So wetlands that are stagnant are less productive than those that flow or are open to flooding rivers. This makes sense because a flow-through system constantly gets more nutrients.
This is what allows them all to be fairly productive. These tend to be the most productive ecosystems in the world. Estimates of the southern coastal plain of the U. The southern marshes do better than the northern ones partly because of the greater influx of solar energy and longer growing season, and partly because of the nutrient rich sediments carried by rivers in that region.
Low or intertidal marshes are more productive than high marshes because of the increased exposure to tidal flow. Belowground production is high. Under unfavorable soil conditions, plants seem to put more energy into root production. Generally, plant production depends on light, water, nutrients, and toxins. If you look at a salt marsh it has full sun, limitless water, and the sedimentary soil is generally rich in nutrients so you'd expect uniformly high production.
In the next lecture we will examine how this energy moves through the rest of the ecosystem, providing fuel for life at higher trophic levels. Summary of Part 1, Primary Production Organisms are characterized as autotrophs and heterotrophs. Autotrophs produce their own food by fixing energy through photosynthesis or, less commonly, chemosynthesis. Heterotrophs must feed on other organisms to obtain energy.
Primary production is the creation of new organic matter by plants and other autotrophs. It can be described per unit area for individual ecosystems or worldwide. Production also is a rate, measured per time unit, while standing crop biomass is the amount of plant matter at a given point in time.
The ratio of standing crop to production is called turnover. The turnover time of a system is important in determining how a system functions. Production rates can be quantified by a simple method by which oxygen or carbon production is measured.
Production can also be quantified by measuring the rate of new biomass accumulation over time. The distinction between gross primary production GPP , net primary production NPP , and net ecosystem production NEP is critical for understanding the energy balance in plants and in whole ecosystems.
Production varies among ecosystems, as well as over time within ecosystems. Rates of production are determined by such factors as climate and nutrient supply. Precipitation is the dominant control worldwide, but nutrient availability often limits primary production in any particular, local system.
The Flow of Energy to Higher Trophic Levels In the section above we examined the creation of organic matter by primary producers. Without autotrophs, there would be no energy available to all other organisms that lack the capability of fixing light energy.
However, the continual loss of energy due to metabolic activity puts limits on how much energy is available to higher trophic levels this is explained by the Second Law of Thermodynamics. Today we will look at how and where this energy moves through an ecosystem once it is incorporated into organic matter.
Most of you are now familiar with the concept of the trophic level see Figure 1. It is simply a feeding level, as often represented in a food chain or food web. Primary producers comprise the bottom trophic level, followed by primary consumers herbivores , then secondary consumers carnivores feeding on herbivores , and so on.
When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores. This does not take into account decomposers and detritivores organisms that feed on dead organic matter , which make up their own, highly important trophic pathways. Figure 1: Trophic levels. What happens to the NPP that is produced and then stored as plant biomass at the lowest trophic level?
On average, it is consumed or decomposed. If NPP was not consumed, it would pile up somewhere. Usually this doesn't happen, but during periods of Earth's history such as the Carboniferous and Pennsylvanian, enormous amounts of NPP in excess of the degradation of organic matter accumulated in swamps.
It was buried and compressed to form the coal and oil deposits that we mine today. When we burn these deposits same chemical reaction as above except that there is greater energy produced we release the energy to drive the machines of industry, and of course the CO 2 goes into the atmosphere as a greenhouse gas. This is the situation that we have today, where the excess CO 2 from burning these deposits past excess NPP is going into the atmosphere and building up over time, dramatically changing our climate.
But let's get back to an ecosystem that is balanced, or in "steady state" "equilibrium" where annual total respiration balances annual total GPP. As energy passes from trophic level to trophic level, the following rules apply: Only a fraction of the energy available at one trophic level is transferred to the next trophic level. Typically the numbers and biomass of organisms decrease as one ascends the food chain. An Example: The Fox and the Hare To understand these rules, we must examine what happens to energy within a food chain.
Suppose we have some amount of plant matter consumed by hares, and the hares are in turn consumed by foxes. The following diagram Figure 2 illustrates how this works in terms of the energy losses at each level. A hare or a population of hares ingests plant matter; we'll call this ingestion.
Part of this material is processed by the digestive system and used to make new cells or tissues, and this part is called assimilation. What cannot be assimilated, for example maybe some parts of the plant stems or roots, exits the hare's body and this is called excretion.
The hare uses a significant fraction of the assimilated energy just being a hare -- maintaining a high, constant body temperature, synthesizing proteins, and hopping about.
This energy used lost is attributed to cellular respiration. The remainder goes into making more hare biomass by growth and reproduction that is, increasing the overall biomass of hares by creating offspring.
