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Eutrophication is the enrichment of an aquatic system by addition of nutrients. It is typically caused by leached phosphorus or nitrogen containing compounds into lakes, rivers, bays, or other semi-enclosed waters (even slow-moving rivers). The addition of these nutrients disrupts the ecosystem, leading to new species introduction, decreased biodiversity, and toxicity.
Theory of eutrophication
A problem to humans and ecosystems
While eutrophication can be a naturally occurring process in aging lakes and estuaries (Bianchi et al., 2000), human activities can accelerate the rate at which nutrients enter aquatic ecosystems from surrounding watersheds. Runoff from agriculture and development, pollution from septic systems and sewers, and other human-related activities increase the flow of organic substances into these aquatic ecosystems.
Eutrophication was recognized as a pollution problem in European and North American lakes and reservoirs in the middle of the twentieth century (Rohde, 1969). Since then, it has become more widespread, causing deterioration of aquatic environments and serious problems when ecosystems are disrupted. Humans may be affected as well; eutrophic conditions interfere with the recreational use of rivers, lakes, and estuaries such that recreation, fishing, hunting, and esthetic enjoyment are hindered. Other health-related hazards, such as hindering drinking water treatment, also arise (Bartram, et al., 1999).
Surveys have shown that in the Asia-Pacific Region, 54% of lakes are eutrophic; in Europe, 53%; in Africa, 28%; in North America, 48%; and in South America, 41% (ILEC/Lake Biwa Research Institute, 1988-1993).
Disturbance through limiting resources
Many ecosystems have a limiting nutrient—that is, a nutrient in decreased abundance that causes competition among organisms. Nitrogen (N) is most limiting to production in estuaries and coastal ecosystems. Phosphorus (P) can also be a limiting nutrient in some coastal systems and most freshwater systems (Carpenter et al., 1998). It has been shown, though, that Carbon, Silica, and Iron may also be limiting nutrients in marine ecosystems (Sundareshwar et al., 2003), but the bulk of eutrophic literature suggests P and N are the most significant limiting nutrients.
Nitrogen is a critical nutrient in estuaries, coasts, and terrestrial ecosystems because plants have high nitrogen requirements. In addition, nitrogen is not readily available in soil because N2, a gaseous form of nitrogen, is more stable. Terrestrial ecosystems rely on microbial nitrogen fixation to break nitrogen down into usable forms. Alternately, marine environments rely more on phosphorus because it is leached from the soil at a slower rate than nitrate (Smith et al., 1999).
Ecosystems are disturbed when these previously sparse limiting nutrients increase in abundance. Plants and animals that could once not grow or become established now take over, causing a form of competitive release.
This competitive release experienced in an ecosystem causes an increase in species that could not previously exist. Several ecological effects can arise, but there are three particularly troubling ecological effects.
When a body of water experiences an increase in nutrients, primary producers reap the benefits first. This means that species such as algae experience a massive population boom (called an algal bloom). Algal blooms tend to disturb the ecosystem by limiting sunlight to bottom dwelling organisms and by reducing the amount of dissolved oxygen available in the environment. Oxygen is required by all respiring plants and animals in an aquatic environment and it is replenished in daylight by photosynthesizing plants and algae. Under eutrophic conditions, dissolved oxygen is reduced by the dense population, and additional oxygen is taken up by microorganisms feeding on dead algae. When dissolved oxygen levels decline, especially at night when there is no photosynthesis, hypoxia occurs and fish or other marine animals may suffocate. As a result, creatures such as fish, shrimp, and especially immobile bottom dwellers die off (Horrigan et al. 2002).
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 introduction
Eutrophication has been shown to cause competitive release by making abundant an otherwise limiting nutrient. This causes shifts in the composition of ecosystems. For instance, an increase in nitrogen might allow new, more competitive species to invade and out compete original species. This has been shown (Bertness et al. 2001) in New England salt marshes, but this study suggests that nitrogen loading isn’t the only disruptive quality about human development. Physical disruptions that humans bring—such as loss of ground cover or increased sunlight—could also be factors that allow new species to invade, above and beyond nitrogen leaching.
Some algal blooms, otherwise called "nuisance algae," are toxic to plants and animals. As stated above, this toxicity can lead to decreased biodiversity, or it can manifest itself in primary producers, making its way up the food chain. As a result of these toxic algae, marine animal mortality has been observed (Anderson 1994). Freshwater algal blooms also pose a threat to livestock. When these blooms die or are eaten, neuro- and hepatotoxins are released which can kill animals and may pose a threat to humans (Lawton and Codd 1991, Martin and Cooke 1994).
