Aquarium Chemistry: Nitrate in the Reef Aquarium

Discussion in 'General Discussions and Advice' started by B00tCamp, 19 Apr 2012.

  1. B00tCamp


    1 Mar 2012
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    Hey Everyone!Found this article on Nitrates in the aquarium...just though i'd share...The link to the original article can be found at the bottom.Please feel free to comment and share your thoughts.

    Aquarium Chemistry: Nitrate in the Reef Aquarium

    By Randy Holmes-Farley

    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 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 → 106 CO2 + 122 H2O + 19 H+ + PO43- + 16 NO3-
    plankton + oxygen → 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+ → 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- → 3 N2 + 5 SO42- + 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+ → Ca2+ + 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 Ca2+ per week since calcium weighs 40 mg/mmol, that's 4.3 ppm Ca2+ per week.
    For comparison, an aquarist adding 2% of the tank volume in saturated limewater daily is adding on the order of 16 ppm of calcium per day. Consequently, this method may not be especially useful for maintaining calcium and alkalinity levels. On the other hand, the acid produced will have a long term lowering effect on the alkalinity, so if you use it, watch the alkalinity.
    As to its actual ability to reduce nitrate, I cannot say for sure. I expect that it can be made to work, but the only aquarist that I have spoken to that uses one has had considerable difficulty with it.
    8. AZ-NO3.

    This product is a material that you add directly to the aquarium, and it has been reviewed by Randy Doniwitz..22 I’ve not been able to determine from the product description what exactly it is or what it claims to do, other than to do something to the nitrate that then allows it to be exported by skimming. In general I am reluctant to recommend things that I do not understand, and consequently do not understand the potential undesirable effects (if any). This product, in particular, claims to have other effects: “AZ-NO3TM provides many other benefits besides nitrate reduction.”
    Nevertheless, a number of aquarists that I have talked to have used the product to reduce nitrate without apparent bad effects.
    9. Nitrate Absorbing Solids.

    Various aluminum oxide and zeolite products have been sold to aquarists for many years for the purpose of binding nitrate out of the solution. Kent’s nitrate sponge is one example. I’ve not tested any myself. Many aquarists report that it does work, but takes a long time and a lot of material.
    10. Polymers And Carbon That Bind Organics

    These are similar to skimming in that they remove organics from the system, preventing them from degrading and contributing to the organic load. Examples are Purigen by Seachem and Poly-Filters by Poly-Bio-Marine. I’ve not used any of these for this purpose, and have not heard of others significantly reducing elevated nitrate levels with them.

    In the past, elevated nitrate was something that many aquarists accepted as a fact of life in keeping marine aquaria. Now, with many ways of reducing nitrate readily available, most aquarists can (and probably should) strive to keep nitrate to more natural levels. I have chosen to keep them low by routinely harvesting macroalgae ( Chaetomorpha sp. and Caulerpa racemosa ) from refugia that also contain deep sand beds. Other aquarists have chosen other routes that better fit their needs. Regardless of what methods you prefer, nitrogen export ought to be one of the design considerations in any reefkeeping setup.
    Happy Reefing!

