Most of the nitrate present in the ocean results from the recycling of organic materials.  The degradation of plankton,² for example, provides nitrate:

(CH₂O)₁₀₆(NH₃)₁₆(H₃PO₄) + 138 O₂ à 106 CO₂ + 122 H₂O + 19 H⁺ + PO₄^--- + 16 NO₃^-

plankton + oxygen à carbon dioxide + water + hydrogen ion + phosphate + nitrate

Other sources of nitrogen to the ocean are volcanic emissions (mostly as ammonia), fixing of N₂ by blue-green algae, and run off from land.  All of these become part of the nitrogen cycle, and a portion will end up as nitrate.

Marine Organisms that Use Nitrate

A wide variety of organisms are capable of absorbing nitrate with which they synthesize a host of nitrogen-containing organic molecules, such as proteins and DNA.¹  Nitrate is primarily used by microorganisms (such as bacteria) and those organisms that get much or all of their energy from photosynthesis, including algae, corals and sea anemones.

In some circumstances and for some organisms that use nitrate, elevated levels of nitrate can result in increased growth.  For example, shoalgrass (Halodule wrightii), and widgeongrass (Ruppia maritime) grow faster in elevated nitrate (0.6 ppm nitrate) than in typical ambient seawater nitrate levels (<0.1 ppm nitrate).³  The various seagrasses have systems for active uptake of nitrate from both leaves and roots.⁴

Marine bacteria,⁵ phytoplankton,^6,7 and macroalgae,^7,8  have also been shown to increase growth rates with elevated nitrate.

In other cases, elevated nitrate does not increase growth.  In these cases, other factors are limiting, such as phosphate, iron, and light.  The growth of the seagrass Zostera marina, for example, is not enhanced by increased nitrate, with growth more often being limited by light.^3,4,9

Obviously, some of the organisms that grow faster in water with elevated nitrate are not necessarily those that aquarists most prefer.  Beyond the obvious concerns about microalgae, dinoflagellates seem to increase growth as the nitrate and other nutrients increase, up to at least 16 ppm.¹⁰  It may come as a comfort to some aquarists to know that the Aiptasia pulchella can only take up nitrate under starvation conditions, and even then not very well.^11,12

Fish, it seems, are not very sensitive to nitrate.  Most researchers find little toxicity.¹³ One group that studied a variety of species of fish larvae report:

“Judging from its effect on 1st-feeding, unionized NH₃ is a potential hazard in the rearing tank; NO₂^- and NO₃^- are nontoxic at levels likely encountered in practical marine fish culture.”¹⁴

Still, many hobbyists report that their fish appear less healthy when they have allowed nitrate levels to get excessively high (over 50 ppm).  Whether that is actually due to nitrate or something else about the water that is coincident with the nitrate rise is unknown.

Finally, the addition of excess nutrients to natural coral reefs has been blamed for a general transition from corals to turf and macroalgae,¹⁵ but what role nitrate plays relative to other nutrients (such as phosphate) is not always clear. 

Effects of Elevated Nitrate in Aquaria

In addition to the concerns described above relating to the growth of potentially undesirable organisms that may be promoted by elevated nitrate (especially algae and dinoflagellates), corals can be impacted by nitrate. Many corals may not be bothered by elevated nitrate, or may even grow more rapidly with the readily available nitrogen.  But in certain corals, especially those that calcify, there may be negative effects from elevated nitrate.

In most cases where nitrate levels have been examined in relation to the growth of calcerous corals, the effects have been reasonably small, but significant.    Elevated nitrate has been shown to reduce the growth of Porites compressa (at less than 0.3-0.6 ppm nitrate),^16,17 but the effect is eliminated if the alkalinity is elevated as well (to 4.5 meq/L).  One explanation is that the elevated nitrate drives the growth of the zooxanthellae to such an extent that it actually competes with the host for inorganic carbon (used in photosynthesis and skeletal deposition).  When the alkalinity is elevated, this competition no longer deprives the host of needed carbon.¹⁷  

