Article 0009 Introduction to the Nitrogen Cycle
What is the Nitrogen Cycle?
The nitrogen cycle is the process by
which toxic ammonia (NH3) is converted into
nitrites (NO2-), and then into nitrates (NO3-) by
the production of beneficial bacteria. In aquatic systems, especially captive environments, the
nitrogen cycle must be allowed to complete before inhabitants are introduced;
this is referred to as cycling the tank. Once the nitrogen cycle has been established in a new tank, it is a continuous conversion process (hence the word
cycle in nitrogen cycle). Because captive environments are typically very
tiny compared to natural habitats, this biological cycle will at some point require some assistance in the form of tank
maintenance (i.e. gravel vacuuming, water changes, filter changes etc.), which will be discussed
later in the article.
When an aquatic tank is first setup, the water is clean and void of the necessary bacteria to enact the nitrogen cycle. The nitrogen cycle begins
when organic matter begins to decay, usually in the form of fish waste and
uneaten food items. Decaying organic matter creates toxic ammonia in a new tank and semi-toxic ammonium ions (NH3 and
NH4+, respectively), which
are converted into potentially toxic nitrites by the nitrifying
bacteria, Nitrosomonas and Nitrococcus. Nitrites are then converted into
nitrates by another nitrifying bacteria, Nitrobacter.
Nitrates are the end result of the nitrogen cycle, and are only harmful in
excessive amounts.
These bacteria are often dubbed 'beneficial bacteria', and will reside in the
substrate of the tank, and in the biological filter (if one is provided).
When organic matter initially decays, i.e. when an aquatic system is first established, the
ammonia
levels increase to potentially lethal levels (they spike), until Nitrosomonas
and Nitrococcus bacteria are established and convert the ammonia
into nitrites. At this point, the nitrite levels will spike to potentially
lethal levels, until Nitrobacter baceria are established and convert the
nitrites into nitrates. During the initial cycling of a new tank,
the water is highly toxic as ammonia and nitrite levels spike, which is potentially fatal to
most fish and amphibian species.
For this reason, tanks are usually cycled with very hardy fish, such as Zebra
Danios (Brachydanio rerio), Guppies (Poecilia reticulata), or feeder gold fish. Still, it is common to incur fatalities even with the
hardiest of fish.
An alternative to using fish to cycle an aquarium is to add small amounts of frozen
or freeze dried foods. This creates decaying organic matter, which will initiate the nitrogen
cycle. This approach has the same end result as using fish, but with no fatalities, and may also be safer for
amphibian tanks, as fish can carry
diseases and such.
pH, Ammonia, Nitrite, and Nitrate
pH is a measurement of the acidity or alkalinity of a solution. pH stands
for potential
of Hydrogen, and is the negative log of the hydrogen ion
concentration in grams, atoms, or moles per liter of a solution. The pH
scale ranges from 1 - 14, where 7.0 is neutral, below 7.0 is acidic, and higher than
7.0 is alkaline (or basic). The base-10 property of the logarithmic function
indicates that each unit of change of pH is equivalent to a tenfold change in
acidity or alkalinity (ion concentration). As the hydrogen ion content in
solution increases, the pH decreases, or becomes more acidic, and vice
versa. It is worth noting that in acid-base relationships, sometimes the pOH,
potential of hydroxide ion, is relevant also. The relationship between pH
and pOH is as follows: pH + pOH = 14. It is also important to know that
acids and bases are found in equilibrium in solution, such that as one
increases, the other decreases, and vice versa. Speaking in terms of water
solution, such as in an aquarium, H+ acts as an acid, while OH-
acts as a base.
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Calculating pH and pOH values |
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pH = -log [H+] and [H+] = 10-pH |
|
pOH = log -[OH-] and [OH-] =
10-pOH |
Ammonia in an aquarium results from decaying organic matter. Two forms are
present in equilibrium, NH3 and NH4+, the
former of which is the toxic, and potentially lethal form. Typical ammonia
tests measure both forms combined, however, individual tests are available.
The amount of NH3 present is proportional to temperature and pH,
but is more dependent on pH. The percentage of NH3 increases with
increased carbonate hardness, which equates to increased pH, as NH4+
ions are converted to toxic NH3 molecules. At lower pH levels, NH3 converts to NH4+ (in
aqueous solution, ammonia acts as a weak base, and ammonium ion as a weak acid). If
the pH increases, the released H+ (hydrogen ion) rapidly forms NH3
ions with the present NH4+ ions, resulting in high
levels of toxic NH3 molecules. In summary, it can be said that at
low pH levels NH4+ ions predominate, while at higher
pH levels, the more toxic NH3 molecules predominate. It should be
clear that higher ammonia test results at lower pH levels are less lethal to
fish and other animals than the same ammonia test results at higher pH
levels, assuming a combined test kit is used.
|
Ammonia Equilibrium |
|
NH3(aq) + H2O(l)
<======> NH4+(aq)
+ OH-(aq) |
|
NH4+(aq)
<======> NH3(aq) + H+(aq) |
It should also be understood that although ammonia test results are
less lethal at lower pH levels, toxic nitrite levels are higher at such pH levels.
