7.7 Trophic State Index

Carlson's Trophic State Index (TSI) is used as the basis for estimating the trophic status of Minnesota lakes. Trophic status ranges from oligotrophic to hypereutrophic (and is viewed as a continuum) on this scale. Carlson's TSI is based on the interrelationships of TP, chlorophyll-a, and Secchi transparency. The individual TSIs are very useful for understanding the relationship of TP, chlorophyll, and transparency for a given lake and provide the best information on the trophic condition of the lake. If the individual TSI values for a lake do not correspond fairly closely (e.g., within 5 TSI units), then the individual values should be inspected and particular attention should be paid to the number of observations used to calculate the mean values and to determine which parameter might be the more accurate predictor of trophic state. The following notes may be helpful in this regard:

If one index value is based on numerous measures while the others are based on a single measure, then the former is probably the better indicator of trophic state.

If there is only a single measurement for each index value, the phosphorus TSI should be favored as it provides an estimate of the "potential" trophic status of a lake.

Secchi or chlorophyll-a TSIs based on single observations should be viewed with caution.

Secchi TSI values in highly colored waters (see note 7) or waters high in inorganic suspended solids (e.g., clay) may provide a poor estimate of trophic state. This is because the dark coloration or high suspended sediments may limit the amount of algae produced and often will be the primary factor limiting transparency.

Lakes dominated by large colonial algae, such as Aphanizomenon sp. (look like clumps of grass clippings), may have high transparencies (low TSI) relative to the phosphorus concentration. This is because these colonies of algae may form "rafts" or scums at the surface of the water which are easily displaced by wind or lowering of a Secchi disk and hence Secchi readings may be deeper than if the algae were dispersed evenly throughout the water column. This is very common in hypereutrophic lakes and hence Secchi may not be the best indicator of trophic status in highly nutrient-rich lakes.

Lakes with extensive macrophyte (rooted submergent and emergent plants) growth may have higher transparency and lower chlorophyll-a (lower TSIs) than expected based on the phosphorus concentration. These plants may compete with algae for available nutrients like phosphorus.

Ecoregion patterns may also give an indication as to which TSI value is a better reflection of the trophic status of a lake.

This data base may not provide an accurate estimate of the current trophic status of a given lake because of the number of observations or the age of the data. In particular, those labeled as "evaluated" should be viewed with caution since the data may be over ten years old.

Carlson's TSI is based on the interrelationships of TP, chlorophyll-a, and Secchi transparency as depicted.

7.8 Eutrophication

Eutrophication is the gradual increase and enrichment of an ecosystem by nutrients such as nitrogen and phosphorus. Eutrophication occurs naturally usually taking thousands of years, but man induced eutrophication can take decades. 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 aesthetic 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 Asia, 54% of lakes are eutrophic; in Europe, 53%; in North America, 48%; in South America, 41%; and in Africa , 28% (ILEC/Lake Biwa Research Institute, 1988-1993).

7.8.1 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 nutrients 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.

7.8.2 Ecological effects

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 (NO 3 ) has been shown to be toxic to human babies. This is because bacteria can live in their digestive tract that convert nitrate to nitrite (NO 2 ). Nitrite reacts with hemoglobin to form methemoglobin, a form that does not carry oxygen. The baby essentially suffocates when its body receives no oxygen.

7.8.3 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

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 source pollution (also known as 'diffuse' or 'runoff' pollution) is that which comes from ill-defined and diffuse sources. Nonpoint sources are difficult to regulate and usually vary spatially and temporally (with season, precipitation, and other irregular events).

There are three reasons that nonpoint sources are especially troublesome:

1. 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 phosphorus lost to surface waters increases linearly with the amount of phosphorus in the soil. Thus much of the nutrient loading in soil eventually makes 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.

2. Runoff to Surface Water and Leaching to Groundwater

Nutrients from human activities tend to travel from land to either surface or ground water. Nitrogen in particular is removed through storm drains, sewage pipes, and other forms of runoff.

Nutrient losses in runoff and leachate are often associated with agriculture. Modern agriculture often involves the application of nutrients onto fields in order to maximize production. However, farmers frequently apply more nutrients than are taken up by crops or pastures. Regulations aimed at minimizing nutrient exports from agriculture are typically far less stringent than those placed on sewage treatment plants and other point source polluters.

3. 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, especially in highly industrialized regions.

Other Causes

7.8.4 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 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 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 ( Arnold , 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

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