    1. The Complete Nitrogen Cycle by Randy Holmes-Farley Aquarium Frontiers
    2. Chemical Oceanography, Second Edition. Millero, Frank J.; Editor. USA. (1996), 496 pp. Publisher: (CRC, Boca Raton, Fla.)
    3. Comparative effects of water-column nitrate enrichment on eelgrass Zostera marina, shoalgrass Halodule wrightii, and widgeongrass Ruppia maritima. Burkholder, JoAnn M.; Glasgow, Howard B., Jr.; Cooke, Jacob E. Dep. Bot., North Carolina State Univ., Raleigh, NC, USA. Marine Ecology: Progress Series (1994), 105(1-2), 121-38.
    4. Review of nitrogen and phosphorus metabolism in seagrasses.
      Touchette, Brant W.; Burkholder, JoAnn M. Department of Botany, North Carolina State University, Raleigh, NC, USA. Journal of Experimental Marine Biology and Ecology (2000), 250(1-2), 133-167.
    5. Inorganic nitrogen utilization by assemblages of marine bacteria in seawater culture. Horrigan, S. G.; Hagstroem, A.; Koike, I.; Azam, F. Mar. Sci. Res. Cent., SUNY, Stony Brook, NY, USA. Marine Ecology: Progress Series (1988), 50(1-2), 147-50.
    6. Some observations on marine phytoplankton kinetics. 2. The effect of nitrate and ammonium concentrations on the growth and uptake rates of the natural population of Ubatuba region, SP (23°S, 045°W). Schmidt, Gilda. Inst. Oceanogr., Univ. Sao Paulo, Brazil. Boletim do Instituto Oceanografico (Universidade de Sao Paulo) (1983), 32(1), 83-90.
    7. Nutrient control of algal growth in estuarine waters. Nutrient limitation and the importance of nitrogen requirements and nitrogen storage among phytoplankton and species of macroalgae. Pedersen, Morten Foldager; Borum, Jens. Freshwater Biological Laboratory, University Copenhagen, Hillerod, Den. Marine Ecology: Progress Series (1996), 142(1 to 3), 261-272.
    8. Nutrient-enhanced growth of Cladophora prolifera in Harrington Sound, Bermuda: eutrophication of a confined, phosphorus-limited marine ecosystem. Lapointe, Brian E.; O'Connell, Julie. Harbor Branch Oceanogr. Inst., Inc., Big Pine Key, FL, USA. Estuarine, Coastal and Shelf Science (1989), 28(4), 347-60.
    9. Seasonal variations in eelgrass (Zostera marina L.) responses to nutrient enrichment and reduced light availability in experimental ecosystems.
      Moore, Kenneth A.; Wetzel, Richard L. The Virginia Institute of Marine Science, School of Marine Science, College of William and Mary, Gloucester Point, VA, USA. Journal of Experimental Marine Biology and Ecology (2000), 244(1), 1-28.
    10. Effects of nitrate and phosphate on growth and C2 toxin productivity of Alexandrium tamarense CI01 in culture. Wang, Da-Zhi; Hsieh, Dennis P. H. Department of Biology, The Hong Kong University of Science and Technology, Kowloon, Hong Kong SAR, Peop. Rep. China. Marine Pollution Bulletin (2002), 45(1-12), 286-289.
    11. Uptake and assimilation of dissolved inorganic nitrogen by a symbiotic sea anemone. Wilkerson, Frances P.; Muscatine, L. Dep. Biol., Univ. California, Los Angeles, CA, USA. Proceedings of the Royal Society of London, Series B: Biological Sciences (1984), 221(1222), 71-86.
    12. Nitrate assimilation by zooxanthellae maintained in laboratory culture. Wilkerson, F. P.; Trench, R. K. Dep. Biol. Sci., Univ. California, Santa Barbara, CA, USA. Marine Chemistry (1985), 16(4), 385-93.
    13. Captive Seawater Fishes : Science and Technology. Spotte, Stephen. (1992), 976 pp. Publisher: Interscience.
    14. Water quality requirements for first-feeding in marine fish larvae. I. Ammonia, nitrite, and nitrate. Brownell, Charles L. Dep. Zool., Univ. Cape Town, Rondebosch, S. Afr. Journal of Experimental Marine Biology and Ecology (1980), 44(2-3), 269-83.
    15. Nutrification impacts on coral reefs from northern Bahia, Brazil.
      Costa, O. S., Jr.; Leao, Z. M. A. N.; Nimmo, M.; Attrill, M. J. Plymouth Environmental Research Centre, University of Plymouth, Plymouth, UK. Hydrobiologia (2000), 440 307-315.
    16. Effects of lowered pH and elevated nitrate on coral calcification.
      Marubini, F.; Atkinson, M. J. Biosphere 2 Center, Columbia Univ., Oracle, AZ, USA. Marine Ecology: Progress Series (1999), 188 117-121.
    17. Bicarbonate addition promotes coral growth. Marubini, Francesca; Thake, Brenda. School of Biological Sciences, Queen Mary and Westfield College, London, UK. Limnology and Oceanography (1999), 44(3), 716-720.
    18. Nitrate increases zooxanthellae population density and reduces skeletogenesis in corals. Marubini, F.; Davies, P. S. Bellairs Research Inst., McGill University, St. James, Barbados. Marine Biology (Berlin) (1996), 127(2), 319-328.
    19. Nutrition of algal-invertebrate symbiosis. II. Effects of exogenous nitrogen sources on growth, photosynthesis and the rate of excretion by algal symbionts in vivo and in vitro. Taylor, D. L. Rosenstiel Sch. Mar. Atmos. Sci., Miami, FL, USA. Proceedings of the Royal Society of London, Series B: Biological Sciences (1978), 201(1145), 401-12.
    20. Effects of elevated seawater temperature and nitrate enrichment on the branching coral Porites cylindrica in the absence of particulate food Nordemar,I.; M Nyström, M.; Dizon, R. Marine Biology (2003) 142:669-677.
    21. An introduction to the chemistry of the sea. Pilson, Michael E. Q. (1998) 431 pp. Publisher: Pearson Education POD.
    22. Nitrate Removal — A New Alternative by Randy Donowitz, Aquarium Frontiers April 1998.

