OK, my beloved wife give me an advise how to solve our problem (or at least come closer to its solution).
The obvious solution to this is to set up a tank full of water with an empty filter, put plants into it and use high light. I would attach the plants to some inert stones so that they stay on its place. This way we can know quite precisely how much nutrients the plants will use up in a set time (say one or two weeks). I'll try to do this kind of test. I'll set up an empty tank with empty filtration (to use it just for a water circulation), and after my plants will establish, then I begin my test. I'll put some known (unlimited) amount of nutrients inside this high-light tank and let it analyse in a lab on the first day, second day, third day and last day. I think it is important to analyse the water not only on the first and last day, but also more often in the early days to know if some nutrients degraded (e.g. chelates). What do you think about this kind of experiment?
I think the basis of your experiment is sound. However it is of vital importance your biosystem (read: the tank and all it's content) are as close to equilibrium as possible. This is important because this is also the case in your "real" tanks. Because you have a fixed dosing schedual, fixed water changes amounts, constant lighting and CO2, feeding of fish etc. As a result of this both bacterial and plants growth are more or less the same over time. There growth speeds are always rate limited. The rate limiting factor can be many things, light, nutrients, pH, temperature but also physical limits that dictate maximum speed of enzymes etc. Once a tank and filter is properly cycled, all factors are kept constant, and amount of in this case plant trimming is similar each week a for of equilibrium is achieved. (note: it's important plants are mature and trimmed to keep them constant in size => bigger plants with bigger/more leaves have more surface area to uptake nutrients and a photons for photosynthesis. Once you feel confident the tank is stable and in equilibrium (I.e. looks the same from one week to another, after trimming) the testing can commence!
...let it analyse in a lab on the first day, second day, third day and last day. I think it is important to analyse the water not only on the first and last day, but also more often in the early days to know if some nutrients degraded (e.g. chelates)
One thing that is excellent from a scientific point of view is taking more than two samples. During my studies we actually had to perform experiments on how to determine at which concentration of a given substrate maximum product formation takes place. This is very important because with this knowledge industrial (bio) processes can be optimised for either speed of product formation and cost reductions. Because now you know at which concentration of a substrate it is just a pure waste to add more, because there is no benefit! But back to the experiment. How are these experiments performed? Without going into too much detail it is quite simple. Because your system is in steady state abd the limiting factor is either lighting, CO2 dosing all other nutrient uptakes are constant en linear over time. So if you take all data from the samples, and plot them against time, you you see straight lines in your plot. However, it is possible that one not the lighting or CO2 are the limiting factor, in which case one of the nutrient concentrations is limiting the growth speed. In this case you would see a decrease of substrate intake over time as the limiting substrate concentration slowly drops, slowing the growth and thus slowing substrate intake further (as substrate intake is dependent on concentration!).
Disclaimer: I do not know if the plants etc take up so much nutrients the data would be significant enough to conclude if the substrate intake is constant or decreasing over time!
If the experiment is performed well this would tell you two things (of this particular tank/bio system!!!):
1) an absolute nutrient uptake in a week giving a rough estimate of how much nutrients plants actually use.
2) it might give insights if all nutrients are available to the plants in excess and CO2 and lighting in this particular tank are the limiting growth factors (it can only be one of them btw). If it is shown that nutrients using recommend EI dosing are still limiting the growth speed it shows that that the EI dosing isn't as crazy as it seems
Things this experiment can't proof (with only one dataset):
1) if nutrients are proven to be in excess we have no idea HOW MUCH these are in excess. It could be as much as 100 mg/L or as little as 1 mg/L. To discover the amount of excess the experiment must be repeated multiple times for each single nutrient until growth speeds decrease. This would probably be a year long study
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2) which factor (light, CO2, which nutrient etc) is the limiting factor in this system will remain unknown.
Not sure if you intended to say so but...why it should be beneficial to have high growth speed? As stated high concentrations of nutrients (especially PO4) gives you high growth, but you don't need this super growth (unless you sell plants). The point is to know what plants need.
From a biological standpoint plants need very little for their so called maintenance. Little light, little nutrients etc is needed for them to survice. Although I am not an expert on aquarium plants and all the water chemistry I think the important point is the competition aquatic plants have with algae as they need compete bother over light, CO2 and the other nutrients. Therefore I think many of the techniques in high tech aquaria are aimed to battle these algae. One of these techniques is frequent water changes, which removes nutrients (aka byproducts from bacteria such as ammonia) algae can use. It's important tot note that almost every chemical that can be found in the aquarium which is the product of one organism is the substraat thus food for another! And for some reason we only want plants, fish, and denitrifying bacteria (for the most part) in our aquarium and not algae and other organisms
😛. The other "technique" against algae (I think!) are healthy plants that grow. When plants grow they use up nutrients and form biomass (i.e. more plant leaves) which remove these nutrients to be used by other organisms such as algae. One thing I do know is that plants are able to directly use ammonium for their growth, and algae spores need ammonia to germinate (please note that I can't find a reference for this to be true!! after looking most scientific articles state ammonia levels inhibit germination...). This could be also true for other nutrients we don't regularly add, and algae need. With rapid plant growth the plants can lower concentrations of these unknown (to me) nutrients and put a lid on algae growth.
Also while doing some research for this post I came across this:
http://prirodni-akvarium.cz/clanky/Factors Affecting Spore Germination in Algae.pdf
There are many interesting things in this publication with this one standing out in particular:
The SG (spore gemination) was decreased not only by the lack of nitrogen, phosphorus or magnesium but also when their (and of calcium) concentration exceed certain levels; e.g., nitrate or phosphate at ≥5-fold level, or mag- nesium at 10-fold level of that present in the basal medium inhibited akinete GRM in Westiellopsis prolifica (Agrawal and Sharma 1994a). Magnesium at ≥5-fold level or calcium at ≥2-fold level also inhibited akinete GRM in Stigeoclonium pascheri (Agrawal and Sarma 1982a). This indicates that SG in algae is sensitive to high levels of inorganic nutrients. Omission of microelements (ZnSO4, MnCl2, MoO3, CuSO4, Co(NO3)2, H3BO3) from the basal medium increased SG in Stigeoclonium pascheri, and by increasing their concentrat- ion to ≥2-fold levels, the condition was reversed (Agrawal and Sarma 1982a). The presence of microele- ments in the basal medium therefore serves as a check in reaching maximum level of SG under control con- ditions. More study is needed to clear the role of micro- and macronutrients in SG.
This could very well mean that the crazy dosing of EI is having a large impact on germination of many algae species, keeping them in check. In this case the EI dosing would be a nice way to fight algae with plant growth as happy side effect
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Disclaimer: most of the above is either a personal theory and not based on personal experience or scientific research that could OR NOT be applicable to the aquarium ecosystem. Use what I said at own risk and if people think I'm calling out some crazy theories that are wrong, please let me know
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