Elevated dissolved organic carbon and coral mortality.

[dtyharry]

Just finished reading Forest Rohwer's book Coral Reefs In The Microbial Seas. It is a brilliant book and discusses in detail the link between elevated doc levels and the decline of the reefs.
In a very unscientific summary by myself, overfishing of reefs leads to increased levels of algae which in turn produce more dissolved organic carbon, which in turn in sufficiently high levels will kill the corals due to vastly increased levels of microbial activity within the coral itself. This has been shown to literally starve the coral of oxygen even when the water all around it is saturated.
It was I interesting to see that exposing corals to elevated levels of phosphate or nitrate had little effect, but dissolved organic carbon always killed them.
This I guess explains coral deaths in tanks overdosed with vodka and such things, but may also be a warning to those with algae scrubbers. If the levels of doc become too high in such an enclosed environment then possible damage could occur.
I am going to try and contact him to see what hIs views are on the subject.
Note that I am not talking about normal levels but elevated levels, which always occurs reefs are overfished and the algae takes over.

[Floyd R Turbo]

That is a good question, what is considered 'elevated' and what DOCs they are talking about. Because there are different types of DOCs. I don't think that in a closed system like we run, which finds a natural balance point, that you could kill off your corals with DOCs produced by an algae scrubber. Otherwise, you would have people coming out of the woodwork saying their algae scrubber killed their corals, and it's just not happening. If anything, it's the opposite. So I think some clarification is definitely needed.

The point about P and N is quite interesting. For some time, I have suspected that P does not in and of itself inhibit coral growth. I say this because the mixed reef tank I run has always had detectable levels of P (for a long time now, never lower than 0.09 and usually 0.13-0.16) yet all the corals are thriving. I have only had a few instances of corals 'clamping' and it has been intermittent. Interestingly, they are usually clamped as the water gets cleaner (toward the end of the weekly ATS cycle) and those are usually the LPS corals, and one SPS. So I have a few odd things going on that I haven't been able to put a finger on, at least not exactly.

[SantaMonica]

A few things:

1. You'll never get too much DOC in a tank. Algae is 90% of non-bacterial biomass in the ocean, and the remaining 10% is everything else...corals, whales, fish, inverts, etc. In order to equal this in your tank, take all the living things... corals, fish, inverts, life in the rocks and sand, etc, and multiply their weight by 9: That is the weight of the mass of the algae you would need to have just to equal what's in the ocean. Forget even trying to be "too much".

2. Bacteria are not getting enough DOC in our tanks; they are carbon limited, even with scrubbers. This is why if you dose carbon on a scrubber tank you get a bacterial bloom; the bacteria are not getting near enough carbon from the algae to multiply.

3. Corals in the ocean have increased temps, increased pollution, and decreased pH. Our tank don't. The book you mention, which I have, details all of this.

4. Vodka over-dosing decreases N and P to levels too low for corals to grow. It's even too low for scrubbers to grow.

5. As Floyd mentioned, nobody has a coral mortality problem. There is never a "coral bleaching" event in scrubber tanks. Not one that I've ever heard of. And over 1000 people have built scrubbers on just my threads alone. Another 10,000 have built scrubbers on other threads, and I would have heard something about them.

[dtyharry]

It has actually been shown that on a pristine reef and not one of the majority that exist today, that actual biomass is highest amongst the top predators and not the producers ie the autotrophic algaes. The reason for this is that the algae is being constantly grazed by the herbivores meaning that a great deal of the algaes energy goes into growth and so little dissolved organic carbon is released. The energy is rapidly transferred up the food chain. Take away the herbivores and the algae grows much larger and requires far less energy for growth. The photosynthesis continues however and the excess dissolved organic carbon which has been shown to cause coral death is released into the water.
It is a little misleading then to say that 90% of the biomass in an aquarium would need to be algae.
The fact that the algae from a scrubber is being harvested on a regular basis replicates the reef herbivores to a certain extent.
It is obviously no coincidence that reefs that are in decline are overrun by algae and as the algae gets worse the situation accelerates.
Ph on a natural reef is far more stable than a small tank full of water.
I suppose unless measurements of scrubbed tanks are taken we will never know for sure what levels of dissolved organic carbon are. Maybe advanced aquarist would like to do an experiment for me!

