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. In doing that, we find that now the most productive systems are open oceans, tropical rain forests, savannas, and tropical seasonal forests see Figure 5.
Figure 5. Average world net primary production of various ecosystems. What accounts for these differences in production per unit area? Basically, the answer is that climate and nutrients control primary productivity. Areas that are warm and wet generally are more productive see Figures 6a and 6b.
Overall, the amount of water available limits land primary production on our world, in part due to the large areas of desert found on certain continents. Agricultural crops are especially productive due to "artificial" subsidies of water and fertilizers, as well as the control of pests. Though relatively simple when broken down into an equation, determining the net primary productivity can prove more challenging than plugging in a few numbers.
No matter how exact you keep your experiment, you will have a hard time factoring in any energy that the plant produced to sustain itself, and you also won't be able to factor in any potential losses to small insects or other invertebrates. With that in mind, scientists can perform experiments to come as close to the correct net productivity as possible. In one type of experiment testing the net productivity of algae in a pond, researchers use glass bottles and samples of water.
They test the amount of dissolved oxygen prior to the experiment, seal and place the bottle back in the water, and then test the oxygen again after a period of time. By taking the final amount of dissolved oxygen from the bottle and subtracting it by the initial amount, they can calculate the net primary productivity.
However, as discussed previously, the exact net productivity will differ slightly from these results, as minor factors such as insect consumption can impact the amount of biomass produced. You could measure the net primary productivity of a corn plant by planting a seed, allowing the plant to grow for an allotted period of time, and then removing the plant, including all leaves and roots, and allowing it to fully dry out to remove all water within the plant.
Water will increase the weight and skew your data. See Lu et al. These methods require correction for atmospheric variation and sometimes require bidirectional reflectance normalization. The input images are composited over multiple days i.
The relationship between satellite measures of reflectance and estimates of NPP will vary depending on the type of vegetation being considered, and thus major land cover type is an important input to calculating NPP. NPP models produce a map of vegetation biomass for a particular spatial and temporal resolution determined by the input data.
The nature of the information depicted on the map e. Remotely sensed NPP estimates are only approximations of true biomass values. The mathematical models used to calculate NPP vary widely, and each model contains assumptions and requires specific inputs. It is important to understand the model assumptions and assess the suitability of the model based on the available data, how well the model characterizes the vegetation compared to field measurements, and the desired output.
Most models work optimally at a particular scale and in a particular ecosystem type, and the application of an existing model to a new location may require changes to the model. NPP is often derived from spectral vegetation indices, such as NDVI, but there is no single equation with a set of coefficients that can be applied to images of different surface types.
Estimation of NPP by satellite imaging requires corrections for atmospheric effects, topography and diurnal variations, and values change rapidly throughout the season with changing phenology. For areas that are continually cloudy, the use of radar or lidar may be necessary to assess vegetation characteristics. The remote sensing data inputs for NPP will vary depending on the method that is used to estimate it.
Generally, an image with visible and near infrared bands is required. If an empirical approach is being used, ground measurements will be needed in order to derive estimates of NPP. You must have an account and be logged in to post or reply to the discussion topics below. Click here to login or register for the site. User Tools Login. Site Tools. Table of Contents Net Primary Productivity.
NPP models for remote sensing vary widely depending on the ecosystem type and desired output, but there are two main ways to estimate vegetation productivity using remotely-sensed images: Linear Modeling - linear modeling approaches attempt to relate reflectance data recorded by a sensor to field measurements of NPP using linear regression techniques. Such approaches may correlate field-measured fractional cover with sensor reflectance bands, or to vegetation indices like NDVI. This method is useful for estimating live biomass.
Physical Models - Physical models use principles of how light energy is absorbed or reflected from different surfaces to estimate physical characteristics of vegetation. Biophysical models incorporate parameters related to how light interacts with processes like photosynthesis, evapotranspiration, stress, and decay of plant material. This method uses theoretical models of radiative transfer theory to estimate the absorbed photosynthetically active radiation see fPAR , and has been successful in predicting biomass across wide scales and different climatic regimes.
Net primary productivity estimates have been used for many purposes: Assessing ecosystem function. Hunt and Miyake compared remotely-sensed estimates of NPP with GIS-based estimates from soil surveys to determine if either approach would be suitable for estimating stocking rates of livestock at a state-wide scale in Wyoming. Reeves et al. Wessels et al. Hunt, Jr, E. Comparison of stocking rates from remote sensing and geospatial data.
Rangeland Ecology and Management Reeves, M. Winslow, and S.
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