Eutrophication

Eutrophication is the gradual increase and enrichment of an ecosystem by nutrients such as nitrogen and phosphorus. 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), there is gathering evidence that terrestrial ecosystems are subject to similarly adverse impacts (APIS, 2005). The increase in available nutrients promotes plant growth, favoring certain species over others and forcing a change in species composition. 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. Human society is impacted as well: eutrophic conditions decrease the resource value of rivers, lakes, and estuaries such that recreation, fishing, hunting, and esthetic enjoyment are hindered. Health-related problems can occur where eutrophic conditions interfere with drinking water treatment (Bartram, et al., 1999).

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. 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).

Contents

1 Concept of eutrophication 2 Ecological effects 2.1 Decreased biodiversity 2.2 New species introduction 2.3 Toxicity 3 Sources of high nutrient runoff 3.1 Point sources 3.2 Nonpoint sources 3.2.1 Soil retention 3.2.2 Runoff to surface water and leaching to groundwater 3.2.3 Atmospheric deposition 3.3 Other causes 4 Prevention and reversal 4.1 Effectiveness 4.2 Minimizing nonpoint pollution: future work 4.2.1 Riparian buffer zones 4.2.2 Prevention policy 4.2.3 Nitrogen testing and modeling 4.3 Natural state of algal blooms 5 See also 6 References

Concept of eutrophication

Eutrophication can be a natural process in lakes, occurring as they age through geological time. Also, estuaries tend to be naturally somewhat eutrophic because land-derived nutrient are concentrated where run-off enters the marine environment in a confined channel (Bianchi et al., 2000). However, 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. Elevated atmospheric compounds of nitrogen can increase soil nitrogen availability.

Chemical forms of nitrogen are most often of concern with regard to eutrophication because plants have high nitrogen requirements—additions of nitrogen compounds stimulate plant growth. Nitrogen is not readily available in soil because N₂, a gaseous form of nitrogen, is highly stable and basically unusable by higher plants. Terrestrial ecosystems rely on microbial nitrogen fixation to convert N₂ into other chemical forms. However, there is a limit to how much additional nitrogen can be utilized. Ecosystems with nitrogen inputs in excess of plant nutritional requirements are referred to as nitrogen saturated. Over-saturated terrestrial ecosystems contribute both inorganic and organic nitrogen to freshwater, coastal, and marine eutrophication, where nitrogen is also typically a limiting nutrient (Hornung, et al., 1995). However, in marine environments, phosphorus may be limiting because it is leached from the soil at a much slower rate than nitrates, which are highly soluble (Smith et al., 1999).

Ecological effects

Adverse effects on lakes, reservoirs, rivers, and coastal oceans caused by eutrophication (from Carpenter et al., 1998; modified from Smith 1998)

Increased biomass of phytoplankton Toxic or inedible phytoplankton species Increases in blooms of gelatinous zooplankton Increased biomass of benthic and epiphytic algae Changes in macrophyte species composition and biomass Loss of coral reef communities Decreases in water transparency Taste, odor, and water treatment problems Oxygen depletion Increased incidence of fish kills Loss of desirable fish species Reductions in harvestable fish and shellfish Decreases in perceived esthetic value of the water body

Numerous ecological effects can arise as primary production is stimulated, but there are three particularly troubling ecological impacts: decreased biodiversity, changes in species composition and dominance, and toxicity effects.

Decreased biodiversity

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.

Toxicity

Some algal blooms, otherwise called "nuisance algae" or "harmful algal blooms," 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

Characteristics of point and nonpoint sources of chemical inputs (from Carpenter et al, 1998; modified from Novonty and Olem 1994)

Point Sources Wastewater effluent (municipal and industrial) Runoff and leachate from waste dispolal systems Runoff and infiltration from animal feedlots Runoff from mines, oil fields, unsewered industrial sites Overflows of combined storm and sanitary sewers Runoff from construction sites >20,000 m² Nonpoint Sources Runoff from agriculture/irrigation Runoff from pasture and range Urban runoff from unsewered areas Septic tank leachate Runoff from construction sites <20,000 m² Runoff from abandoned mines Atmospheric deposition over a water surface Other land activities generating contaminants

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

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 sources

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):

Soil retention

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).

Atmospheric deposition

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).

Other causes

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.

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, 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.

Prevention policy

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.

See also

• list of environment topics

References

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