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Thread: Scrubbers DO NOT export Phosphorus

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    Scrubbers DO NOT export Phosphorus

    Now that I have your attention :-)

    Full disclosure - I have an ATS, I have posted my build in this forum, and I have contributed to the knowledge base here. This thread is not about bashing any method. It is about trying to understand what is really happening, so that we can better our aquaria.

    Abbreviations: P=total phosphorus, Po=organic phosphorus, Pi=inorganic phosphorus

    Over at Nano-Reef we had a great, informative, entertaining, sometimes confrontational, but overall a very thought provoking thread on the concept that Macroalgae in a reef tank (in a fuge, or ATS) can NOT export net phosphorus. The origin of the Nano-Reef thread is one titled "Algae - What's Under The Hood..." over at The Reef Tank forum (you have to join to view). I won't re-hash everything in those 2 threads, and I encourage others to go through those on your own - they are tedious at times, but enlightening.

    At first it seems impossible. How does an ATS not export P? What about all that algae I harvest every week that is thrown out, it has P in it, right? Yes it does...but during that week of growth, the algae has been dumping as much, or maybe even more, equivalent mass of Po (which we cannot measure) into the tank as it has been consuming Pi, feeding the bacteria that use the Po to make more Pi, which then feeds the algae, and so on...it is a cycle. The way I just described it is obviously very simplified, but for our purposes it will be suitable. There is research documenting this. The best one I have seen quantifies the net P biomass lost as 8% of the total algae P biomass - but this 8% comes during an incubation test period of about 4-5 hrs if I recall, and the algae were in a state of starvation so they were conserving Po. The references are in the above threads, I can post them later.

    Algae will reach a homeostatic balance of Pi input and Pi export based on how much Pi is in the tank. It is like an algal survival mechanism. If it sucks up all the Pi available, it would starve (=not good). The algae won't keep extracting Pi at a steady rate until there is none left. It will balance its use depending on availability. It is mathematically impossible for an ATS to reduce total P in a tank.

    There are other benefits to an ATS. But for this discussion, I want to only focus on the P export aspect of it.

    What can the ATS do then, with regards to P? It can create a competitive situation whereby Pi is utilized by the ATS in favor of the nuisance algae in the display, which esthetically at least is a benefit. It can also act as a buffer, if you will. If you are a heavy feeder, the algae will increase its rate of Pi use, again preventing assimilation into nuisance algae, to a limited degree of course.

    This leads us back now to the issue of the Po that the algae is leaking into the tank. On the one hand, Po does not really harm the tank inhabitants. It can even be viewed as a food source for coral. But it is also a preferred food source for bacteria. It is less energetically expensive for a bacteria to break P bonds in dissolved Po, than it is for them to break the bonds of Po that is bound to the rock or substrate (these are calcium-P bonds). Bacteria will utilize the high levels of dissolved Po in favor of the calcium-bound Po. What does this mean? The substrate and live rock can now more easily bind excess Po floating around. Depending on how big your P sink is (how much rock, how much sand), eventually, without outside help, it will be saturated ---> tank crash?

    Water changes help removing Po. Siphoning and detritus removal take Po with it. A skimmer can be good at removing Po. Somewhere, in between all these methods of Po export, there is a balance that can be reached with an ATS in the system.

    But the danger is if we rely to heavily on the ATS, then we are reaching a saturation point more quickly.

    Now let's assume we are doing water changes, skimming, etc. along with an ATS in an adequate enough manner where Po becomes an overall algae limiter in the system (via reduced bacterial conversion of Po->Pi)...then theoretically, all things being equal, the ATS has reached maximum efficiency. This is good. It means that we have reached a point where the ATS will work the best in your favor to compete with nuisance algae.

    I have seen numerous instances on this forum where algae growth seems to be reduced, or changes quality and color, and we keep pushing more light and more flow. Maybe this is incorrect - maybe in some of those cases, they reached the system max because they are exporting Po correctly? Maybe we need to reduce ATS growth because the screen/ light/ flow are exceeding the algal capacity? I would love to say, let's feed more, but then we are just making it harder to deal with all the Po.

    I don't have nearly all the answers. I do have lots of questions, as I'm sure others here will too. Given this info about how P cycles in a tank, let's discuss and strategize.

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    So when we are measuring 0.00 on Hanna, what does this means in real world ? At the same time I never was 100% free of gha.

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    Hmm. You cannot remove P from a system and still have the same amount left in the system. Therefore a net export, which is what we are after. If people (like me) choose to feed more then good. Great start to a thread by the way. Looking forward to other views.

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    A few readings of marine biology nutrient studies, or especially, phycology assimilation studies will change your mind

    It would be impossible for algal biomass to grow without consuming net P. Just ask the biofuel people who have to add millions of pounds of P to their water in order to grow sufficient mass. Your model would mean that their P would remain constant.

    Couple of corrections:

    Bacteria will utilize the high levels of dissolved Po in favor of the calcium-bound Po.
    Your Po (known as POC: Particlate Organic Carbon; also known as "food") does not bind to rocks; only your Pi (orthophosphate, or DIP: Dissolved Inorganic Phosphorus) does. And aerobic bacteria does not consume this bound P from the rocks until the P redissolves, due to a lowered pH.