A similar acid/base equilibrium occurs between nitrite (NO2-)
and nitrous acid (HNO2), where the acidic form is predominate in
acidic water, and vice versa. However, in the case of nitrites, the opposite
is true in terms of toxicity, that is, the acidic form is more toxic than
the basic form.
pH, Ammonia, Nitrite, and Nitrate
The bio-load is the amount of organic matter present in an aquarium, in regards
to the nitrogen cycle, and is measured by its proportionality to the volume of water for which it is contained.
For example, a 55 gallon aquarium housing 5 Cynops pyrrhogaster will have a larger bio-load
than a 100 gallon aquarium with the same inhabitants because the amount of organic matter
present at any time is more concentrated in the 55
gallon, and more diluted in the 100 gallon. Also, the bio-load is increased or
decreased when life forms are added or subtracted, and the bacteria
present in the tank must compensate when the bio-load changes. Each time a new member is introduced, the bio-load
increases, and the nitrogen cycle attempts to compensate by increased bacterial
growth. Similarly, when a member is removed, there is a slight
excess in bacteria for which the nitrogen cycle will attempt to compensate. In turn, this disruption of the
nitrogen cycle can cause spikes in ammonia, nitrites, and nitrates, and reduced pH levels.
To avoid shocking the system, it is necessary to introduce new inhabitants
moderation (i.e. few at a
time).
Introducing New Inhabitants - The Bio Load
The bio-load is the amount of organic matter present in an aquarium, in regards
to the nitrogen cycle, and is measured by its proportionality to the volume of water for which it is contained.
For example, a 55 gallon aquarium housing 5 Cynops pyrrhogaster will have a larger bio-load
than a 100 gallon aquarium with the same inhabitants because the amount of organic matter
present at any time is more concentrated in the 55
gallon, and more diluted in the 100 gallon. Also, the bio-load is increased or
decreased when life forms are added or subtracted, and the bacteria
present in the tank must compensate when the bio-load changes. Each time a new member is introduced, the bio-load
increases, and the nitrogen cycle attempts to compensate by increased bacterial
growth. Similarly, when a member is removed, there is a slight
excess in bacteria for which the nitrogen cycle will attempt to compensate. In turn, this disruption of the
nitrogen cycle can cause spikes in ammonia, nitrites, and nitrates, and reduced pH levels.
To avoid shocking the system, it is necessary to introduce new inhabitants
moderation (i.e. few at a
time).
Cycling and Maintenance of Aquatic Tanks
Beneficial bacteria will reside in two key places in an aquarium: the substrate, and biological filter. Once
the bacteria is established, it
should never be completely removed by thorough cleaning of the gravel or removal of the biological filter. If the
bacteria is completely
removed from the tank, the nitrogen cycle will restart, which is potentially fatal to fish and
amphibians. However, bacteria can grow in excess in small aquarium environments, and will require some
maintenance to ensure stable levels. Excess food and waste should be removed from the gravel
by a 1/3 gravel vacuum every month or so, depending on the size
of the tank, the number of inhabitants, and the type of inhabitants. This will keep
ammonia levels at 0 ppm (parts per million), which will in turn help
regulate nitrite and nitrate levels, as well as influence the stability of pH levels. The biological filter
should be lightly cleaned of clogging debris and algae every month, or so, depending on the size
of the tank, the number of inhabitants, and the type of inhabitants.
General Hardness (GH), Carbonate Hardness (KH),
Alkalinity, CO2, and pH
Aquarium water contains dissolved ions in varying amounts, which determine
the hardness of the water. Water hardness is a general term that refers to
water hardness as a whole, however, in the aquarium world, there are two
measurements of water hardness: Carbonate hardness (KH), and general
hardness (GH).In a freshwater system, such as an aquarium, cations (ions with a
positive charge) of magnesium and calcium are present in varying amounts,
and form salts with anions (ions with a negative charge) present in the
water. General hardness (GH) is a measurement of the amount of dissolved
magnesium, and calcium in the water source. For clarity, iron, aluminum,
strontium, and barium may also be present, but provide negligible
contributions to water hardness.
Some confusion of terms has resulted from the original translation of the
German word, Gesamthaerte, abbreviated GH, which means "Total
Hardness". Somewhere in the translation to English, the definition was
obscured to "General Hardness", still abbreviated GH, which is
used in the U.S. In summary, "General Hardness" is synonymous with
"Total Hardness", both abbreviated by GH. Similarly, the
abbreviation KH was derived from the German word Karbonathaerte, meaning
"carbonate hardness".
The terms KH, and carbonate hardness are used synonymously with the term
alkalinity, which often causes confusion in the aquarium world. The terms KH
and carbonate hardness should be used carefully, as they are not accurate descriptions of what is
actually measured by a
typical KH test. A KH test is supposed to measure the buffering capacity of
only the carbonate/bicarbonate buffering relationship in a given water source.
