Carbon dioxide in the Earth's atmosphere
Carbon dioxide in the Earth's atmosphere is present in a low concentrations. Regardless, it is essential to photosynthesis in plants and other photoautotrophs, and is also a prominent greenhouse gas due to its radiative forcing strength.
Concentration
Atmospheric CO2 concentrations measured at Mauna Loa Observatory.
As of January 2007, the CO2 concentration in Earth's atmosphere was about 0.0383% by volume, or 383 ppmv. This is 99 ppm (38%) above the 1832 ice core levels of 284 ppm. The National Oceanic and Atmospheric Administration describes current levels "...as a dry mole fraction defined as the number of molecules of carbon dioxide divided by the number of molecules of dry air multiplied by one million (ppm)." and the chart as of July 2007 shows a level of about 387 ppm.
There is an annual fluctuation of about 3-9 ppm in the measurements, which roughly follows the Northern Hemisphere's growing season. The Northern Hemisphere dominates the annual cycle of CO2 concentration because it has much greater land area and plant biomass than the Southern Hemisphere. Concentrations peak in May as the spring greenup begins and reach a minimum in October when the quantity of biomass undergoing photosynthesis is greatest.
Despite its relativly small concentration overall in the atmosphere, CO2 is an important component of Earth's atmosphere because it absorbs infrared radiation at wavelengths of 4.26 µm (asymmetric stretching vibrational mode) and 14.99 µm (bending vibrational mode), thereby playing a role in the greenhouse effect. See also "Carbon dioxide equivalent".
The three vibrational modes of carbon dioxide: (a) symmetric, (b) asymmetric stretching; (c) bending. In (a), there is no change in dipole moment, thus interaction with photons is impossible, while in (b) and (c) there is optical activity.
The initial carbon dioxide in the atmosphere of the young Earth was produced by volcanic activity. This was essential for a warm and stable climate conducive to life. Volcanic activity now releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year, which is less than 1% of the amount released by human activities.
Global fossil carbon emissions 1800 – 2000.
From 1832 to 2004, the atmospheric CO2 concentration increased from 284 ppmv to 377 ppmv, or about 33%, with most of the change occurring since 1970. Burning fossil fuels such as coal and petroleum is the leading cause of increased man-made CO2; deforestation is the second major cause. Around 24 billion tonnes of CO2 are released from fossil fuels per year worldwide, equivalent to about 6 billion tonnes of carbon. (See List of countries by carbon dioxide emissions.)
Carbon dioxide is released to the atmosphere by a variety of natural sources, and over 95% percent of total CO2 emissions would occur even if humans were not present on Earth. For example, the natural decay of organic material in forests and grasslands, such as dead trees, results in the release of about 220 billion tons of carbon dioxide every year. These natural sources are balanced by natural sinks, which remove carbon dioxide from the atmosphere.The increase in carbon dioxide concentration arises because the increase from human activity is not balanced by a corresponding sink.
Smoke and ozone pollution from Indonesian fires, 1997.
In 1997, Indonesian peat fires may have released 13% – 40% as much carbon as fossil fuel burning does in a single year.Various techniques have been proposed for removing excess carbon dioxide from the atmosphere in carbon dioxide sinks. Not all the emitted CO2 remains in the atmosphere; some is absorbed in the oceans or biosphere. The ratio of the emitted CO2 to the increase in atmospheric CO2 is known as the airborne fraction (Keeling et al., 1995); this varies for short-term averages but is typically 57% over longer (5 year) periods.
Increased amounts of CO2 in the atmosphere enhance the greenhouse effect. It is currently the majority scientific opinion that carbon dioxide emissions are the main cause of global warming observed since the latter half of the 20th century. The effect of combustion-produced carbon dioxide on climate is occasionally called the Callendar effect, after engineer and inventor Guy Stewart Callendar who was one of the first to propose this association.