    Here is the link to the original article:
  2. Guest

  3. Mc


    8 Jun 2011
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    Nice article.;)
  4. dallasg

    dallasg Moderator

    14 Dec 2008
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    RHF is the king!
    i love his articles and subscribe to AA
  5. LCornelius

    LCornelius Moderator

    5 Mar 2009
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    Durbanville (CPT)
    Very cool, thanks!
  6. OP


    1 Mar 2012
    Likes Received:
    Glad you guys like it. Hopefully it helps answer a few questions for fellow reefers ;)

    Here's another article by the same author on Phosphates:

    Chemistry And The Aquarium: Phosphorus: Algae's Best Friend
    By Randy Holmes-Farley

    This article describes some of the issues around phosphorus in reef tanks, including the forms that it takes, its origins, ways to test for it, and most importantly, ways to export it.

    Phosphorus is one of the basic building blocks of living matter. It is present in every living creature, and in the water of every reef tank. Unfortunately, it is present in excess in many reef tanks, and that excess has the potential to cause two big problems for reef keepers. The first is that is can drive excessive growth of undesirable algae. The second is that it can directly inhibit calcification by corals and coralline algae. Since most reef keepers don’t want either of these things to happen, they strive to keep phosphorus levels under control.
    Fortunately, there are some effective ways of keeping phosphorus concentrations to acceptable levels. Unfortunately, the means for testing for total phosphorus are not trivial. One can readily test for one of the common forms of phosphorus in reef tanks, inorganic orthophosphate, but testing for organic phosphorus compounds is considerably more tedious. Moreover, if there is an algae “problem”, then the algae may be consuming the phosphate as fast as it enters the water, masking the issue. Consequently, reef keepers may not recognize that they have a phosphorus problem, only that they have an algae problem.
    This article describes some of the issues around phosphorus in reef tanks, including the forms that it takes, its origins, ways to test for it, and most importantly, ways to export it.
    Phosphate in Seawater

    The “simplest” form of phosphorus in reef tanks is inorganic orthophosphate (sometimes called Pi by biologists). It is also present in natural seawater, although other forms do exist there as well. Its concentration in seawater varies greatly from place to place, and also with depth and with the time of day. Surface waters are greatly depleted in phosphate, relative to deeper waters, due to biological activities that serve to sequester phosphate in organisms. Typical phosphate ocean surface concentrations are very low by reef keeping standards, sometimes as low as 0.005 ppm.
    [​IMG] The structure of orthophosphate, with a central phosphorus atom (purple) and four oxygen atoms (red) arranged in a tetrahedron.

    At concentrations below about 0.03 ppm, the growth rate of many species of phytoplankton is dependent on the phosphate concentration (assuming that something else is not limiting growth, such as nitrogen or iron). Above this level, the growth rate is independent of phosphate concentration for many organisms. So if you want to deter algae growth by controlling phosphate, you need to keep phosphate levels quite low.
    In order to best understand how to maintain appropriate phosphate levels, we must first understand our quarry. Orthophosphate consists of a central phosphorus atom surrounded by four oxygen atoms in a tetrahedron (Figure 1). Orthophosphate exists in various forms in seawater, depending on the pH. At pH 8.1, seawater contains 0.5% H2PO4-, 79 % HPO42-, and 20% PO43-. At higher pH the equilibrium shifts toward more PO43- and less HPO42-.
    The shift in distribution with pH may seem esoteric, but it actually has important implications for such things as the binding of phosphate to rock and sand. It may also surprise some people that so much of the phosphate is present as PO43- while in fresh water only 0.1% is present in that form at the same pH. There are a number of reasons for this difference between salt water and fresh water that involve the effects of other ions in the seawater on the phosphate (such as calcium and magnesium ion-pairs), and these have been described previously.
    Other Forms of Inorganic Phosphate

