Nitrate in Ecosystems and Drinking Water
Valerie Chase, PhD
Staff Biologist, National Aquarium in Baltimore
501 E. Pratt Street, Baltimore, MD 21202

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No other human-caused global change has occurred so abruptlyas the increased nitrogen content of our natural ecosystems—notincreased carbon dioxide, not increased human population, notdeforestation. That the first edition of Issues in Ecology, anew publication from the Ecological Society of America, focuses on howpeople have altered the global nitrogen cycle indicates the importanceprofessional ecologists place on this problem.

Some of the earliest public attention to the issueappeared in agricultural publications, which addressed the reproductiveproblems of cows and sheep associated with nitrates in water supplies;this was followed by attention to human health concerns.

Marine and estuarine ecologists weighed in next,demonstrating the role of nitrogen in the overproduction of algae andthe changes in algal species in coastal and bay waters"enriched" with nitrogen. In turn, the death and decompositionof algae have led to low oxygen levels (anoxia), which causes mortalityin shellfish and fish.

Ecologists have noted the role of nitrous oxidesin acid rain and photochemical smog. More recently, they have discoveredreduced biodiversity of species in terrestrial ecosystems that areheavily fertilized.

The nitrogen cycle is based on natural events inwhich atmospheric nitrogen is fixed into compounds, largely as ammoniumand nitrate, which are essential mineral nutrients for the growth,repair, and reproduction of plants and algae. Nitrogen enters the foodchain through plants, where it appears in a variety of compounds likeamino acids. Then it is converted back to atmospheric nitrogen bydenitrifying bacteria that live in anaerobic (oxygen-free) mud, or it isreturned to the soil as a mineral nutrient by decomposers (Fig.1).

Nitrogen is fixed in a number of naturalprocesses. Some algae and bacteria convert atmospheric nitrogen intoplant-useable nitrogen. The best known of these live in a symbioticrelationship with legume roots. Lightning is a much smaller source offixed nitrogen. Estimates of the annual natural nitrogen fixation priorto the widespread planting of legume crops range from 90 to 140teragrams (Tg). (A teragram is 1 trillion grams or 1 million metrictons.)

Since nitrogen is often the mineral nutrientlimiting crop plant growth, humans have expended great energy increasingits production in plant-usable form. One method is the widespreadcultivation of legume crops and forage, like soybeans, clover, andalfalfa. More recently, the manufacture of inorganic nitrogen fertilizerhas skyrocketed to double the amount fixed by nitrogen-fixing crops.These 2 sources add approximately 120 Tg of nitrogen per year.

Burning forests, grasslands, and fossil fuelsreleases nitric oxides and other gases. The high-temperature combustionalso oxidizes some of the atmospheric nitrogen. Nitrous and nitric oxidegases add the insult of forming acid rain and photochemical smog.

Even remote regions of the world receive somenitrogen enrichment from atmospheric pollution, but because inorganicfertilizer and legume crops, cars, factories, and power plants are notevenly distributed across the Earth, some regions are more affected bynitrogen release than others. Since nitrate, ammonium, nitric oxide, andnitrous oxide dissolve in water, all end up eventually in groundwaterand in aquatic ecosystems (Fig. 2). The water in developed or highlyagricultural regions generally has the highest levels. Denitrifyingbacteria living in anoxic places (like the mud in wetlands) are the onlymeans by which fixed nitrogen naturally returns to the atmosphere as aninert gas.

Nitrogen fixed in plant products is eaten byanimals and often returns to natural systems in the form of sewage ormanure, and some of the nitrogen is recycled into crops. Unless theamount and timing of the manure or sludge spreading are carefullymonitored, much of the nitrogen from these sources also enters streams,rivers, bays, or coastal seas. Large-scale aquaculture using inorganicfertilizer can also produce severe nutrient loading.

Nitrate in drinking water can be dangerous foryoung animals, including humans. In oxygen-poor stomach environmentsnitrate is reduced to nitrite. Most adult humans convert only about 5%to nitrite, but babies have less stomach acid, a condition that favorsnitrite production.

Fetuses and infants have a different form ofhemoglobin from children and adults. Nitrite combines irreversibly withfetal hemoglobin, preventing it from carrying oxygen. For this reasonthe US Environmental Protection Agency (EPA) sets a limit of 10 partsper million (ppm) of nitrogen as nitrate for public drinking water.There is no evidence that babies have died in the United States fromnitrate pollution, but in Europe, where levels of 2,000 ppm have beenmeasured, some have. Miscarriages and brain damage are also possible.

American farm animals have been affected, however.Ruminants like sheep and cows, with their elaborate, highly anoxicstomachs convert nitrate in water and nitrogen-rich feed stock tonitrite. Reproductive failure in cows and sheep has been traced to highnitrate ingestion on farms where wells have tested as high as 700 ppm ofnitrogen as nitrate.