Origins
Natural sources of atmospheric carbon dioxide include volcanic outgassing, the combustion of organic matter, and the respiration processes of living aerobic organisms; man-made sources of carbon dioxide come mainly from the burning of fossil fuels for heating, power generation and transport. It is also produced by various microorganisms from fermentation and cellular respiration. Plants convert carbon dioxide to carbohydrates during a process called photosynthesis. They produce the energy needed for this reaction through the photolysis of water. The resulting gas, oxygen, is released into the atmosphere by plants, which is subsequently used for respiration by heterotrophic organisms, forming a cycle.
Historical variation
CO2 concentrations over the last 400,000 years
The most direct method for measuring atmospheric carbon dioxide concentrations for periods before direct sampling is to measure bubbles of air (fluid or gas inclusions) trapped in the Antarctic or Greenland ice caps. The most widely accepted of such studies come from a variety of Antarctic cores and indicate that atmospheric CO2 levels were about 260 – 280 ppmv immediately before industrial emissions began and did not vary much from this level during the preceding 10,000 years (10 kyr).
The longest ice core record comes from East Antarctica, where ice has been sampled to an age of 800 kyr BP (Before Present).During this time, the atmospheric carbon dioxide concentration has varied by volume between 180 – 210 ppm during ice ages, increasing to 280 – 300 ppm during warmer interglacials. The data can be accessed here.
Some studies have disputed the claim of stable CO2 levels during the present interglacial of the last 10 kyr. Based on an analysis of fossil leaves, Wagner et al.argued that CO2 levels during the period 7 – 10 kyr ago were significantly higher (~300 ppm) and contained substantial variations that may be correlated to climate variations. Others have disputed such claims, suggesting they are more likely to reflect calibration problems than actual changes in CO2. Relevant to this dispute is the observation that Greenland ice cores often report higher and more variable CO2 values than similar measurements in Antarctica. However, the groups responsible for such measurements (e.g., Smith et al.) believe the variations in Greenland cores result from in situ decomposition of calcium carbonate dust found in the ice. When dust levels in Greenland cores are low, as they nearly always are in Antarctic cores, the researchers report good agreement between Antarctic and Greenland CO2 measurements.
Changes in carbon dioxide during the Phanerozoic (the last 542 million years). The recent period is located on the left-hand side of the plot, and it appears that much of the last 550 million years has experienced carbon dioxide concentrations significantly higher than the present day.
On longer timescales, various proxy measurements have been used to attempt to determine atmospheric carbon dioxide levels millions of years in the past. These include boron and carbon isotope ratios in certain types of marine sediments, and the number of stomata observed on fossil plant leaves. While these measurements give much less precise estimates of carbon dioxide concentration than ice cores, there is evidence for very high CO2 volume concentrations between 200 and 150 myr BP of over 3,000 ppm and between 600 and 400 myr BP of over 6,000 ppm.On long timescales, atmospheric CO2 content is determined by the balance among geochemical processes including organic carbon burial in sediments, silicate rock weathering, and vulcanism. The net effect of slight imbalances in the carbon cycle over tens to hundreds of millions of years has been to reduce atmospheric CO2. The rates of these processes are extremely slow; hence they are of limited relevance to the atmospheric CO2 response to emissions over the next hundred years. In more recent times, atmospheric CO2 concentration continued to fall after about 60 myr BP, and there is geochemical evidence that volume concentrations were less than 300 ppm by about 20 myr BP. Low CO2 concentrations may have been the stimulus that favored the evolution of C4 plants, which increased greatly in abundance between 7 and 5 myr BP. Present carbon dioxide levels are likely higher now than at any time during the past 20 myr[17] and certainly higher than in the last few hundred thousand.