    Phosphorus can also take other inorganic forms, such as the polyphosphates which are rings and chains of phosphate ions strung together by P-O-P bonds. While these are not significant in natural seawater, they can be present in things that get added to our tanks. There are many of these compounds, but most will likely break down into orthophosphate when added to a reef tank.
    Polyphosphates are used industrially to bind metals, such as in some laundry detergents. In that application, they form soluble complexes with calcium and magnesium, softening the water and permitting better cleaning action. The amount of phosphate getting into natural waterways from laundry detergents, however, is high enough that algae blooms sometimes result, and the practice is now illegal in many places.
    Organic Phosphates

    Unfortunately for reef keepers, the world of organic phosphorus compounds is far more complex than inorganic phosphates. Many common biochemicals contain phosphate esters. Every living cell contains some. Molecules such as DNA, ATP, phospholipids (lecithin), and many proteins contain phosphate groups. In these molecules, the basic phosphate structure is covalently attached to the remainder of the organic molecule through one or more phosphate ester bonds to a carbon atom.
    These bonds are stable for some period of time in water, but will eventually break down to release inorganic orthophosphate from the organic part of the molecule, a process that can be sped up through the action of enzymes in a reef tank. Many of these organic phosphate compounds will be readily removed from a tank by skimming. Export of organic phosphates is the major way that skimming can result in reduced inorganic orthophosphate levels in a tank. Orthophosphate ions themselves are not significantly removed via skimmate (since they do not adsorb onto an air/water interface), but organic phosphates can be removed before they are converted into inorganic orthophosphate.
    An important point about organic phosphates is that they will mostly not be impacted by phosphate-binding materials sold to the aquarium hobby. Consequently, while these products may do a fine job of reducing inorganic orthophosphate, they may not help an algae problem that is caused primarily by organic phosphates.
    A final point is that organic phosphates will not be detected by most test kits. Those that do detect organic phosphates (e.g., Hach PO-24) break the phosphate off of the organic compound and thereby convert it into inorganic orthophosphate prior to testing. However, these kits are tedious and expensive, and not for every hobbyist.
    Phosphate Sources in Reef Tanks

    Organic phosphorus compounds, as well as orthophosphate, are so prevalent that any natural food will contain significant concentrations of phosphorus. Flake fish food is typically about 1% phosphorus (3% phosphate equivalent) by weight. Consequently, if 5 grams of flake food is added to a 100 gallon tank, there is the potential for the inorganic orthophosphate level to be raised by 0.4 ppm in that SINGLE FEEDING. That fact can be a significant issue for reef keepers: what to do with all of that phosphorus?
    If the food is completely converted into tissue mass then there will be no excess phosphate. But much of the food that any organism consumes goes to provide energy, leaving a residue of CO2, phosphate, and a variety of nitrogen-containing compounds. A fish, whether it is an adult or a growing juvenile will consequently excrete much of the phosphorus that it takes in with food as phosphate in its waste. Of course, overfeeding will result in more delivery of phosphate than will lower feeding levels.
    Additionally, many types of seafood available at the grocery store have various inorganic phosphate salts intentionally added to them as preservatives. These foods include canned and frozen seafood, as evidenced by the label, and even some fresh seafood. In these cases, rinsing the food before using it may help reduce the phosphate load added to the tank.

    Finally, tap water can be a significant source of phosphate. The tap water supplied by the Massachusetts Water Resources Authority to me is acceptably low in phosphate, or at least it was the last time that I measured it a few years ago (I use RO/DI due to excessive silica in it). In other water supplies, however, phosphate levels can be too high. I’d recommend anyone with an algae problem who uses tap water to test to see if phosphate in the water is a possible issue.
    On the other side of the issue are those tanks without fish. Since phosphorus is required for growing tissue, it is mandatory that there be some phosphorus source for corals growing in a reef tank. Finding a source is trivial if there are fish in the tank that require feeding, but in tanks without fish, reef keepers must somehow add phosphorus. The answer to this question is rather easy: either add fish food even though there are no fish, or add a source of phosphorus such as a plant fertilizer (and don’t forget about a source of nitrogen as well).
    How to Export Phosphate