The conversion of assimilated energy into new tissue is termed secondary production in consumers, and it is conceptually the same as the primary production or NPP of plants. In our example, the secondary production of the hare is the energy available to foxes who eat the hares for their needs. Clearly, because of all of the energy costs of hares engaged in normal metabolic activities, the energy available to foxes is much less than the energy available to hares.
Just as we calculated the assimilation efficiency above, we can also calculate the net production efficiency for any organism. This efficiency is equal to the production divided by the assimilation for animals, or the NPP divided by the GPP for plants. The "production" here refers to growth plus reproduction.
These ratios measure the efficiency with which an organism converts assimilated energy into primary or secondary production. These efficiencies vary among organisms, largely due to widely differing metabolic requirements.
The reason that some organisms have such low net production efficiencies is that they are homeotherms , or animals that maintain a constant internal body temperature mammals and birds.
This requires much more energy than is used by poikilotherms , which are also known as "cold-blooded" organisms all invertebrates, some vertebrates, and all plants, even though plants don't have "blood" that do not regulate their temperatures internally. Just as we can build our understanding of a system from the individual to the population to the community, we can now examine whole trophic levels by calculating ecological efficiencies.
You might think of it as the efficiency of hares at converting plants into fox food. Note that the ecological efficiency is a "combined" measure that takes into account both the assimilation and net production efficiencies. You can also combine different species of plants and animals into a single trophic level, and then examine the ecological efficiency of for example all of the plants in a field being fed on my all of the different grazers from insects to cows.
Thinking about the overall ecological efficiency in a system brings us back to our first rule for the transfer of energy through trophic levels and up the food chain. For example, If hares consumed kcal of plant energy, they might only be able to form kcal of new hare tissue.
For the hare population to be in steady state neither increasing nor decreasing , each year's consumption of hares by foxes should roughly equal each year's production of new hare biomass. So the foxes consume about kcal of hare biomass, and convert perhaps 10 kcal into new fox biomass.
The overall loss of energy from lower to higher trophic levels is important in setting the absolute number of trophic levels that any ecosystem can contain. From this understanding, it should be obvious that the mass of foxes should be less than the mass of hares, and the mass of hares less than the mass of plants. Generally this is true, and we can represent this concept visually by constructing a pyramid of biomass for any ecosystem see Figure 3. A pyramid of biomass showing producers and consumers in a marine ecosystem.
Pyramids of Biomass, Energy, and Numbers A pyramid of biomass is a representation of the amount of energy contained in biomass, at different trophic levels for a given point in time Figure 3, above, Figure 4-middle below. The amount of energy available to one trophic level is limited by the amount stored by the level below. Because energy is lost in the transfer from one level to the next, there is successively less total energy as you move up trophic levels.
In general, we would expect that higher trophic levels would have less total biomass than those below, because less energy is available to them.
We could also construct a pyramid of numbers , which as its name implies represents the number of organisms in each trophic level see Figure 4-top. For the grassland shown in Figure 4-top, the bottom level would be quite large, due to the enormous number of small plants grasses. For other ecosystems such as the temperate forest, the pyramid of numbers might be inverted: for instance, if a forest's plant community was composed of only a handful of very large trees, and yet there were many millions of insect grazers which ate the plant material.
Just as with the inverted pyramid of numbers, in some rare exceptions, there could be an inverted pyramid of biomass, where the biomass of the lower trophic level is less than the biomass of the next higher trophic level.
The oceans are such an exception because at any point in time the total amount of biomass in microscopic algae is small. Thus a pyramid of biomass for the oceans can appear inverted see Figure 4b-middle.
You should now ask "how can that be? This is a good question, and can be answered by considering, as we discussed above, the all important aspect of "time". Even though the biomass may be small, the RATE at which new biomass is produced may be very large. Thus over time it is the amount of new biomass that is produced, from whatever the standing stock of biomass might be, that is important for the next trophic level.
We can examine this further by constructing a pyramid of energy , which shows rates of production rather than standing crop. Once done, the figure for the ocean would have the characteristic pyramid shape see Figure 4-bottom.
Algal populations can double in a few days, whereas the zooplankton that feed on them reproduce more slowly and might double in numbers in a few months, and the fish feeding on zooplankton might only reproduce once a year. Thus, a pyramid of energy takes into account the turnover rate of the organisms, and can never be inverted. Note that this dependence of one trophic level on a lower trophic level for energy is why, as you learned in the lectures on predation, the prey and predator population numbers are linked and why they vary together through time with an offset.
Figure 4: Pyramids of numbers, biomass, and energy for various ecosystems. The Residence Time of Energy. We see that thinking about pyramids of energy and turnover time is similar to our discussions of residence time of elements.