Relationship with oceanic concentration
Air-sea exchange of CO2
The Earth's oceans contain a huge amount of carbon dioxide in the form of bicarbonate and carbonate ions — much more than the amount in the atmosphere. The bicarbonate is produced in reactions between rock, water, and carbon dioxide. One example is the dissolution of calcium carbonate:
CaCO3 + CO2 + H2O ⇌ Ca2+ + 2 HCO3-
Reactions like this tend to buffer changes in atmospheric CO2. However, since it produces an acidic compound, the pH of sea water is thought to go down with increasing carbon dioxide levels. Reactions between carbon dioxide and non-carbonate rocks also add bicarbonate to the seas, which can later undergo the reverse of the above reaction to form carbonate rocks, releasing half of the bicarbonate as CO2. Over hundreds of millions of years this has produced huge quantities of carbonate rocks.
The vast majority of CO2 added to the atmosphere will eventually be absorbed by the oceans and become bicarbonate ion, but the process takes on the order of a hundred years because most seawater rarely comes near the surface.
As the oceans warm, carbon dioxide solubility in the surface waters decreases markedly. However, the overall system is quite complex, as indicated above, and further details may be found in the article on the carbon solubility pump.
An unknown, though probably large, quantity of CO2 is in the ocean sediments as a methane-carbon dioxide-water clathrates, one of the family of gas hydrates.
Friday, September 28, 2007
Thursday, September 27, 2007
Eutrophication
Eutrophication, strictly speaking, means an increase in chemical nutrients -- typically compounds containing nitrogen or phosphorus -- in an ecosystem. It may occur on land or in water. The term is however often used to mean the resultant increase in the ecosystem's primary productivity -- in other words excessive plant growth and decay -- and even further impacts, including lack of oxygen and severe reductions in water quality and in fish and other animal populations.
Lakes, rivers, and oceans
The bright green water in the Potomac River estuary is the result of a dense bloom of cyanobacteria.Eutrophication is frequently a result of nutrient pollution such as the release of sewage effluent and run-off from lawn fertilizers into natural waters (rivers or coasts) although it may also occur naturally in situations where nutrients accumulate (e.g. depositional environments) or where they flow into systems on an ephemeral basis (e.g. intermittent upwelling in coastal systems). Eutrophication generally promotes excessive plant growth and decay, favors certain weedy species over others, and is likely to cause severe reductions in water quality . In aquatic environments, enhanced growth of choking aquatic vegetation or phytoplankton (that is, an algal bloom) disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of oxygen in the water, needed for fish and shellfish to survive. The water then becomes cloudy, colored a shade of green, yellow, brown, or red. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment.
Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the mid-20th century.Since then, it has become more widespread. Surveys showed that 54% of lakes in Asia are eutrophic; in Europe, 53%; in North America, 48%; in South America, 41%; and in Africa, 28%.
Eutrophication can be a natural process in lakes, as they fill in through geological time, though other lakes are known to demonstrate the reverse process, becoming less nutrient rich with time. Estuaries also tend to be naturally eutrophic because land-derived utrients are concentrated where run-off enters the marine environment in a confined channel.
Human activities can accelerate the rate at which nutrients enter ecosystems. Runoff from agriculture and development, pollution from septic systems and sewers, and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems (including coral reefs). Elevated atmospheric compounds of nitrogen can increase soil nitrogen availability.
Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to point source pollution from sewage. The concentration of algae and the trophic state of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication. Humankind has increased the rate of phosphorus cycling on Earth by four times, mainly due to agricultural fertilizer production and application. Between 1950 and 1995, 600,000,000 tonnes of phosphorus were applied to Earth's surface, primarily on croplands (Carpenter et al. 1998). Control of point sources of phosphorus have resulted in rapid control of eutrophication, mainly due to policy changes.