    So now that we know where phosphate comes from, and how much, we can proceed to ask where it goes and how to maximize those export processes. Certainly, some phosphorus goes into the bodies of growing organisms, including bacteria, algae, corals, and fish. Some of these organisms stay permanently in the tank, and others may be removed by harvesting of algae, skimming of small organisms, and even pruning of corals.
    A less frequently discussed mechanism for phosphate reduction may simply be the precipitation of calcium phosphate, Ca3(PO4)2. The water in many reef tanks will be supersaturated with respect to this material, as the equilibrium saturation concentration in normal seawater is only 0.002 ppm phosphate. As with CaCO3, the precipitation of Ca3(PO4)2in seawater may be limited by kinetic factors more than equilibrium factors, so it is impossible to say how much might precipitate under reef tank conditions (without, of course, somehow determining it experimentally). This precipitation may be especially likely where calcium and high pH additives (like limewater) enter the tank water. The locally high pH converts much of the HPO42- to PO43-. Combined with the locally high calcium, the locally high PO43- may push the supersaturation of Ca3(PO4)2 to unstable levels, causing precipitation.
    Likewise, phosphate can precipitate onto the surface of calcium carbonate, such as onto live rock and sand. The absorption of phosphate from seawater onto aragonite is pH dependent, with the maximum binding taking place around pH 8.4 and with less binding at lower and higher pH values. If the calcium carbonate crystal is static (not growing), then this process is reversible, and the aragonite can act as a reservoir for phosphate. This reservoir can make it difficult to completely remove excess phosphate from a tank that has experienced very high phosphate levels, and may permit algae to continue to thrive despite cutting off all external phosphate sources. In such cases, removal of the substrate may even be required.
    The relationship of calcium carbonate to the phosphate cycle has been studied by Frank Millero in the Florida Bay ecosystem (click here for Millero's studies). If aragonite crystals are growing, as they often are in some parts of our systems, then I’d expect some of this phosphate to get buried and locked into the aragonite crystals.
    A side effect of the adsorption of phosphate onto aragonite may well be the reported impact of phosphate on the calcification of corals. The presence of phosphate may inhibit the formation of calcium carbonate crystals via surface adsorption, and this effect may very well be the factor that inhibits calcification of corals at high phosphate levels.
    Many reef keepers accept the concept that limewater addition reduces phosphate levels. This may be true, but the mechanism remains to be demonstrated. Craig Bingman has done a variety of experiments related to this hypothesis, and published them in Aquarium Frontiers. While many may not care what the mechanism is, knowing it would help to understand the limits to this method, and how it might best be employed.
    Habib Sekha (the owner of Salifert) has pointed out that limewater additions may lead to substantial precipitation of calcium carbonate in reef tanks. This idea makes perfect sense. After all, it is certainly not the case that large numbers of reef tanks will exactly balance calcification needs by replacing all evaporated water with saturated limewater. And yet, many find that calcium and alkalinity levels are stable over long periods with just that scenario. The only way that can be true is if such additions typically dump excess calcium and alkalinity into the tank that is subsequently removed by precipitation of calcium carbonate (such as on heaters).
    It is this ongoing precipitation of calcium carbonate, then, that may reduce the phosphate levels: phosphate binds to these growing surfaces, and becomes part of the solid precipitate. If true, this mechanism may be attained with other high pH additive systems (like some of the two-part additives such as the original B-ionic) if enough is added. However, it will not be as readily attained with low pH systems, such as calcium carbonate/carbon dioxide reactors because the low pH inhibits the precipitation of excess calcium and alkalinity.
    Uptake of Phosphate by Organisms