A second study on Porites porites and Montastrea annularis tends to support this hypothesis.  They showed that elevated nitrate caused an increase in photosynthesis, in the density of zooxanthellae, and in their chlorophyll a and c2, and total protein, while skeletal growth decreased considerably.¹⁸  This effect may not be generally true, however, since elevated nitrate does not appear to have decreased calcification in Acropora cervicornis (though the experiments were carried out under very different conditions).¹⁹ 

One very recent study ²⁰ on Porites cylindrica has reported that elevated nitrate (0.9 ppm) did not increase the rate of photosynthesis or zooxanthellae density, but actually decreased it, contrary to the previous literature.  They do not provide an explanation of why their results were different, though they indicated that the corals may have been expelling zooxanthellae, which would confound some of the results.  Additionally, all of the corals in the study were stressed in that they lost significant biomass during the study compared to when first collected in the wild.  Because of that effect, I do not put much faith in how this study may relate to aquaria where corals are growing rapidly.

Measuring Nitrate in Aquaria

Nitrate is fairly easily measured in marine aquaria at levels higher than about 0.5 ppm.  I have found the nitrate kits from LaMotte and Salifert to be quite easy to use, and in my limited testing appear to be accurate enough for aquarium purposes.  Below 0.5 ppm, quantitation is difficult with existing kits.  Habib Sekha, the owner of Salifert, has indicated that it may not be difficult to make kits with lower detection limits if there is a demand for them. So if you want such a kit to be produced commercially, you might contact him.

Other brands of test kits may be suitable, or not.  One group of aquarists carried out tests on a variety of different kits, and the results are shown at this web site (in German).

Sources of Nitrate in Reef Tanks

The primary source of nitrate in reef aquaria is food added to the system.  All proteins contain nitrogen, as do a wide variety of other biomolecules.  When metabolized, much of this nitrogen can end up as nitrate in a process similar to that shown for plankton above.

Other inputs can include the die off of organisms, which also degrade in a fashion similar to that shown above for plankton. 

Finally, the use of unpurified water can lead to significant addition of nitrate to aquaria.  In the United States, drinking water is permitted to contain up to 44 ppm nitrate.  Daily addition of such water to replace evaporated water will provide a significant amount of nitrate.  In many municipal water systems, however, the level of nitrate is much lower.  In my water supply, the level is typically only 0.1 ppm nitrate.

Lowering  Nitrate in Aquaria

The bottom line for many aquarists is that they have nitrate levels in their aquaria that are higher than they prefer.  I strive to keep the nitrate levels in my aquaria below 1 ppm, and preferably undetectable with current hobby kits (less than about 0.5 ppm).  If the ability of the kits to measure lower nitrate levels is enhanced, then I might move my target levels down.  Obviously, the higher the nitrate is, the greater the concern. 

This section outlines a variety of actions that can be taken to reduce nitrate levels in aquaria.  Note that I don’t include any discussion of water changes, though obviously they work to some extent.  The problem is that it is very hard to reduce the nitrate concentration to natural levels in that fashion unless the system is constantly flushed with clean water.

The first activity is to measure nitrate with a quality test kit. Then follow one or more of the actions below and monitor the nitrate over time to see if it is helping.

1.  Reduce the inputs of nitrogen to the aquaria.  If you are overfeeding, stop.  I’m not, however, suggesting that folks starve any organisms in their aquaria for the sake of reducing nitrate levels.  There are better options available.  If you are using tap water, test it for nitrate to see if it is a source, and if so, purify it first.  A reverse osmosis/deionizing system( RO/DI) is best for a variety of reasons, but a simple RO or DI system will likely be adequate for this purpose.

2.  Increase nitrogen export by skimming, or skimming more extensively.  Such skimming alone does not usually permit aquaria to eliminate a nitrate problem, but it can be a significant help, and also has other benefits, such as aeration and phosphorus removal.

3.  Increase nitrogen export by growing and harvesting macroalgae or turf algae (or any other organism of your choice).  The more that you grow and harvest, the more nitrogen will be exported, cutting down on the amount that ends up as nitrate.  The procedure is often effective at driving nitrate levels below those detectable by most aquarium nitrate kits (about 0.5 ppm).  This process also has the advantage of exporting phosphorus.