Terrestrial ecosystems
Although traditionally thought of as enrichment of aquatic systems by addition of fertilizers into lakes, bays, or other semi-enclosed waters (even slow-moving rivers), terrestrial ecosystems are subject to similarly adverse impacts.[4] Increased content of nitrates in soil frequently leads to undesirable changes in vegetation composition and many plant species are endangered as a result of eutrophication in terrestric ecosystems, e.g. majority of orchid species in Europe. Ecosystems (like some meadows, forests and bogs that are characterized by low nutrient content and species-rich, slowly growing vegetation adapted to lower nutrient levels) are overgrown by faster growing and more competitive species-poor vegetation, like tall grasses, that can take advantage of unnaturally elevated nitrogen level and the area may be changed beyond recognition and vulnerable species may be lost. Eg. species-rich fens are overtaken by reed or reedgrass species, spectacular forest undergrowth affected by run-off from nearby fertilized field is turned into a thick nettle and bramble shrub.
Chemical forms of nitrogen are most often of concern with regard to eutrophication because plants have high nitrogen requirements so that additions of nitrogen compounds stimulate plant growth (primary production). Nitrogen is not readily available in soil because N2, a gaseous form of nitrogen, is very stable and unavailable directly to higher plants. Terrestrial ecosystems rely on microbial nitrogen fixation to convert N2 into other physical forms (such as nitrates). However, there is a limit to how much nitrogen can be utilized. Ecosystems receiving more nitrogen than the plants require are called nitrogen-saturated. Saturated terrestrial ecosystems contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient.[5] However, because phosphorus is generally much less soluble than nitrogen, it is leached from the soil at a much slower rate than nitrogen. Consequently, phosphorus is much more important as a limiting nutrient in aquatic systems..
Ecological effects
Eutrophication is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit.
Many ecological effects can arise from stimulating primary production, but there are three particularly troubling ecological impacts: decreased biodiversity, changes in species composition and dominance, and toxicity effects.
• Increased biomass of phytoplankton
• Toxic or inedible phytoplankton species
• Increases in blooms of gelatinous zooplankton
• Decreased biomass of benthic and epiphytic algae
• Changes in macrophyte species composition and biomass
• Decreases in water transparency (increased turbidity)
• Color, smell, and water treatment problems
• Dissolved oxygen depletion
• Increased incidences of fish kills
• Loss of desirable fish species
• Reductions in harvestable fish and shellfish
• Decreases in perceived aesthetic value of the water body
Decreased biodiversity
When an ecosystem experiences an increase in nutrients, primary producers reap the benefits first. In aquatic ecosystems, species such as algae experience a population increase (called an algal bloom). Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. Oxygen is required by all respiring plants and animals and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off. In extreme cases, anaerobic conditions ensue, promoting growth of bacteria such as Clostridium botulinum that produces toxins deadly to birds and mammals. Zones where this occurs are known as dead zones.
New species invasion
Eutrophication may cause competitive release by making abundant a normally limiting nutrient. This process causes shifts in the species composition of ecosystems. For instance, an increase in nitrogen might allow new, competitive species to invade and out-compete original inhabitant species. This has been shown to occur in New England salt marshes.
Toxicity
Some algal blooms, otherwise called "nuisance algae" or "harmful algal blooms," are toxic to plants and animals. Toxic compounds they produce can make their way up the food chain, resulting in animal mortality. Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten, neuro- and hepatotoxins are released which can kill animals and may pose a threat to humans.[10][11] An example of algal toxins working their way into humans is the case of shellfish poisoning. Biotoxins created during algal blooms are taken up by shellfish (mussels, oysters), leading to these human foods acquiring the toxicity and poisoning humans. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Other marine animals can be vectors for such toxins, as in the case of ciguatera, where it is typically a predator fish that accumulates the toxin and then poisons humans. Nitrogen can also cause toxic effects directly. When this nutrient is leached into groundwater, drinking water can be affected because concentrations of nitrogen are not filtered out. Nitrate (NO3) has been shown to be toxic to human babies. This is because bacteria can live in their digestive tract that convert nitrate to nitrite (NO2). Nitrite reacts with hemoglobin to form methemoglobin, a form that does not carry oxygen. The baby essentially suffocates as its body receives insufficient oxygen.
