carbon dioxide uptake: Global distribution and covariation with climate.

Science , — Field, C.