Energy at the Surface of the Earth: An Introduction to the Energetics of Ecosystems

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The temporary tends to be those which are larval forms of adults living on the seashore, e. In freshwater the temporary plankton is very limited. These are the producers of energy and may be the base of many food webs. The principle component is the diatom, a form of alga. The basic form is a single-celled organism with the outer wall made from a high level of silica.

This is one of the most important nutrients needed for growth. They multiply by cell division and if there is sufficient silica in the water see below the cells will grow and then split again. Sometimes the cells do not separate and remain together so that chains of diatoms form. Those like Asterionella form a circle and are quite distinctive under the microscope. Phytoplankton is very productive. Although at any one time the amount of primary production is limited called standing crop over a period like a year the net productivity can be higher than a meadow.

This is down to the ability for diatoms to quickly multiply, less than half an hour to double. Only a few diatoms need survive and they can quickly reproduce. Diatoms have diurnal rhythms in the water column. As soon as the sun is up they commence photosynthesis. The process generates oxygen and so the diatom becomes buoyant and begins to rise in the water.

This causes it to move closer to the surface and gain more light for maximum photosynthesis. Of course, when the sun sets food production halts, no oxygen is formed and is used up. Buoyancy is lost and it starts to sink. If it sinks below the lit zone before photosynthesis starts the next day it will continue to sink and dies on the bottom.

The animal plankton will need to follow the phytoplankton. Some of these animals will be herbivores eating the diatoms. Daphnia moves using its antennae but the degree of jerky movement is quite limited and so is referred to as plankton. To help them with their buoyancy during summer months they develop an enlarged head! Other animals in the plankton are carnivores, e. Cyclops, and will eat the small Daphnia.

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The density of zooplankton is linked to the density of diatoms and often the release of animal young is synchronised with the time of year and blooms. Consumers are organisms that ingest organic compounds to obtain energy.

An organism that eats a primary producer is called a primary consumer. An organism that eats a primary consumer is called a secondary consumer. There is rarely enough energy or stored biomass available in an ecosystem for more than a quaternary consumer. These relationships can be represented in a food chain or web. Not all energy is transferred from one trophic level to the next.

The number of organisms, biomass and energy at each trophic level can be represented as a pyramid. Detritivores or decomposers feed on dead or decaying organic matter to obtain energy. They form an important part of any food web as they release energy back into the ecosystem. Just like on land, there are many animals involved in the decay process.

Various shortened terms can be used. The table shows how a fallen leaf can be converted into FPOM, suitable for filter feeders to consume. Primary productivity can be high in ponds. In turn this leads to high numbers of consumers and detritivores. For much of freshwater the primary source of energy is sunlight. But the amount of sunlight which penetrates the water can vary due to:.

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On the equator in the middle of the day light rays may hit the water at a right angle. However, as latitude increases, and as the angle becomes more acute, especially in the amount reflected increases while the amount absorbed decreases. If the water is crystal clear, light may penetrate over 40 metres down into a lake. However the intensity will be extremely low, just a few percent of what was at the surface.

By 10 metres depth already two thirds of the light has disappeared. As well as light intensity, the wavelength of light is also important. On land, yellow-green light the middle bands of light are of least importance to plants whilst blue and red light the two extremes of the visible spectrum are the most important. This latter group penetrate water quickly but do not go deep. Green penetrates the furthest. This means that the green plants need to remain near the upper regions of the photic zone as that is where the light is most suitable for photosynthesis.

Some plants, especially algae, will be able to survive lower down as they produce specialised accessory pigments to pick up the wavelengths of light that will pass into the deeper areas. They appear brown or even black.

Early life on land and the first terrestrial ecosystems

Turbidity may be generated by organic matter in the form of detritus from decay or it could be in a peaty area where humic acid flows into the lake. This can create very dark water. Alternatively high nutrient levels will encourage high densities of plankton to grow. The dissolved minerals present will depend very much on the geology of the land from which the water inflow crosses. In uplands of granite and other igneous rock this may be quite low. Calcium ions are essential for maximising population densities as it is needed for skeletal tissue, cell walls and shells in molluscs. Nitrogen enters in the form of ammonia or nitrates.

The latter may be due to run-off from agricultural land and will encourage eutrophication. This means a substantial growth of plant material. Phosphate occurs naturally in small amounts and combines with iron to form ferric phosphate, precipitating to the benthic region. However, phosphate is now a significant pollutant of water entering from farmland and will also result in eutrophication. Organic matter is an important source of nutrients. Decomposition releases valuable nutrients and is vital for the recycling of materials within freshwater ecosystems.

This is called autochthonous material - materials obtained through recycling. Leaves falling from trees in autumn may be blown significant distances but the moment they touch the surface tension they stop and eventually sink to be trapped and decomposed. Nutrients derived from matter originating outside freshwater are called allochthonous. Freshwater animals use oxygen that is dissolved in the water. Oxygen is sometimes in short supply. The lower the temperature the higher the saturation of oxygen. Conversely the warmer the water gets the lower the amount of oxygen that can dissolve in it.

