Real reef levels of N and P seem to be well below our target levels

UK_Pete · Jan 24, 2015

I was doing a bit of reading today and chanced on a reefers page whos talking about how low nutrients are on real reefs (hes disagreeing with the ULNS description as inaccurate) I did some digging and realised he seems to be right. Nutrient levels on real reefs in the GBR seem to be more like 3 to 10 ppb phosphate and 10 ppb nitrate (thats parts per billion, so 0.01 ppm NO3 and down to 0.003 ppm PO4). I was wondering if this had any significant to reef tanks (and specifically SPS).

One reference, here

http://www.aslo.org/lo/toc/vol_48/issue_6/2266.pdf

Seems to suggest DIN of around 0.3 to 1 umol/L which I make 21 ppb to 62 ppb, but page 23 of this (table 2.8)

http://www.gbrmpa.gov.au/__data/assets/pdf_file/0004/7690/MMP_AIMS_August_2008.pdf

Seems to suggest an annual mean DIN of 2.5 ppb, or 10 ppb NO3. The same table suggests a DIP of 3 ppb, or 10 ppb PO4. Values of DON are 73 ppb (320 ppb NO3), and DOP of about 3 ppb (9 ppb PO4).

So nitrate of 10 ppb seems well below the current recommended values for reef tanks, and phosphate of 10 ppb seems a bit below too, but by less of a margin. Tables further up the report show even lower numbers for many areas, both in nitrogen and especially phosphate (table 2.5, averaging about 3ppb PO4, not P).

Since people seem to report problems if nutrients drop this low I am wondering whats the reason for all this. Are tanks actually better off with these levels of nutrients perhaps. They also list organic nitrates and phosphates and only nitrate is significantly boosted by organic sources to about 250 ppb. Is that where aminos come in, adding organic nitrogen?

Considering this I am wondering if trying to maintain a tank at zero measurable phosphates and nitrates would be best, for SPS anyway, by carbon dosing, aggressive GFO (or iron chelate dosing) and a denitrifier, and adding aminos for organic N, maybe this is a known 'method' but I have always thought that every method tried to maintain measurable N and P. I did think that too aggressive use of GFO for instance would result in failure but even with aggressive GFO, is it really possible to drive PO4 below 3 ppb for instance?

heathd.hd · Jan 24, 2015

Love these type of questions waiting for smart people to add there responses. Lol

Randy Holmes-Farley · Jan 24, 2015

I don't disagree with the numbers in surface seawater. In my phosphate article I say it can be lower than 0.005 ppm phosphate and my nitrate articles refer to less than 0.1 ppm nitrate.

One problem with trying to maintain those levels is that we cannot accurately measure either, so under shooting can have serious consequences.

I also think that corals also do well in areas not so depleted (where deeper water wells up with much more in the way of nutrients). So it is not everywhere that low, even on reefs.

The corals in some ULNS systems that are pastel colored from reduced zoox do not appear natural to me, and if those low zoox levels are attained by low nutrients, then either the levels are lower than in the ocean where those corals were not so colored, or they were receiving other nutrients in the ocean that are lacking in a tank (such as plankton).

Thus, if we do not have other foods sources for the corals and zoox, driving nutrients as low or lower than the ocean may not be optimal.

Nano sapiens · Jan 24, 2015

As Randy mentioned, plankton volume on a typical reef is much higher than what we can safely provide in our tanks. I've had the good fortune of visiting the same fringing reef over many seasons and years. Snorkeling in the afternoon, especially, was sometimes like swimming through split-pea soup! If I was to provide my reef tank with this much planktonic nutrition on a daily basis, I'd need some extememly heavy-duty filtration and/or constant water changes.

Luckily, corals can use a multitude of nutritional sources, so having a bit higher nitrate and phosphate (as compared to a natural pristine reef) can help make up for the planktonic deficit.