Prevention and reversal
Eutrophication poses a problem not only to ecosystems, but to humans as well. Reducing eutrophication should be a key concern when considering future policy, and a sustainable solution for everyone, including farmers and ranchers, seems feasible. While eutrophication does pose problems, humans should be aware that natural runoff (which causes algal blooms in the wild) is common in ecosystems and should thus not reverse nutrient concentrations beyond normal levels.
Effectiveness
Cleanup measures have been mostly, but not completely, successful. Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. These efforts have had a 90% removal efficiency. Still, some targeted point sources did not show a decrease in runoff despite reduction efforts.
Minimizing nonpoint pollution: future work
Nonpoint pollution is the most difficult source of nutrients to manage. The literature suggests, though, that when these sources are controlled, eutrophication decreases. The following steps are recommended to minimize the amount of pollution that can enter aquatic ecosystems from ambiguous sources.
Riparian buffer zones
Studies show that intercepting non-point pollution between the source and the water is a successful mean of prevention (Carpenter et al., 1998). Riparian buffer zones,an interface between a flowing body of water and land, have been created near waterways in an attempt to filter pollutants; sediment and nutrients are deposited here instead of in water. Creating buffer zones near farms and roads is another possible way to prevent nutrients from traveling too far. Still, studies have shown that the effects of atmospheric nitrogen pollution can reach far past the buffer zone. This suggests that the most effective means of prevention is from the primary source.
Prevention policy
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems,[6] but it is generally agreed that a policy regulating agricultural use of fertilizer and animal waste must be imposed. In Japan the amount of nitrogen produced by livestock is adequate to serve the fertilizer needs for the agriculture industry.Thus, it is not unreasonable to command livestock owners to clean up animal waste — which when left stagnant will leach into ground water.
Nitrogen testing and modeling
Soil Nitrogen Testing (N-Testing) is a technique that helps farmers optimize the amount of fertilizer applied to crops. By testing fields with this method, farmers saw a decrease in fertilizer application costs, a decrease in nitrogen lost to surrounding sources, or both.By testing the soil and modeling the bare minimum amount of fertilizer needed, farmers reap economic benefits while the environment remains clean.
Natural state of algal blooms
Although the intensity, frequency and extent of algal blooms has tended to increase in response to human activity and human-induced eutrophication, algal blooms are a naturally-occurring phenomenon. The rise and fall of algae populations, as with the population of other living things, is a feature of a healthy ecosystem.Rectification actions aimed at abating eutrophication and algal blooms are usually desirable, but the focus of intervention should not necessarily be aimed at eliminating blooms, but towards creating a sustainable balance that maintains or improves ecosystem health.
Lakes, rivers, and oceans
The bright green water in the Potomac River estuary is the result of a dense bloom of cyanobacteria.Eutrophication is frequently a result of nutrient pollution such as the release of sewage effluent and run-off from lawn fertilizers into natural waters (rivers or coasts) although it may also occur naturally in situations where nutrients accumulate (e.g. depositional environments) or where they flow into systems on an ephemeral basis (e.g. intermittent upwelling in coastal systems). Eutrophication generally promotes excessive plant growth and decay, favors certain weedy species over others, and is likely to cause severe reductions in water quality . In aquatic environments, enhanced growth of choking aquatic vegetation or phytoplankton (that is, an algal bloom) disrupts normal functioning of the ecosystem, causing a variety of problems such as a lack of oxygen in the water, needed for fish and shellfish to survive. The water then becomes cloudy, colored a shade of green, yellow, brown, or red. Human society is impacted as well: eutrophication decreases the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and aesthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment.
Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the mid-20th century.Since then, it has become more widespread. Surveys showed that 54% of lakes in Asia are eutrophic; in Europe, 53%; in North America, 48%; in South America, 41%; and in Africa, 28%.