4.  Use a deep sand bed.  These beds can develop low oxygen regions where nitrate is used by certain organisms to act as an electron acceptor in place of oxygen (O₂). The end result is that nitrate is converted into N₂, and the N₂ blows off of the tank to the atmosphere.  The reactions that take place can be complex.²¹  In oxygen-containing environments, the reaction looks very similar to that shown above for plankton (ignoring phosphorus here):

organic + 175 O₂ --> 122 CO₂ + 16 NO₃^- + 16 H⁺ + 138 H₂O

where organic stands for a typical organic material ((CH₂O) ₈₀ (CH2)₄₂(NH₃)₁₆).  In the absence of O₂, and taking the nitrogen species completely to N₂ (which may happen in several reaction steps), we have the following overall reaction:

organic + 124 NO₃^- + 124 H⁺ --> 122 CO₂ + 70 N₂ + 208 H₂O

In many aquaria, a deep sand bed by itself is adequate to keep nitrate at levels below 0.5 ppm.  In others, it has not been adequate.  Success may depend on the size of the bed, it’s composition (sand type, particle size distribution, and life forms in it), and the demands put on it in terms of nitrate processing.

5. Remove existing filters designed to facilitate the nitrogen cycle.  Such filters do a fine job of processing ammonia to nitrite to nitrate, but do nothing with the nitrate.  It is often non-intuitive to many aquarists, but removing such a filter altogether may actually help reduce nitrate.   So slowly removing them and allowing more of the nitrogen processing to take place on and in the live rock and sand can be beneficial.

It is not that any less nitrate is produced when such a filter is removed, it is a question of what happens to the nitrate after it is produced.

When it is produced on the surface of media such as bioballs, it mixes into the entire water column, and then has to find its way, by diffusion, to the places where it may be reduced (inside of live rock and sand, for instance).

If it is produced on the surface of live rock or sand, then the local concentration of nitrate is higher there than in the first case above, and it is more likely to diffuse into the rock and sand to be reduced to N₂.

6.  Use a carbon-driven denitrator.  There are a variety of different commercial systems available, none of which are especially popular in the United States at this time.  However, they can do a good job of removing nitrate and some aquarists quite like them.

In one of these types of systems, a carbon source is added to a portion of tank water in a low oxygen environment.  In many cases, the carbon source is methanol.  The methanol is mixed with aquarium water in a controlled situation (such as fluid pumped through a coil) and the methanol is consumed by bacteria that use nitrate as an electron acceptor instead of oxygen:

12 NO₃^- + 10 CH₃OH + 12 H⁺  à 10 CO₂ + 6 N₂ + 26 H₂O

The end result is that nitrate is removed from the aquarium.  The typical drawback to such a system is the need for careful control over the conditions, and the consequent complexity that often accompanies such a reactor.

7.  Use a sulfur denitrator.  In  these systems, bacteria use elemental sulfur and produce N₂ from it and nitrate according the following equation (or something similar):

2 H₂O + 5 S + 6 NO₃^-   à 3 N₂ + 5 SO₄^-- + 4 H⁺

It has also been suggested to pass the effluent of such a reactor through a bed of aragonite to use the acid (H⁺) produced to dissolve the calcium carbonate, and thereby provide calcium and alkalinity to the aquarium.

While that is a fine idea, it doesn’t add much calcium and alkalinity to most aquaria.

To estimate the magnitude of the effect, we start with a liberal estimate of how much nitrate might be removed. Say 10 ppm of nitrate per week.

10 ppm nitrate = 0.16 mmole/L of nitrate

Since 4 moles of H⁺ are produced for every 6 moles of nitrate consumed, this will produce

0.107 mmoles/L of H⁺ per week

How much calcium this could produce?
Assume that it takes one proton to dissolve one calcium carbonate:

CaCO₃ + H⁺ ßà Ca⁺⁺ + HCO₃^-

Clearly, this is a substantial overestimate because much of the acid will be used up driving the pH down to the point where CaCO₃ can even begin to dissolve. Consequently, we have an upside limit of

0.107 mmoles of Ca⁺⁺ per week

since calcium weighs 40 mg/mmol, that's

4.3 ppm Ca⁺⁺ per week.