Nitrate Levels in Aquaria vs Wild Reef Nutrition

[TexasReefer82]

I often wonder how wild reefs are able to grow and thrive with such low levels of dissolved nutrients - namely nitrates and phosphates.

I have an SPS/Acropora dominant tank and I maintain KNO3 solution on a dosing pump just to keep the corals alive - I try to maintain nitrates up to 5ppm. Without dosing NO3 to my tank it quickly trends toward zero and I observe mild bleaching and STN in my Acropora. The same goes for phosphate although it seems a bit easier to maintain as long as I don't have a bryopsis breakout.

Many hobbyists add amino acids to the water for direct absorption by corals.

Wild reefs have essentially zero nitrates and phosphates in the water column (at least as far as our hobbyist grade test kits are concerned) and there aren't free amino acids floating around either. Presumably these corals are acquiring their Nitrogen and Phosphorus form some other source than direct absorption from the water. But what is this other source? LPS corals I can see consuming plankton at night when feeding tentacles are out. However, most Acropora and SPS in general don't eat anything in my experience.

Does anyone have an insight into how wild reefs acquire their Nitrogen and Phosphorus?

 

[P-Dub]

I often wonder how wild reefs are able to grow and thrive with such low levels of dissolved nutrients - namely nitrates and phosphates.

I have an SPS/Acropora dominant tank and I maintain KNO3 solution on a dosing pump just to keep the corals alive - I try to maintain nitrates up to 5ppm. Without dosing NO3 to my tank it quickly trends toward zero and I observe mild bleaching and STN in my Acropora. The same goes for phosphate although it seems a bit easier to maintain as long as I don't have a bryopsis breakout.

Many hobbyists add amino acids to the water for direct absorption by corals.

Wild reefs have essentially zero nitrates and phosphates in the water column (at least as far as our hobbyist grade test kits are concerned) and there aren't free amino acids floating around either. Presumably these corals are acquiring their Nitrogen and Phosphorus form some other source than direct absorption from the water. But what is this other source? LPS corals I can see consuming plankton at night when feeding tentacles are out. However, most Acropora and SPS in general don't eat anything in my experience.

Does anyone have an insight into how wild reefs acquire their Nitrogen and Phosphorus?

Well, not quite zero but... Remember that we have a closed system. In that closed system, there are a number of organisms competing for a finite amount of nutrients. In the ocean, there are a finite amount of organisms competing for an infinite amount of nutrients, albeit low but constantly replenished. I hope that makes sense. Think of it this way. The ocean is a neverending buffet while our reef tanks are piecemeal. Additionally, different areas or types of reefs contain different levels of nutrients. Inner lagoons generally have higher nutrient levels than the reef drop-offs.

 

[TexasReefer82]

A couple thoughts on that...

I understand that the ocean's total content of dissolved Nitrate and Phosphate is immense even though the actual concentration on a reef is very low. I've heard this line of thought before - but it's not accurate to equate total ocean content to availability. For availability of nutrients (dissolved NO3, PO4, amino's, etc) to a coral the only thing that matters is the aqueous concentration of that nutrient - it's the concentration that the coral "sees." That the ocean has an immense virtually inexhaustible supply only serves to keep that concentration constant. In a 40 gallon aquarium the supply of any dissolved nutrient or ion certainly isn't inexhaustible but we imitate the ocean's enormous content by using dosing pumps to keep the concentration constant vs time.

It's easy enough to find online references to wild reef concentrations of <0.1 ppm NO3 and 0.005 ppm PO4. These are both below the limit of detection of the hobbyist grade test kits. I know that when my test kits read zero on either NO3 or PO4 I've got a problem and I need to raise these concentrations into measurable ranges.

How are corals on wild reefs able to live and thrive with such low availability/concentrations? While the same coral in my tank, if kept in the same concentrations, will suffer greatly?

Is it possible that we've been lied to about the actual concentration of NO3 and PO4 on wild reefs? Maybe the wild concentrations aren't so exceedingly low as we've been told - and maybe maintaining our tanks at 0.05 ppm PO4 and 5 ppm NO3 isn't so unnatural after all.