Eutrophication can be a natural process in lakes, as they fill in through geological time, though other lakes are known to demonstrate the reverse process, becoming less nutrient rich with time. Estuaries also tend to be naturally eutrophic because land-derived utrients are concentrated where run-off enters the marine environment in a confined channel.
Human activities can accelerate the rate at which nutrients enter ecosystems. Runoff from agriculture and development, pollution from septic systems and sewers, and other human-related activities increase the flux of both inorganic nutrients and organic substances into terrestrial, aquatic, and coastal marine ecosystems (including coral reefs). Elevated atmospheric compounds of nitrogen can increase soil nitrogen availability.
Phosphorus is often regarded as the main culprit in cases of eutrophication in lakes subjected to point source pollution from sewage. The concentration of algae and the trophic state of lakes correspond well to phosphorus levels in water. Studies conducted in the Experimental Lakes Area in Ontario have shown a relationship between the addition of phosphorus and the rate of eutrophication. Humankind has increased the rate of phosphorus cycling on Earth by four times, mainly due to agricultural fertilizer production and application. Between 1950 and 1995, 600,000,000 tonnes of phosphorus were applied to Earth's surface, primarily on croplands (Carpenter et al. 1998). Control of point sources of phosphorus have resulted in rapid control of eutrophication, mainly due to policy changes.
Terrestrial ecosystems
Although traditionally thought of as enrichment of aquatic systems by addition of fertilizers into lakes, bays, or other semi-enclosed waters (even slow-moving rivers), terrestrial ecosystems are subject to similarly adverse impacts.[4] Increased content of nitrates in soil frequently leads to undesirable changes in vegetation composition and many plant species are endangered as a result of eutrophication in terrestric ecosystems, e.g. majority of orchid species in Europe. Ecosystems (like some meadows, forests and bogs that are characterized by low nutrient content and species-rich, slowly growing vegetation adapted to lower nutrient levels) are overgrown by faster growing and more competitive species-poor vegetation, like tall grasses, that can take advantage of unnaturally elevated nitrogen level and the area may be changed beyond recognition and vulnerable species may be lost. Eg. species-rich fens are overtaken by reed or reedgrass species, spectacular forest undergrowth affected by run-off from nearby fertilized field is turned into a thick nettle and bramble shrub.
Chemical forms of nitrogen are most often of concern with regard to eutrophication because plants have high nitrogen requirements so that additions of nitrogen compounds stimulate plant growth (primary production). Nitrogen is not readily available in soil because N2, a gaseous form of nitrogen, is very stable and unavailable directly to higher plants. Terrestrial ecosystems rely on microbial nitrogen fixation to convert N2 into other physical forms (such as nitrates). However, there is a limit to how much nitrogen can be utilized. Ecosystems receiving more nitrogen than the plants require are called nitrogen-saturated. Saturated terrestrial ecosystems contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient.[5] However, because phosphorus is generally much less soluble than nitrogen, it is leached from the soil at a much slower rate than nitrogen. Consequently, phosphorus is much more important as a limiting nutrient in aquatic systems..
Ecological effects
Eutrophication is apparent as increased turbidity in the northern part of the Caspian Sea, imaged from orbit.
Many ecological effects can arise from stimulating primary production, but there are three particularly troubling ecological impacts: decreased biodiversity, changes in species composition and dominance, and toxicity effects.
• Increased biomass of phytoplankton
• Toxic or inedible phytoplankton species
• Increases in blooms of gelatinous zooplankton
• Decreased biomass of benthic and epiphytic algae
• Changes in macrophyte species composition and biomass
• Decreases in water transparency (increased turbidity)
• Color, smell, and water treatment problems
• Dissolved oxygen depletion
• Increased incidences of fish kills
• Loss of desirable fish species
• Reductions in harvestable fish and shellfish
• Decreases in perceived aesthetic value of the water body
Decreased biodiversity
When an ecosystem experiences an increase in nutrients, primary producers reap the benefits first. In aquatic ecosystems, species such as algae experience a population increase (called an algal bloom). Algal blooms limit the sunlight available to bottom-dwelling organisms and cause wide swings in the amount of dissolved oxygen in the water. Oxygen is required by all respiring plants and animals and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen greatly increases during the day, but is greatly reduced after dark by the respiring algae and by microorganisms that feed on the increasing mass of dead algae. When dissolved oxygen levels decline to hypoxic levels, fish and other marine animals suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off. In extreme cases, anaerobic conditions ensue, promoting growth of bacteria such as Clostridium botulinum that produces toxins deadly to birds and mammals. Zones where this occurs are known as dead zones.