But here we are talking about the residence time of "energy". This difference in residence time between aquatic and terrestrial ecosystems is reflected in the pyramids of biomass, as discussed above, and is also very important to consider in analyzing how these different ecosystems would respond to a disturbance, or what scheme might best be used to manage the resources of the ecosystem, or how you might best restore an ecosystem that has been degraded e.
Humans and Energy Consumption All of the animal species on Earth are consumers, and they depend upon producer organisms for their food. For all practical purposes, it is the products of terrestrial plant productivity and some marine plant productivity that sustain humans.
What fraction of the terrestrial NPP do humans use, or, "appropriate"? It turns out to be a surprisingly large fraction, which launches us immediately into the question of whether this appropriation of NPP by humans is sustainable.
Let's use our knowledge of ecological energetics to examine this very important issue. Why NPP? Because only the energy "left over" from plant metabolic needs is available to nourish the consumers and decomposers on Earth. In a cropland NPP and annual harvest occur in the same year. In forests, annual harvest can exceed annual NPP for example, when a forest is cut down the harvest is of many years of growth , but we can still compute annual averages.
Note that the following estimates are being successively revised in the literature, but the approach to the problem is always the same. Outputs: 2 Scenarios Total productivity of lands devoted entirely to human activities.
This includes total cropland NPP, and also energy consumed in setting fires to clear land. A high estimate is obtained by including lost productive capacity resulting from converting open land to cities, forests to pastures, and due to desertification and other overuse of land. This is an estimate of the total human impact on terrestrial productivity. Table 1 provides estimates of total NPP of the world. A simple but important example of this potential for "co-limitation" comes from polar regions, where oblique solar insolation combines with deep mixing of surface waters to yield low light availability.
In such environments, higher iron supply can increase the efficiency with which phytoplankton capture light energy Maldonado et al. More broadly, it has been argued that phytoplankton should generally seek a state of co-limitation by all the chemicals they require, including the many trace metal nutrients Morel In contrast to the terrestrial biosphere, most marine photosynthesis is conducted by single-celled organisms, and the more abundant of the multicellular forms are structurally much simpler than the vascular plants on land.
This size range is composed mostly of eukaryotes, organisms whose cells contain complex membrane-bound structures "organelles" , including the cell's nucleus and chloroplasts. Well-studied forms of eukaryotic phytoplankton include the opal-secreting diatoms, prymnesiophytes including the CaCO 3 -secreting coccolithophorids , and the organic wall-forming dinoflagellates.
The centrality of these organisms in early oceanographic thought was due to their accessibility by standard light microscopy. Only with recent technological advances have smaller organisms become readily observable, revolutionizing our view of the plankton.
In particular, the cyanobacteria, which are prokaryotes lacking a nucleus and most other organelles found in eukaryotes , are now known to be important among the phytoplankton. Initially, the cyanobacteria were identified largely with colonial forms such as Trichodesmium that play the critical role of "fixing" nitrogen see below. It is now recognized that two cyanobacterial genera — Synechoccocus and Prochlorococcus — dominate phytoplankton numbers and biomass in the nutrient-poor tropical and subtropical ocean Waterbury et al.
In addition, new methods, both microscopic and genetic, are revealing a previously unappreciated diversity of smaller eukaryotes in the open ocean. Mapping ecological and biogeochemical functions onto the genetic diversity of the phytoplankton is an active area in biological and chemical oceanography. Based on observations as well as theory, the smaller phytoplankton such as the unicellular cyanobacteria are thought to dominate regenerated production in many systems, whereas the larger eukaryotes appear to play a more important role in new production i.
The food source of a given form of zooplankton is typically driven by its own size, with microzooplankton grazing on the prokaryotes and smaller eukaryotes and multicellular zooplankton grazing on larger eukaryotes, both phytoplankton and microzooplankton. Because of their relative physiological simplicity, microzooplankton are thought to be highly efficient grazers that strongly limit the biomass accumulation of their prey.
In contrast, the multicellular zooplankton , because they typically have more complex life histories, can lag behind the proliferation of their prey, allowing them to bloom and sometimes avoid predation altogether and sink directly. The multicellular zooplankton also often facilitate the production of sinking organic matter, for example, through the production of fecal pellets by copepods.
In the nutrient-poor tropical and subtropical ocean, the small cyanobacteria tend to be numerically dominant, perhaps because they specialize in taking up nutrients at low concentrations. Small phytoplankton have a greater surface area-to-volume ratio than do large phytoplankton. A greater proportional surface area promotes the uptake of nutrients across the cell boundary, a critical process when nutrients are scarce, likely explaining why small phytoplankton dominate the biomass in the nutrient-poor ocean.
