BY: RANDY HOLMES-FARLEY
Nitrate in the Reef Aquarium
Nitrate is an ion that has long dogged aquarists. The nitrogen that it is formed from comes in with foods, and in many aquaria it builds up and can be difficult to keep at natural levels. A decade or two ago, many aquarists performed water changes with nitrate reduction as one of the primary goals. Fortunately, we now have a large array of ways to keep nitrate in check, and modern aquaria suffer far less from elevated nitrate than they have in the past.
Nitrate is often associated with algae, and indeed the growth of algae is often spurred by excess nutrients, including nitrate. The same can be said for other potential pests in aquaria, such as dinoflagellates. Nitrate itself is not particularly toxic at the levels usually attained in aquaria, at least as it is so far known in the scientific literature. Nevertheless, elevated nitrate can excessively spur the growth of zooxanthellae, which in turn can actually decrease the growth rate of the host coral.
For these reasons, most reef aquarists strive to keep nitrate levels down. Some are very successful, and others are not. This article provides background on nitrate in the ocean and in aquaria, and describes a number of techniques that aquarists have successfully used to keep nitrate levels down to more natural levels in reef aquaria.
Nitrate in the Ocean
Nitrogen takes many forms in the ocean,1 one of which is nitrate. Other forms include dinitrogen (N2), ammonia (NH3/NH4+), nitrite (NO2-), and a myriad of nitrogen-containing organic compounds. Of the inorganic species, nitrate is often, but not always the highest in concentration. Concentrations in the ocean vary considerably from location to location, and also with depth.2 Surface waters are much lower in concentration due to scavenging by various organisms, and are often less than 0.1 ppm nitrate (not that all concentrations in this article are in ppm nitrate ion, and not in ppm nitrate nitrogen). Deeper waters typically range from 0.5 to 2.5 ppm nitrate. Surface regions where upwelling of deeper water takes place will also have these higher values.
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 Ã 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.â€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.
(CONT.)
Nitrate in the Reef Aquarium
Nitrate is an ion that has long dogged aquarists. The nitrogen that it is formed from comes in with foods, and in many aquaria it builds up and can be difficult to keep at natural levels. A decade or two ago, many aquarists performed water changes with nitrate reduction as one of the primary goals. Fortunately, we now have a large array of ways to keep nitrate in check, and modern aquaria suffer far less from elevated nitrate than they have in the past.
Nitrate is often associated with algae, and indeed the growth of algae is often spurred by excess nutrients, including nitrate. The same can be said for other potential pests in aquaria, such as dinoflagellates. Nitrate itself is not particularly toxic at the levels usually attained in aquaria, at least as it is so far known in the scientific literature. Nevertheless, elevated nitrate can excessively spur the growth of zooxanthellae, which in turn can actually decrease the growth rate of the host coral.
For these reasons, most reef aquarists strive to keep nitrate levels down. Some are very successful, and others are not. This article provides background on nitrate in the ocean and in aquaria, and describes a number of techniques that aquarists have successfully used to keep nitrate levels down to more natural levels in reef aquaria.
Nitrate in the Ocean
Nitrogen takes many forms in the ocean,1 one of which is nitrate. Other forms include dinitrogen (N2), ammonia (NH3/NH4+), nitrite (NO2-), and a myriad of nitrogen-containing organic compounds. Of the inorganic species, nitrate is often, but not always the highest in concentration. Concentrations in the ocean vary considerably from location to location, and also with depth.2 Surface waters are much lower in concentration due to scavenging by various organisms, and are often less than 0.1 ppm nitrate (not that all concentrations in this article are in ppm nitrate ion, and not in ppm nitrate nitrogen). Deeper waters typically range from 0.5 to 2.5 ppm nitrate. Surface regions where upwelling of deeper water takes place will also have these higher values.
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 Ã 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.â€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.
(CONT.)