Ultimately, these toxins can work their way up to humans, as is the case in shellfish poisoning (Shumway 1990). Biotoxins created during algal blooms can become manifested in shellfish, leading to a variety of poisoning in humans. Such examples include paralytic, neurotoxic, and diarrhoetic shellfish poisoning. Theoretically, other marine animals could also be vectors for such toxins.
There are also toxic effects caused directly by nitrogen. 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 when its body receives no oxygen.
Sources of high nutrient runoff
In order to gauge how to best prevent eutrophication from occurring, specific sources that contribute to nutrient loading must be identified. There are two common sources of nutrients and organic matter: point and nonpoint sources.
Point sources are directly attributable to one influence. In point sources the nutrient waste travels directly from source to water. For example, factories that have waste discharge pipes directly leading into a water body would be classified as a point source. Point sources are relatively easy to regulate.
Nonpoint pollution is that which could potentially come from large areas. Nonpoint sources are difficult to regulate and usually vary temporally (with season, precipitation, and other irregular events ).
General human presence brings with it a variety of nonpoint sources. It has been shown that N transport is correlated with various indices of human activity in watersheds (Cole et al. 1993, Howarth et al. 1996) and with amount of development (Bertness et al. 2001). Agriculture and development are activities that contribute most to nutrient loading.
There are three reasons that nonpoint sources are especially troublesome (Smith et al., 1999):
Nutrients from human activities tend to accumulate in soils and remain there for years. It has been shown (Sharpley et al. 1996) that the amount of P lost to surface waters increases linearly with the amount of P in the soil. Thus any nutrient loading onto soil will eventually make its way to water. Furthermore, phosphorus has the capacity to be released from the soil after a lag time of 10 years. Nitrogen, similarly, has a turnover time of decades or more.
Runoff to surface water and leaching to groundwater
Nutrients from human activities tend to travel from land to either surface or ground water. Quite simply, this is because water is human's primary waste disposal system; nitrogen is removed through storm drains, sewage pipes, and other forms of runoff.
This is most common in agriculture. Common agricultural practices require a large input of nutrients into fields in order to sustain production. Farmers frequently over-fertilize, and nutrient inputs to crops far exceed outputs (Buol 1995). It has also been shown that regulations on agricultural runoff are far less stringent than those placed on sewage treatment (Carpenter et al., 1998).
Nitrogen is released into the air because of ammonia volatilization and nitrous oxide production. The combustion of fossil fuels is a large human-initiated contributor to atmospheric nitrogen pollution. Atmospheric deposition (e.g., in the form of acid rain) can also effect nutrient concentration in water (Paerl 1997).
Any factor that causes increased nutrient concentrations can potentially lead to eutrophication. In modeling eutrophication, the rate of water renewal plays a critical role; stagnant water is allowed to collect more nutrients than bodies with replenished water supplies. It has also been shown that the drying of wetlands causes an increase in nutrient concentration and subsequent eutrophication booms (Mungall and McLaren).
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.
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, which involved removal of phosphorus, have had a 90% removal efficiency (Raike et al., 2003). 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 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 (Agnold, 1997) that the effects of airborne nitrogen pollution can reach far past the buffer zone. This suggests that the most effective means of prevention is from the primary source.
Laws regulating the discharge and treatment of sewage have led to dramatic nutrient reductions to surrounding ecosystems (Smith et al., 1999), but there needs to be policy regulating agricultural use of fertilizer and animal waste. In Japan the amount of nitrogen produced by livestock is adequate to serve the fertilizer needs for the agriculture industry (Kumazawa, 2002). Thus, it is not unreasonable to command livestock owners to clean up animal waste—which when left stagnant will leach into ground water—and convert it into fertilizer. In fact, this law, the 1999 Law on the Appropriate Treatment and Optimization of Livestock Manure, is a step in the right direction. As with the Finnish prevention measures, these prevention and cleanup efforts are successful!
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 (Huang et al., 2001). 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
Algal blooms may be destructive, but they are not unnatural. In fact, a natural cycle where populations rise and crash, such as in the Baltic Sea, can be a part of a healthy marine ecosystem (Bianchi et al., 2000). In this case regulation is desirable but reversal measures, if excessive, can be just as counterproductive. Thus, the aim of restoration efforts must then be not to eliminate the blooms, but return them to their original frequency.
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