New species invasion
Eutrophication may cause competitive release by making abundant a normally limiting nutrient. This process causes shifts in the species composition of ecosystems. For instance, an increase in nitrogen might allow new, competitive species to invade and out-compete original inhabitant species. This has been shown to occur in New England salt marshes.
Toxicity
Some algal blooms, otherwise called "nuisance algae" or "harmful algal blooms," are toxic to plants and animals. Toxic compounds they produce can make their way up the food chain, resulting in animal mortality. Freshwater algal blooms can pose a threat to livestock. When the algae die or are eaten, neuro- and hepatotoxins are released which can kill animals and may pose a threat to humans.[10][11] An example of algal toxins working their way into humans is the case of shellfish poisoning. Biotoxins created during algal blooms are taken up by shellfish (mussels, oysters), leading to these human foods acquiring the toxicity and poisoning humans. Examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Other marine animals can be vectors for such toxins, as in the case of ciguatera, where it is typically a predator fish that accumulates the toxin and then poisons humans. Nitrogen can also cause toxic effects directly. When this nutrient is leached into groundwater, drinking water can be affected because concentrations of nitrogen are not filtered out. Nitrate (NO3) has been shown to be toxic to human babies. This is because bacteria can live in their digestive tract that convert nitrate to nitrite (NO2). Nitrite reacts with hemoglobin to form methemoglobin, a form that does not carry oxygen. The baby essentially suffocates as its body receives insufficient oxygen.
Prevention and reversal
Eutrophication poses a problem not only to ecosystems, but to humans as well. Reducing eutrophication should be a key concern when considering future policy, and a sustainable solution for everyone, including farmers and ranchers, seems feasible. While eutrophication does pose problems, humans should be aware that natural runoff (which causes algal blooms in the wild) is common in ecosystems and should thus not reverse nutrient concentrations beyond normal levels.
Effectiveness
Cleanup measures have been mostly, but not completely, successful. Finnish phosphorus removal measures started in the mid-1970s and have targeted rivers and lakes polluted by industrial and municipal discharges. These efforts have had a 90% removal efficiency. Still, some targeted point sources did not show a decrease in runoff despite reduction efforts.
Minimizing nonpoint pollution: future work
Nonpoint pollution is the most difficult source of nutrients to manage. The literature suggests, though, that when these sources are controlled, eutrophication decreases. The following steps are recommended to minimize the amount of pollution that can enter aquatic ecosystems from ambiguous sources.
Riparian buffer zones
Studies show that intercepting non-point pollution between the source and the water is a successful mean of prevention (Carpenter et al., 1998). Riparian buffer zones,an interface between a flowing body of water and land, have been created near waterways in an attempt to filter pollutants; sediment and nutrients are deposited here instead of in water. Creating buffer zones near farms and roads is another possible way to prevent nutrients from traveling too far. Still, studies have shown that the effects of atmospheric nitrogen pollution can reach far past the buffer zone. This suggests that the most effective means of prevention is from the primary source.
Prevention policy
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems,[6] but it is generally agreed that a policy regulating agricultural use of fertilizer and animal waste must be imposed. In Japan the amount of nitrogen produced by livestock is adequate to serve the fertilizer needs for the agriculture industry.Thus, it is not unreasonable to command livestock owners to clean up animal waste — which when left stagnant will leach into ground water.