[SantaMonica]

You are confused. Benthic algae is a very tiny part of the total algal biomass of the ocean (or lakes). Pelagic algae, primarily cyanobacteria and diatoms, do almost all the primary production in the water masses, and about half the primary production on earth. How else do you think a trophic struction of 10% per level can be achieved if 90% is not at the base? You could get rid of all benthic on the planet, and the microbial loops would still work the same. Do a little research.

[dtyharry]

Refer to chapter 8 of the book I mentioned at the start of this link. I am well aware that on an ocean wide basis the biomass is in the proportions you mention but you are getting confused between physical biomass and the amount of primary energy being produced. Yes ocean wide algae of all types produce by far the most biomass as you would expect, but don't forget that all the coral reefs in the world cover just a fraction of one percent of the ocean surface. There is vastly more life on a coral reef than on an equivalent patch of open ocean for example. The energy has to come from somewhere and it is from the turfs and other fleshy algaes. It is counterintuitive but also true that the physically largest biomass on a pristine reef is the top predators. As I mentioned previously this is because the algae is kept at very low levels by the herbivores and the energy is rapidly moved up the food chain. Most of the energy produced by the algae goes into reqrowth and is not leaked into the water in the form of dissolved organic carbon. Exactly the same principle as mowing your lawn, lots of energy produced but lovely short grass not overrunning your property. It is only when the algae is allowed to grow unchecked because of overfishing that the problems occur.
Don't take my word for it, I am just referring to forest rohwer's research and chapter eight, microbialisation of Christmas atoll.
Assuming 10% efficiency you would calculate that 1lb of shark requires it to eat 10lb of prey fish. These 10lb of prey fish would need to eat 100lb of fleshy and turf algae etc. If you are saying that for every 1lb of shark swimming on a reef you have to have physically present 100lb of algae you are confused. 100lb of algae yes has to be produced but the constant aggressive grazing keeps it's physical size very low. I am no expert but when you think about it it does make sense. It is the extra physical presence of all the turf and other algaes on a sick reef which eventually kills any remaining corals by increased levels of dissolved organic carbon.

[SantaMonica]

1. You are assuming there is no connection to the open ocean. Eckman transport will bring in ocean waters that are 100% based on a phytoplankton first trophic level. In other words, the DOC chemistry of the ocean will be transfered to the reef. And a shark or other top predator is far more than two trophic levels up from phytoplankton. Try about 10.

2. The coral mortality that the book talks about includes the other coral-weakening factors of increased pollution, temps, and lowered pH. I hope you understand that our tanks don't have these problems.

3. You are also not taking into account the much higher percentage of coral biomass in a tank. On a real reef, even a shallow 30 foot deep one, the mass of the corals is a tiny fraction of a percentage of the water volume; so there is not much coral biomass to consume DOC and the bacteria that consume the DOC. In a reef tank however, the coral biomass percentage is hundreds or thousands times more. And this extra coral biomass consumes DOC and bacteria out of the water faster than it can be produced. This is again shown by vodka dosing providing a missing carbon component, which would not work if the water were already "saturated" in DOC.

4. And you are still disregarding the empirical evidence that not one mass bleaching event has ever occurred in a scrubbed tank. If you want to make one happen, turn your temp up to 80 and wait a few days.

I'll review that chapter to see if I forgot anything, but even if I did it would not matter because empirical testing (over 1000 scrubbed tanks on my threads alone) overrules any natural reef theory.

[dtyharry]