Even when it is fully saturated water contains little oxygen. At 5 degrees and at normal air pressure one litre of water contains only 8. At higher altitudes this lessens but the lowering level of oxygen may be compensated for by the reduction in temperature that allows a higher amount to saturate the water.

The amount of oxygen consumed by animals will increase as temperature rises. Oxygen is essential for most of the life in freshwater. Some will be able to live anaerobically in the mud. They may either be permanent anaerobes, e. But the majority of organisms need a steady supply of oxygen and if the temperatures rises this will be a problem. The rate of decomposition within the benthos will be determined by the amount of oxygen present.

In oligotrophic lakes the oxygen levels may well be moderately high in the benthos compared to eutrophic ones that can be deoxygenated. The most significant aspect to pH is the amount of carbonic acid present. Carbon dioxide disolves in water to produce this and so it is a measurement of the level of CO 2 available for photosynthesis.

The ions dissociate to yield hydrogen and hydrogen carbonate. Measurement of the pH of a pond over a 24 hour period will demonstrate this well. At dawn the water will be acidic due to the high level of carbon dioxide released through respiration by all the living organisms present. However, as the sunlight becomes available photosynthesis occurs and the level of carbon acid declines. By mid-day the water may become increasingly alkaline until the sun sets.

With the reduction in light photosynthesis halts and only respiration is occuring in the organisms. Some animals are specific to calcium-rich, alkaline water like the crayfish. Snails need calcium for their shells and so will be limited to water rich in this mineral. Plankton may also be specific to alkaline waters.

Temperature is one of the major factors affecting freshwater ecosystems. Water has a high heat storage capacity, which means that freshwater systems have a smaller temperature range on both a yearly and a hour timescale compared to terrestrial ecosystems. However, the main effects of temperature are:. The reason that organisms are affected by temperature is largely due to the fact that they are cold blooded or poikilothermic.

The external environment will determine their internal temperature and therefore metabolic activity. The water in a pond and lake is warmed by solar energy. If the pond is very shallow there could be a substantial increase in temperature over a diurnal period with cooling at night. With the high heat storage capacity the larger the water body the less effect the sun will have over the day. However, the upper region of a lake will warm in the sun and, if wind turbulence is low, an underlying cool layer will be present.

This is thermal stratification and is very important in determining other abiotic and biotic factors. Remembering that very cold water is at its most dense the warm water at the surface called the epilimnion will be least dense and "float" over this substantially cooler, dense water layer called hypolimnion. The water between the two will show a rapid change in temperature over a relatively short distance.

This sharp change in temperature is called the thermocline. This stratification tends to be present during summer and winter when disturbance is at a minimum. In spring and autumn there is the greatest chance of wind-induced wave action. 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.

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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.

Ecosystem Energetics: Reviewed and Revisited

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. Primary production of the biosphere: Integrating terrestrial and oceanic components. Gough, C. The legacy of harvest and fire on ecosystem carbon storage in a north temperate forest. Global Change Biology 13 , — Controls on annual forest carbon storage: Lessons from the past and predictions for the future. Bioscience 58 , — Haberl, H.

Quantifying and mapping the human appropriation of net primary production in earth's terrestrial ecosystems. Melillo, J. Global climate-change and terrestrial net primary production. Nature , — Potter, C. Terrestrial ecosystem production - a process model-based on global satellite and surface data. Global Biogeochemical Cycles 7 , — Prince, S. Global primary production: A remote sensing approach. Journal of Biogeography 22 , — Roy, J. Terrestrial Global Productivity. Zhao, M. Drought-induced reduction in global terrestrial net primary production from through Introduction to the Basic Drivers of Climate.

Terrestrial Biomes. Coral Reefs. Energy Economics in Ecosystems. Biodiversity and Ecosystem Stability. Biological Nitrogen Fixation. Ecosystems Ecology Introduction. Factors Affecting Global Climate. Rivers and Streams: Life in Flowing Water. The Conservation of Mass. The Ecology of Carrion Decomposition. Causes and Consequences of Biodiversity Declines. Earth's Ferrous Wheel. Alternative Stable States.

Recharge Variability in Semi-Arid Climates. Secondary Production. Food Web: Concept and Applications. Terrestrial Primary Production: Fuel for Life. Aa Aa Aa. Measuring Gross and Net Primary Production. Terrestrial Primary Production and Global Change. Ecosystem ecologists have long been interested in quantifying and understanding what controls terrestrial primary production.


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While gross primary production GPP is the total influx of carbon into an ecosystem through the photosynthetic fixation of CO 2 , net primary production NPP is this gross carbon influx discounted for plant respiratory costs of growth and maintenance. Net primary production forms the base of ecological food chains and is heavily manipulated by humans in the production of food, fiber, wood, and increasingly biofuels. Climate, disturbance, and ecological succession exert influences on terrestrial NPP and GPP, suggesting that mounting anthropogenic influences on global climate and land-use will have substantial effects on the future primary production of terrestrial ecosystems.

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