Nitrogen testing and modeling
Soil Nitrogen Testing (N-Testing) is a technique that helps farmers optimize the amount of fertilizer applied to crops. By testing fields with this method, farmers saw a decrease in fertilizer application costs, a decrease in nitrogen lost to surrounding sources, or both.By testing the soil and modeling the bare minimum amount of fertilizer needed, farmers reap economic benefits while the environment remains clean.
Natural state of algal blooms
Although the intensity, frequency and extent of algal blooms has tended to increase in response to human activity and human-induced eutrophication, algal blooms are a naturally-occurring phenomenon. The rise and fall of algae populations, as with the population of other living things, is a feature of a healthy ecosystem.Rectification actions aimed at abating eutrophication and algal blooms are usually desirable, but the focus of intervention should not necessarily be aimed at eliminating blooms, but towards creating a sustainable balance that maintains or improves ecosystem health.
Monday, September 17, 2007
Forest ecology
Forest ecology is the scientific study of patterns and processes in forests. The management of forests is known as forestry.
Scope of Forest Ecology
Forest ecology is one branch of a biotically-oriented classification of types of ecological study (as opposed to a classification based on organizational level or complexity (e.g population or community ecology)). Thus, forests can be, and are, studied at any number of organizational levels, from the individual organism to the ecosystem. However, as the term forest connotes an area inhabited by more than one organism, forest ecology most often concentrates on the level of the population, community or ecosystem. Logically, trees are an important component of forest research, but the wide variety of other life forms and abiotic components in most forests means that other elements, such as wildlife or soil nutrients, are often the focal point. Thus, forest ecology is a highly diverse and important branch of ecological study.
Unique Aspects of Forest Ecology
Forest ecology studies share characteristics and methodological approaches with other areas of terrestrial plant ecology. However, the presence of trees makes forest ecosystems and their study unique in at least four ways.
Community diversity & complexity
First, since trees grow to much larger sizes than other plant life-forms, there is the potential for a wide variety of forest structures (or physiognomies). The infinite number of possible spatial arrangements of trees of varying size and species makes for a highly intricate and diverse micro-environment in which environmental variables such as solar radiation, temperature, relative humidity, and wind speed can vary greatly over large and small distances. In addition, an important proportion of a forest ecosystem's biomass is often underground, where variations in soil structure, water quality and quantity, and levels of various soil nutrients can vary greatly. Thus, forests are often highly heterogeneous environments compared to other terrestrial plant communities. This heterogeneity in turn greatly affects how forest studies are designed and executed. It also affects the design of forest inventory sampling strategies, the results of which are sometimes used in ecological studies.
Energy potential
Second, forests accumulate large amounts of standing biomass, and many are capable of accumulating it at high rates, i.e. they are highly productive. Such high levels of biomass and tall vertical structures represent large stores of potential energy that can be converted to kinetic energy under the right circumstances. Two such conversions of great importance are fires and treefalls, both of which radically alter the biota and the physical environment where they occur. Also, in forests of high productivity, the rapid growth of the trees themselves induces biotic and environmental changes, although at a slower rate and lower intensity than relatively instantaneous disturbances such as fires.
Death in the forest ecosystem
Third, the woody materials in many forests decay relatively slowly in comparison to most other organic materials, due to a combination of environmental factors and wood chemistry (see lignin). Trees growing in arid and/or cold environments do so especially slowly. Thus, tree trunks and branches can remain on the forest floor for long periods, affecting such things as wildlife habitat, fire behavior, and tree regeneration processes. This is also an important factor in the science of dendrochronology.
Water
Lastly, forest trees store large amounts of water because of their large size and anatomical/physiological characteristics. They are therefore important regulators of hydrological processes, especially those involving groundwater hydrology and local evaporation and rainfall/snowfall patterns. Thus, forest ecological studies are sometimes closely aligned with meteorological and hydrological studies in regional ecosystem or resource planning studies
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