UK_Pete · Jan 25, 2015

Hi Randy, that was not to suggest the numbers in your articles differed - I think its just that I didnt appreciate when I read them that these were normal values rather than extreme ones. From the second PDF linked to, it seems that these values are seen across all the monitored reefs, which is what surprised me. We hear a lot about eutrophication nowadays but exactly what nutrient level is considered eutrophicated I don't know.

As for unnatural color of corals, the most unnatural color I've seen is zeo systems, but I just recently realised its because they are using heavy metals to bleach the corals. So this dosent count really. The next ones down the chain of brightness seem to be carbon dosers and ultralith but those don't seem unnaturally bright to me compared to the reefs I've dived on, about right mainly (to qualify that, not all corals are bright on a real reef IME, but the bright ones peppered in with the browner ones are probably the ones which are selected for harvesting and sent to shops etc). I wonder if zeo was run without the heavy metal bleaching agents, would colors look about right compared to the reefs.

@nano, as for plankton, very interestingly levels are reported in those tables too. PN (particulate nitrogen) of 15 ppb and PP (particulate phosphorus) of 3 ppb. If you use say the nitrogen to calculate the actual wet weight of the plankton, I believe (Randy correct if wrong) you would say about 3% of the weight of the fish is nitrogen, so wet fish weight would be about 33x the particulate nitrogen. So 33*15 ppb = 0.5 ppm plankton, or about half a miligram per litre. For my 165 litre tank this means about 82 miligrams - 0.09 grams food. That does not corespond to daily feeding, as its the amount required to be in the water constantly despite plankton being filtered by skimmer, sedimentation etc, but allowing a factor of 10 to 100 times would probably suffice to compensate for skimming and other losses, resulting in a figure of between 0.8 and 8 grams of food a day, fed slowly and constantly. Food being something like diy liquidised fish mix for instance. This is probably about what many people do feed a day, a chunk of food about 2 cm x 2cm x 2cm would be 8 grams, so its little more than a cube in a 55 per day. I also observe the high amount of sediment in the water when I dive but I think when you are actually in the water, looking through several meters of water, it looks more. Additionally if you compare the suspended matter with nitrogen levels in table 2.8, you can see that much of the suspended matter is not food (perhaps 3/4 not food).

@Randy, regarding driving nutrients too low to measure having serious consequences, I am wondering if its at all possible to drive them below these real reef levels with our equipment. If its not, then is the mantra of not letting nutrients get too low actually beneficial? Are we deliberately choosing eutrophicated levels of nutrients and making colors duller than natural maybe. If corals are happy in nature with 3 ppb PO4 and 10 ppb NO3, would achieving these levels in aquaria be good or bad. More interesting to me is taking nitrogen, almost all of the nitrogen on real reefs seems to be organic it seems, so I wonder about aiming for very low inorganic N and using organic forms of nitrogen (aminos and allowing organic N like urea to build up, which it probably does anyway), together with heavier feeding, and aggressive inorganic nutrient removal.

It seems to me technologically harder to maintain a low level of nutrients than it is to just run everything full bore, GFO and a denitrifier for instance. Trying to maintain reduced flow through various reactors to keep nutes above detection limits is tricky IMO. I wonder if running everything at full flow and supplying nutes in the more natural way via particulates and organic nutes, which might also be less available to algae (havent checked that though) might actually be easier.

Randy Holmes-Farley · Jan 26, 2015

I guess I really don't know how low people can drive phosphate in a real reef tank with GFO, and whether that low level might actually be "natural", with the corals suffering from lack of other sources of N and P that they could get in the ocean.

Perhaps some day we will have adequate tools to answer these questions, but not, apparently, in the near term.

I agree that perhaps the zeo colors are coming partly from other effects, but I'm not convinced that most corals tanks are less colorful than they would be in the ocean due to tank phosphate at 0.01 or 0.02 ppm and nitrate at, say 1 ppm. Perhaps the zoox in the ocean corals grow better due to the more available organics. On those few occasions when I've snorkeled around tropical reefs, most corals seem pretty brown overall.