The microzooplankton effectively graze these small cells, preventing their biomass from accumulating and sinking directly. Moreover, these single-celled microzooplankton lack a digestive tract, so they do not produce the fecal pellets that represent a major mechanism of export. Instead, any residual organic matter remains in the upper ocean, to be degraded by bacteria.
All told, microzooplankton grazing of phytoplankton biomass leads to the remineralization of most of its contained nutrients and carbon in the surface ocean, and thus increases recycling relative to organic matter export. In contrast, larger phytoplankton , such as diatoms, often dominate the nutrient-rich polar ocean, and these can be grazed directly by multicellular zooplankton. By growing adequately rapidly to outstrip the grazing rates of these zooplankton , the diatoms can sometimes accumulate to high concentrations and produce abundant sinking material.
In addition, the zooplankton export organic matter as fecal pellets. Figure 3 The most broadly accepted paradigm for the controls on surface nutrient recycling efficiency.
NPP is supported by both new nutrient supply from the deep ocean and nutrients regenerated within the surface ocean. In the nutrient-poor tropical and subtropical ocean a , the small cyanobacteria tend to be numerically dominant. The microzooplankton that graze these small cells do so effectively, preventing phytoplankton from sinking directly.
Moreover, these single-celled microzooplankton do not produce sinking fecal pellets. Instead, any residual organic matter remains to be degraded by bacteria. In nutrient-rich regions b , large phytoplankton are more important, and these can be grazed directly by multicellular zooplankton. By growing adequately rapidly to outstrip the grazing rates of zooplankton, the large phytoplankton can sometimes accumulate to high concentrations and produce abundant sinking material.
The relationships between nutrient supply, phytoplankton size, and sinking thus dominate this view of upper ocean nutrient cycling. Satellites can measure the color of the surface ocean in order to track the concentration of the green pigment chlorophyll that is used to harvest light in photosynthesis Figure 4.
Higher chlorophyll concentrations and in general higher productivity are observed on the equator, along the coasts especially eastern margins , and in the high latitude ocean Figure 4a and b.
Figure 4 Composite global ocean maps of concentrations of satellite-derived chlorophyll and ship-sampled nitrate NO 3 - ; the dominant N-containing nutrient.
Northern hemisphere summer is shown in the left panels and southern hemisphere summer on the right. In the vast unproductive low- and mid-latitude ocean, warm and sunlit surface water is separated from cold, nutrient-rich interior water by a strong density difference that restricts mixing of water and thereby reduces nutrient supply, which becomes the limiting factor for productivity.
These "ocean deserts" are dissected by areas, mainly at the equator and the eastern margins of ocean basins, where the wind pushes aside the buoyant, warm surface lid and allows nutrient-rich deeper water to be upwelled.
In the high latitude ocean, surface water is cold and therefore the vertical density gradient is weak, which allows for vertical mixing of water to depths much greater than the sunlit "euphotic zone" as a result, the nutrient supply is greater than the phytoplankton can consume, given the available light and iron, see text.
Sea ice cover impedes measurement of ocean color from space, reducing the apparent areas of the polar oceans in the winter hemisphere upper panels. There are caveats regarding the use of satellite-derived chlorophyll maps to deduce productivity, phytoplankton abundance, and their variation. Second, chlorophyll concentration speaks more directly to the rate of photosynthesis i.
Fourth, the depth range sensed by the satellite ocean color measurements extends only to the uppermost ten's of meters, much shallower than the base of the euphotic zone Figure 2.
Compared to nutrient-bearing regions, nutrient-deplete regions e. Thus, satellite chlorophyll observations tend to over-accentuate the productivity differences between nutrient-bearing and -depleted regions. Despite these caveats, satellite-derived ocean color observations have transformed our view of ocean productivity.
In some temperate and subpolar regions, productivity reaches a maximum during the spring as the phytoplankton transition from light to nutrient limitation. In the highest latitude settings, while the "major nutrients" N and P remain at substantial concentrations, the trace metal iron can become limiting into the summer Boyd et al.
In at least some of these polar systems, it appears that light and iron can "co-limit" summertime photosynthesis Maldonado et al. Our planet's climate has changed throughout its long history among various extremes and on different time scales, ranging from millions of years, to just a few millennia, to just a few centuries.
Discover oceanic processes, productivity of life in the ocean, and how ocean organisms and circulation respond to climate change. Our planet's surface is created by tectonic processes, but later molded into shape by water, wind, and ice. Discover the many terrestrial landscapes Earth contains and the processes that create them. Citation: Sigman, D.
Nature Education Knowledge 3 10 Productivity fuels life in the ocean, drives its chemical cycles, and lowers atmospheric carbon dioxide.
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