Fertilization can change the relative competitiverelationships among species, favoring a few at the expense of manyothers. Experiments on a Minnesota native grassland measured speciesdiversity from 20 to 30 species per square meter; this was reduced todominance by a single introduced grass after a number of years offertilization. Changes in species composition can occur in enrichedaquatic systems, also, although typically nitrogen is more likely to bethe growth-limiting nutrient in estuarine and marine systems whilephosphate is the growth limiter in freshwater environments.

Nutrient enrichment is implicated in red tideepisodes and in recent Pfiesteria events in North Carolina andMaryland (suspected to be precipitated by phosphate in hog and chickenmanure runoff). In both cases, naturally occurring toxic dinoflagellatesincurred huge population growth over a short period of time, causingfish kills. A spring 1996 red tide in Florida also produced manateedeaths from inhalation of toxic compounds.

Toxins from Pfiesteria produceneurological problems in humans, and the organism itself attacks fishand human flesh. Thus, the impact of nutrient loading may range from aloss of biodiversity to fish kills and human health hazards.

Nitrate enrichment can alter an entire estuary.Phytoplankton growth is so dense in the summer in the Chesapeake Baythat light penetrates less than 0.5 m. Sea grasses rooted in the bottomhave insufficient light for growth. As the phytoplankton sink to thebottom, they are decomposed by aerobic bacteria, which can completelydeplete the oxygen in bottom waters during calm weather. Deeper watersof the Bay experience anoxic events each summer. Sea grasses havedeclined in many areas, also, mainly because of nutrient loading.

Just as the nature of the problem—sewage,fertilizers, manure, or power plants—varies with the region, so doesthe mix of actions to address these problems. Nitrogen that enterssystems from a specific source—such as a smokestack or a sewageoutfall—can be addressed with regulations and technological solutions.

Most of the nitrogen sources are not within thecurrent scope of regulation, however. No one measures or regulates howmuch fertilizer homeowners, farmers, or golf courses use. Although somesources are very hard to regulate, we might curtail fertilizer usethrough education, incentives, and taxes.

Because nitrate in water is easily and safelymeasured and because its presence is a widespread problem, it makes anexcellent topic for class projects in ecology, environmental science, oragriculture. During the summer of 1990, I started an environmentalscience course for secondary teachers. Over a 2-week period we workedour way across Maryland from the mountains to the sea, examining adifferent environmental issue each day. The first mucky, algae-riddenpond in the mountains barely registered a trace of nitrate (though wedid find a stream with a pH of 4.3).

Then we came down out of the mountains. The firststop in dairy country produced 8 ppm of nitrogen as nitrate from a tapat a fast-food restaurant. All across central Maryland we found 6 to 9ppm in drinking water and in streams and rivers during that dry summer.Algae and aquatic plants can reduce nutrient levels in ponds, lakes, andbays, but groundwater and runoff record the story of nitrate pollution.

From that class on, we have tested nitrate inwater. Over the years, the highest nitrate measured in teacher courseswas 15 ppm from a well in a very expensive residential area. Ironically,that teacher brought her own well water to class to avoid drinking"nasty" Baltimore tap water, which measured only 3 ppm at thattime.

Since nitrate is stable, teachers and students canmake water collections that represent wide geographic regions anddemonstrate nitrate pollution from a variety of sources. They can evenbuy and test bottled water from all over the world.

Recently, a new nitrate test kit has beendeveloped by LaMotte Company that uses zinc reduction (eliminatingcadmium, a toxic heavy metal) and 2 simple tablets to measure nitrogenas nitrate in ppm. Accurate as long as the chemicals are fresh, it isboth safe and easy to use. Consequently, when we rewrote Living inWater in its third edition, we included nitrate as an issue andadded 5 new exercises for nitrate, plus additional watershed activities.Nonpoint source and point source nitrate pollution are measured andmodeled in the third edition of Living in Water.

Ideally, first have students study nitratepollution in class. They then identify a nitrate-enriched environmentwith these test kits (Fig. 3). Encourage them to ask questions about thesource of the pollution and the possible solutions. Classes may plan anddo projects that reduce nitrate pollution either through direct action,such as planting forest buffer strips along waterways, or through publiceducation, such as encouraging individuals to reduce fertilizer use. Thekits can then be used to monitor the results of the student projectsover time.

Since each region varies, detailed ideas for classprojects should be researched locally. Have your students use theInternet, email, or letters to contact state and local agencies as wellas conservation organizations and environmental educators. The Internetmay also lead your students to federal agencies such as the EPA, whichhas information on nitrate pollution.

Nitrate pollution is a large-scale problem inwhich each of us plays some role. While governments have a major role insolving this problem, individual education and action are essential toimprove the situation.

Note: Figs. 2 and 3 are adapted from Living in Water with permission from the National Aquarium in Baltimore.