Although carbonate and bicarbonate are the main contributors to the
buffering ability of a water source, there are other contributing ions
present, as well. In actuality, a KH test measures the total alkalinity, or
buffering capacity, of the water source because it also takes into account
other contributing ions. Essentially, a KH test measures alkalinity as a
whole, and does not discriminate between carbonate/bicarbonate ions and
other contributing ions. This is because alkalinity is measured by acid
titration, which does not discriminate between ions; any significant base
present in the solution will be titrated. A true carbonate hardness test
would measure only the buffering capabilities of the carbonate and
bicarbonate system. Although the alkalinity in an aquarium is usually a
reflection mainly of carbonate & bicarbonate, this is not always the
case.
When using pH/KH/CO2 tables to determine CO2
concentration, errors can occur as a result of carbonate hardness test
results that include other ion measurements, in addition to bicarbonate.
This is because such tables are based on true carbonate hardness
measurements, not alkalinity. In general, KH test results of soft water,
where phosphates or other salts are not present or are negligible,
bicarbonate is the major buffer component, and so levels of KH are very
close to the alkalinity value. In this case, because KH is equal, or nearly
equal to alkalinity, pH/KH/CO2 tables are virtually accurate.
However, in the case of hard water, where phosphates or other buffers are
present, the measurement of alkalinity and carbonate hardness will be
significantly different values. In this case, a KH test will read higher
than accurate, and inaccurate values from pH/KH/CO2 tables will
result.
Although there is not a direct method of measuring true carbonate
hardness, the value can be calculated indirectly by using a pH/KH/CO2 tables,
in conjunction with accurate CO2 and pH tests. After determining
the pH and CO2, the tables can be used to determine the true KH
measurement.
Permanent and temporary hardness are terms that also appear ambiguously
in the aquarium world. Permanent hardness is essentially GH minus the hardness resultant from salts of magnesium and calcium, other
than carbonates and bicarbonates. Temporary hardness is essentially the GH
minus the permanent hardness, that is, the hardness resultant from magnesium
and calcium carbonates and bicarbonates. This is not to say that you should
actually subtract units, however, this may give a mental perspective of what
is actually meant by the terms permanent and temporary hardness.
| Carbonate Hardness (KH) = measurement of alkalinity, or
buffering capacity, resultant only from
carbonates and bicarbonates of calcium and magnesium. Carbonate Hardness is
also referred to as Temporary Hardness because it can easily be manipulated.
Carbonate Hardness is also value of the Total Hardness minus the Permanent
Hardness. Carbonate Hardness, or KH, tests actually measure the alkalinity
of a solution, which is the hardness resultant from all contributing
substances in the solution, including carbonates and bicarbonates. KH, measured in ppm, or DH, measured in degrees, where one unit DH
is equal to 17.8 ppm. Permanent Hardness = measurement of waters total,
or general, hardness minus the carbonate hardness. The permanent
hardness is a measurement of salts of calcium and magnesium, other
than carbonates and bicarbonates.
General Hardness (GH) = measurement of total water hardness.
Alkalinity = the ability of a solution to resist changes
to pH; buffering capacity. The higher the alkalinity, the more
stable the pH. The larger the
buffering capacity, the more stable the pH. When a solution becomes saturated (low buffering
capacity), the pH changes rapidly.
pH = negative logarithm of the hydrogen ion concentration present in
a solution; potential of Hydrogen. This equates to the measurement of acidity
and alkalinity of a solution. On a scale of 1.0 - 14.0, where each increment
represents a 10 fold change in pH and 7.0 is neutral, below 7.0 is acidic,
and above 7.0 is alkaline. Acidic solutions are the result of high hydrogen
ion concentrations, and alkaline solutions are the result of low hydrogen
ion concentration.
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| |
|
CO2 + H20 <======> H2CO2
(Carbon Dioxide + water = Carbonic Acid) |
|
H2CO2 <======> H+
+ HCO3- (Carbonic Acid => Hydrogen ion
& Bicarbonate) |
|
HCO3- <======> H+
+ CO32- (= Bicarbonate => Hydrogen ion
& Carbonate) |
To soften hard water, diluted de-ionized or spring waters are sometimes, and
are usually more effective than chemical alternatives. De-ionized water should
never be used alone with amphibians because it is void of ions, and can
disrupt the chemical composition of amphibian cells (see
article
0006 - Water Quality and Amphibians for more information). Instead,
de-ionized, distilled, or spring waters are usually mixed in some ratio with
hard water to form a softer solution.
To maintain
hardened water, crushed coral can be added to the substrate, or placed in mesh
bags and added to the tank or filter. A few natural additions, such as driftwood
or peat, can be used to lower pH values. With any method, the pH should be incremented very slowly, so as not to
shock the inhabitants. A change of no more than .3 units per day is recommended.
References:
Green, David M, Stanley K. Sessions. Amphibian
Cytogenetics and Evolution.
Academic Press, 1997.
McDiarmid, Roy W., and Ronald Altig. Tadpoles: The
Biology of Anuran Larvae.
University of Chicago Press, 2000.
The Nitrogen Cycle (2001). Algone.com, Dedicated to the Aquarium Hobbiest and
Fish Enthusiest. http://www.algone.com/cycle.htm
(Accessed: 2001).
CO2 & Water Hardness. The Krib.
http://www.thekrib.com/Plants/CO2
(accessed: 2002).
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