    How organisms obtain phosphate is, in nearly all cases, poorly understood. Even the absorption mechanisms used by humans are still the subject of intense research (one of my research areas involves drugs to modify this absorption, such as Renagel). It’s not surprising then that phosphate absorption mechanisms in coral reef creatures would also be poorly understood.
    A handful of Chaetomorpha sp. macroalgae being harvested from the authors refugium.​
    One frequently hears that limiting phosphate will limit algae growth in reef tanks. That is almost certainly true, but some species of microalgae thrive more readily under phosphate limitation than others (click here for phosphate limitation studies). Some species of microalgae can, in fact, significantly regulate their inorganic phosphate transport capabilities to deal with variable phosphate levels (click here for Upregulation of Phosphate Transport).
    Finally, one must also consider organic phosphates. Many organisms can enzymatically break down organic phosphates prior to absorption. Consequently, we are left not having a very good understanding of what organisms in our tanks use what forms and concentrations of phosphorus. Further complicating matters, our tanks are usually greatly skewed from natural seawater in terms of other nutrients (e.g., nitrogen and iron), so one cannot readily extrapolate from phosphate studies in seawater to our tanks.
    Nevertheless, growing and harvesting macroalgae (Figure 2) remains one of the best ways to reduce phosphate levels in reef tanks (along with other nutrients). Tanks with large amounts of thriving macroalgae rarely have microalgae problems or excessive phosphate levels that might inhibit calcification of corals. Whether the reduction in phosphate is the cause of the microalgae reduction is not obvious; other nutrients can also become limiting. But in a certain sense it makes no difference. If rapidly growing macroalgae absorb enough phosphorus to keep the orthophosphate concentrations in the water column acceptably low, and at the same time keep microalgae under control, most reef keepers will be satisfied.
    For those interested in knowing how much phosphorus is being exported by macroalgae, this free pdf article in the journal Marine Biology has some important information. It gives the phosphorus and nitrogen content for 9 different species of macroalgae, including many that reefkeepers maintain. For example, Caulerpa racemosa collected off Hawaii contains about 0.08 % by dry weight phosphorus and 5.6% nitrogen. If one were to harvest 10 grams (dry weight) of this macroalgae from a tank, it would be the equivalent of removing 24 mg of phosphate. That amount is the equivalent of reducing the phosphate concentration from 0.2 ppm to 0.1 ppm in a 67 gallon tank. All of the other species tested gave similar results (plus or minus a factor of 2). Interestingly, using nitrogen data in the same paper, it would also be equivalent to reducing the nitrate content by 2.5 grams, or 10 ppm in that same tank.
    Commercial Products

    There are, of course, many commercial products for reducing phosphate concentrations. Typically, these only reduce inorganic orthophosphate, but they can do that effectively, if not inexpensively. Two of the main types are those based on aluminum oxide (such as Seachem’s Phosguard) and those based on iron oxides and hydroxides (such as Rowaphos). Many people have successfully used these products (including myself), but others claim problems from the aluminum products that they blame on aluminum toxicity. I’ve seen no basis for these claims in my own tank, but I did not use them long term.
    My advice on these products is that they can be used successfully, but that there may be better, and certainly less expensive and more interesting ways to reduce phosphate levels (such as setting up a refugium with macroalgae in it).
    Summary of Phosphate Reduction Methods

    Here is a list of ways that people can reduce phosphate levels. They are listed in order of preference that I have for addressing these issues in my own system:
    1. The big winner is macroalgae growth. Not only does it do a good job of reducing phosphate levels, but it reduces other nutrients as well (e.g., nitrogen compounds). It is also inexpensive and may benefit the tank in other ways, such as a haven for the growth of small life forms that help feed and diversify the tank. It is also fun to watch. I’d also include in this category the growth of any organism that you routinely harvest, whether corals or something else.
    2. Skimming is another big winner, in my opinion. Not only does it reduce organic forms of phosphate, but it reduces other nutrients and increases gas exchange. Gas exchange is an issue that many people don’t recognize, but that can contribute to pH problems.
    3. The use of limewater, and possibly other high pH alkalinity supplements, is also a good choice. It can be very inexpensive, and it solves two other big issues for reef keepers: maintaining calcium and alkalinity.
    4. Commercial phosphate binding agents clearly are effective.
    5. Simply keeping the pH high in a reef tank (8.4) may help keep phosphate that binds to rock and sand from reentering the water column. Allowing the pH to drop into the 7’s, especially if it drops low enough to dissolve some of the aragonite, may serve to deliver phosphate to the water column. In such systems (typically those with carbon dioxide reactors), raising the pH may help control soluble phosphate.

    Issues involving phosphorus can be among the most difficult to diagnose in a reef tank, especially if the live rock and sand have been exposed to very high phosphate levels and may be acting as a phosphate reservoir. Fortunately, there are steps that can be taken even in the absence of any algae problem that will benefit reef tanks in a variety of ways, not the least of which is reduction of phosphate levels. All reef keepers, and especially those designing new systems, should have a clear idea in mind about how they expect phosphorus to be exported from their system. If allowed to find its own way out, it will more than likely end up in microalgae that the reef keeper is constantly battling.
  7. OP


    1 Mar 2012
    Likes Received:
    And it posted
    Last edited: 19 Apr 2012
  8. Clownfish9906


    5 Mar 2012
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    Midrand, Gauteng
    AA = Aquarists Anonymous...:whistling:

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