 

[BeejReef]

It's a really fascinating question. Well, if the concentration of tic tacs in the ocean is nearly undetectable.. 1/3600 seconds, u get one tic tac per hour.
If it's nearly undetectable in your tank, you should get the same tic tac per hour, but there's also the chance of it actually being zero tic tacs. U can eat the last tic tac.

Since it's such a wide-open question, I'll hazard my guess. It may be because the ocean has far higher concentrations of organisms that the corals (even sps) feed on that do contain N and P. It is just bound within their cellular structures and therefore not testable. Phytoplankton comes to mind. I found the video I kind of was remembering. NSW performed as well if not better than most coral foods.

 

[madweazl]

What makes you believe nitrates of 5-10ppm work better than .01ppm in aquaria? I firmly believe you only need measurable amounts of nitrate and phosphate to have success, anything else is simply excess. The natural reef also has all kinds of "food" raining on it 24/7; predators chowing down on smaller critters, fish waste, particulate being drummed up from off the reef structure, all manner of energy sources.

 

[TexasReefer82]

What makes you believe nitrates of 5-10ppm work better than .01ppm in aquaria? I firmly believe you only need measurable amounts of nitrate and phosphate to have success, anything else is simply excess. The natural reef also has all kinds of "food" raining on it 24/7; predators chowing down on smaller critters, fish waste, particulate being drummed up from off the reef structure, all manner of energy sources.

I only believe that because I have observed it in my own aquarium. The term "Measurable" amounts is subject to the quality of the analysis used to measure. What exact concentrations are you referring to when you say measurable amounts? Why would anything above measurable amounts be considered excess and not useful?

One major unknown for all of us is when a hobbyist grade test kit reads zero: the actual concentration of the sample tested could be anything between zero and the limit of detection of that test kit. The problem is that wild reef values fall within this unknown range.

I agree that there is far more "food" on the wild reef than in our tanks. But do we know for sure that corals eat this - particularly acropora which I've never observed and am unaware of it eating anything.

 

[TexasReefer82]

It's a really fascinating question. Well, if the concentration of tic tacs in the ocean is nearly undetectable.. 1/3600 seconds, u get one tic tac per hour.
If it's nearly undetectable in your tank, you should get the same tic tac per hour, but there's also the chance of it actually being zero tic tacs. U can eat the last tic tac.

Since it's such a wide-open question, I'll hazard my guess. It may be because the ocean has far higher concentrations of organisms that the corals (even sps) feed on that do contain N and P. It is just bound within their cellular structures and therefore not testable. Phytoplankton comes to mind. I found the video I kind of was remembering. NSW performed as well if not better than most coral foods.

Looks like a fascinating video - will watch at home this evening!

 

[madweazl]

I only believe that because I have observed it in my own aquarium. The term "Measurable" amounts is subject to the quality of the analysis used to measure. What exact concentrations are you referring to when you say measurable amounts? Why would anything above measurable amounts be considered excess and not useful?

If you're addressing acropora specifically, you aren't seeing better results with 5ppm nitrates than you are with .1ppm (they may be the same however). The term measurable isn't subjective in our case; it wouldn't do much good for us to say measurable is = to x if we didn't have the kits to do it now would it (I used .1ppm in my previous post for a reason)? I used .1ppm specifically because most of us can see that pretty easy with something like the Salifert kit.

 

[P-Dub]

The levels across the different zones of a reef system from the mangroves to the fringing drop-offs vary. The variability within each reef zone will be dependant on the tides, temperature, weather, and human impact, to name just a few. I believe that there are more bioavailable nutrients in nature than just what we are testing for as @William DeCoursey and @madweazl hit on and also a far more complex ecosystem than what we have in our enclosed systems. I'm compelled to take some readings and will share what I find.
In the meantime, a great read...
https://www.sciencedirect.com/science/article/pii/S1877343513001917