    Water changes help removing Po. Siphoning and detritus removal take Po with it. A skimmer can be good at removing Po.
    Yes, these do remove food particles. But when are you going to put them back in? If dosing aminos or vitamins, you'll need to dose more. If feeding liquid or powdered coral foods, you'll need to feed more.

    I have seen numerous instances on this forum where algae growth seems to be reduced, or changes quality and color, and we keep pushing more light and more flow. Maybe this is incorrect
    You do not add light if the growth becomes nutrient limited, i.e., bright yellow. Flow, yes. Iron, yes.

    Maybe we need to reduce ATS growth because the screen/ light/ flow are exceeding the algal capacity?
    This is the reason for the reduced screen size recommendation; to keep what nutrients there are confined to long gha instead of spread out over large areas of brown slime.

    maybe in some of those cases, they [algae] reached the system max because they are exporting Po correctly?
    Algae don't export your Po (known as POC: Particlate Organic Carbon; also known as "food"), they only export your Pi (orthophosphate, or DIP: Dissolved Inorganic Phosphorus).

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    Probably the best response I've seen to be honest. However, this does not explain the accumulation of dissolved organic carbon. When you stop exporting through a skimmer, for instance, it is extremely easy (through bubbles on a screen) to form a foam. This indicates to me that the exudates from algae and corals are not being used as fast as previously thought, hence an excess. This excess then can (in certain circumstances) fuel dino's and other unpleasant natural phenomena.

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    Quote Originally Posted by kotlec View Post
    So when we are measuring 0.00 on Hanna, what does this means in real world ? At the same time I never was 100% free of gha.
    Hanna and other kits are only measuring inorganic phosphate, which is consumed so quickly when it is available that testing kits are not really accurate. There is no good way for hobbyists to measure organic P sources, unfortunately.

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    Quote Originally Posted by xerophyte_nyc View Post
    Hanna and other kits are only measuring inorganic phosphate, which is consumed so quickly when it is available that testing kits are not really accurate. There is no good way for hobbyists to measure organic P sources, unfortunately.
    And dino's for example can tap into the organically bound P. therefore making inorganic readings defunct.

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    Garf said, you cannot remove P from the system and have the same amount left.

    Technically that is correct. The instant that algae is flushed down the toilet, there is some P that is bound within the algae being exported. At that point, the bacteria in the tank suddenly lost a source of food, because the algae was providing Po all week long. Some bacteria will die, liberating more Po. Some bacteria will now switch to using the P bound in rock for food. The sum of all that bacterial activity is more net Pi back into the water, so that algae can grow again. The cycle continues with back and forth swings until it is in balance again. Since we continually feed our tanks, there is always more P incoming. If you stopped feeding, then you would see that the algae won't grow as fast, it will just gain biomass in harmony with bacteria to match the nutrient levels in the tank.

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    Took a while to get to grips with this. I agree, got an old thread relating to the exudates of algae (which are basically a carbon dosing [sugars]). And all carbon dosing methods correctly state that you must export (skim).

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    Quote Originally Posted by SantaMonica View Post
    A few readings of marine biology nutrient studies, or especially, phycology assimilation studies will change your mind
    Maybe, maybe not

    It would be impossible for algal biomass to grow without consuming net P. Just ask the biofuel people who have to add millions of pounds of P to their water in order to grow sufficient mass. Your model would mean that their P would remain constant.
    The moment the algae is harvested, yes technically there is P being thrown out. But from the time that growth started from a freshly harvested screen, for that week while algae was growing, there was MUCH more P going in and out of the algae biomass. And there is excess Po being shed into the water. This Po forms detritus and pollutes the substrate and binds to Calcium. This is the bad part because we cannot measure it, only after the substrate is full are there problems. As aquarists and ATS proponents we need to understand this better, and account for it in our husbandry.

    And aerobic bacteria does not consume this bound P from the rocks until the P redissolves, due to a lowered pH.
    This part doesn't even matter (for now).


    Yes, these do remove food particles. But when are you going to put them back in? If dosing aminos or vitamins, you'll need to dose more. If feeding liquid or powdered coral foods, you'll need to feed more.
    It is a conundrum. We can't have great food from Po and not have to account for it binding to calcium in the substrate and rock at the same time.


    Algae don't export your Po (known as POC: Particlate Organic Carbon; also known as "food"), they only export your Pi (orthophosphate, or DIP: Dissolved Inorganic Phosphorus).
    What about all those amino acids and all the other great things the ATS adds to the water? Bacteria will get to this before coral will. It is better to feed coral particulate food that we add, than to rely on waste products. I'm not saying that the dissolved food is a bad thing - I'm just suggesting there is too much of it, which is bad for the tank long term without another means of export like skimming, water changes, detritus removal.

    Bacteria are the primary source of Pi in aquaria. They process dissolved nutrients, liberating Pi. The dissolved nutrients come from food, waste, and algal waste. They can also digest the P bound to rock (but that is much harder for it to do). The Pi liberated by bacteria is what fuels algae. It is a tightly knit cycle. In the oceans, we have abyssal plains and geothermal processes that bind the phosphate away. In an aquarium, we do not. We have skimmers, water change, etc.

    I'm not discounting scrubbers. I just want us to better understand how to manage our aquariums, by accounting for all this organic P.

    I have literature and links that describe the phosphate cycle, etc. I will provide them later for everyone to read and understand. Looking forward.

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