Point three is a very good one. The average volume of aquarium water contains far more coral than an equivalent one of a reef. The two trophic level was for the sake of simplicity, of course there are more than two levels from algae to sharks. At ten levels at ten percent efficiency one pound of shark would require a billion or so pounds of algae to be produced so there is obviously some error there! Even so the point is that an awful lot of algae has to be produced in order to feed a thriving reef. This is completely different to that amount of algae being present at any one time. If that were the case the reefs would be overrun. Aggressive harvesting by herbivores keeps it in check on a healthy reef.
Although global factors like reduced ph and increasing temperatures pose a global threat to coral reefs, the immediate concerns are overfishing and nutrient enrichment. A healthy reef can deal with excess nutrients to a certain extent via increased algae growth, AS LONG as there are sufficient herbivores to keep algae levels low. Add overfishing to the equation and you get today's algae ridden coral poor reefs. Ocean levels of doc are lower than over reefs anyway and so mixing by ocean currents would have no doc increasing effect.
I don't think I mentioned coral bleaching at all but 80 would not cause it. There are many ways for a coral to die apart from bleaching and increased doc levels has been shown to increase the level of disease.
I am not arguing that scrubbers reduce nutrients, that is clear to see, my main point was that on a natural reef, a healthy one has very low levels of algae and an unhealthy one has very high levels. Algae and coral are not good bed fellows, too much algae will indirectly kill the reef.
I may be mistaken but did the regular contributor floyd at some time mention that his corals seemed to do not so well towards the time of a cleaning, a time when you would expect the water to be at it's best. Maybe all the algae growing on that very effective screen was releasing elevated levels of doc into the water, bit of a curve ball and maybe wrong but what the hell!

[Floyd R Turbo]

I've posted that issue on my tank thread several times, and after close observation the 'meatier' LPS corals I have seem to not like the water towards the end of the cycle. The theory here is that these corals like the water a little 'dirtier', but what part of dirty I do not know. You may be on to something though, I'm just not sure what that might be. The softies don't seem to be affected, and I have no problem keeping Xenia and Anthelia alive, no matter how I try to kill it. My mega-fuzzy-mushroom, which was 6" in diameter, just split into 4 pieces the other day. GSP going nuts. Meanwhile my Frogspawns and Branching Hammers are withering away (especially after the dino outbreak) but the screen is at maximum growth again after an off-week with the accidental 24/7 lighting. So is it the thick algae growth or what I do not know. Too many variable to pin it down to one thing.

[SantaMonica]

I don't think I mentioned coral bleaching at all but 80 would not cause it.

You are correct; I meant 88.

Ocean levels of doc are lower than over reefs anyway and so mixing by ocean currents would have no doc increasing effect.

You may want to look again. DOC is kept low by bacteria; there is no clear pattern of "higher DOC on reefs"...

Bacterioplankton carbon growth yield and DOC turnover in some coral reef lagoons. Proceedings of the 8th International Coral Reef Symposium, 1997.
http://www.reefbase.org/download/downlo ... 00001879_1

Table 3: DOC concentrations in Pacific surface waters and in some coral reef lagoons:

Pacific near Eniwetok.....................................86
Pacific near Houtman Abrolhos atoll lagoon. (Aust)...13-33
West Pacific........................................... ......101
Equatorial Pacific (1989).................................125-225
Station Aloha Hawaii......................................90-115
Equatorial Pacific (1992).................................63-67
Pacific near Miyako Island...............................93
Pacific surface near Tikehau............................87
Pacific (0-40 meters deep) near G. Astro. Rf......110
Mauro Atoll lagoon.........................................145
Ponape Island lagoon.....................................223
Eniwetok atoll lagoon.....................................100
Houtman Abrolhos atoll lagoon (Australia)..........130-305
Kaneohe Bay, Hawaii......................................148
Bora Bay, Miyako Island..................................84
Wreck Reef.............................................. ....64
Tikehau lagoon............................................ .105
Takapoto lagoon...........................................1 21
Great Astrolabe Reef lagoon............................114

"The comparison between total DOC decrease and bacterial biomass increase allowed the estimation of bacterial Carbon Growth Yield consistent in three different coral lagoon waters. The low CGY values confirm the importance of bacterioplankton in the mineralization processes occurring in coral reef waters. Such low CGY values are an index of bottom-up limitation of bacterioplankton growth in the coral reef lagoons, and are in agreement with the low bacterial growth rates (turnover times of 4 to 8 days), considering an average temperature of 30 degrees C. This could be due to a poor availability of either organic carbon, or inorganic nutrients."

A healthy one has very low levels of algae

No. A healthy reef has extremely high levels of algae, as much at 20 pounds wet weight of standing stock per square meter. And this is in addition to phytoplankton, which at 5 meters and deeper start becoming significant. I have this data too if you want it.

too much algae will indirectly kill the reef.

No. Too much coral death will allow more algae to grow in their place. If "algae killed corals", then the corals would have started dying a lot sooner that 10-20 years ago, which is when it started.