Several parts of that sentence are opinions (IMO)
I can see that. If you have a good imagination.
It may very well be correct, but I've never seen any study that actually showed the net release of problematic nutrients from a sand bed of any sort.
http://www.google.com/url?q=http://...414893&usg=AFQjCNHLTbKIUK89peAoFNKQAmAnU6pfOg
http://www.google.com/url?url=http:...iments&usg=AFQjCNEadoN5p-j75TH1byVeEUhXSPO8aQ
http://www.ozcoasts.gov.au/indicators/sediment_org_matter.jsp
There is absolutly no question as to weather or not nutrients like nitrogen and phosphorus are released from sediments. The fact that we're alive to talk about it is proof that nutrients are released from sediments. The net release will vary greatly from habitate to habitat, season to season, composition of the sediments, and even in samples taken from feet or inches apart, so it's hard to quantify.
In our systems where inorganic sedimentation is very very very low, and organic sedimentation is very high, the release of nutrients like nitrogen and
phosphorus from sediments will be very high. The rate of decomposition will be highest near the surface of our sediments where oranic particulate matter, and oxygen levels are at their highest.
We typically feed at a rate that exceeds the rate of decomposition. In other words, before one feeding has the time to break down, we feed again. This means that without our intervention, the amount of rotting organic matter within the system increases with time. Thanks to gravity, and the simple laws of physics, the majority, or at least a large portion, of this rotting organic matter ends up in/on the sediments. When a system like Shemik's DSB is emploied, our intervention is prohibited. According to the proper management of his methods, we're not permited to manually go in and remove this rotting organic matter. With Leng Sy's Miracle Mud, we are advised to manually remove the sediments, along with the rotting organic matter it contains, but only after six months to a year of organic matter accumulation.
When organic matter accumulats and rotts, the nutrients it contains, like nitrogen and phosphorus, are converted into liquid form, and released into the
surounding environment. If we don't manually go in and remove this rotting organic matter, our sediments become little more than a fertilizer plant constantly pumping out nutrients.
You assume in your opinion that such release is "unhealthy". You might be focussing on certain corals to give that opinion, but it is unlikely to apply to all organisms we keep.
You are correct. If we look at the photos Ivans75 posted, the only coral I see that has its polyps extended appears to be xenxa, or at least some form of octocoral. IMHO, this is due to the nutrient level in the water, where these corals are kept.
We disscussed earlier in this thread how the organisms living on reefs change as nutrient levels change. This is true throughout nature.
When a coral reef goes from being a healthy, growing, reef, to one that is in decline, the organisms living there change. Stony corals become less abundant, and sponges, sea fans, and algae become more abundant. We keep a wide variety of organisms in this hobby that thrive under different nutrient levels. We typically don't have a problem providing our systems with enough nutrients though. The common problem in our systems is keeping nutrient levels low enough. As I said before, check the number of algae problem threads on this site, and that becomes clear.
Those organisms we typically keep that prosper with higher nutrients than stony corals, still have their upper limit. Just as on the reef, the organisms we keep successfully, change as nutrient levels change. If we employ a system like Shemik's DSB, or Leng Sy's Miracle mud (and do not go through the expense to change it out regularly), we have a system that becomes more nutrient rich as time progresses. We have seen this in hobbyists tanks over, and over, and over again. The typically scenario is for stony corals to do well while the system is young and clean. As the system ages, and nutrient levels rise, stony corals begin to suffer, and oranisms like xenia and GSP begin to prosper. As rotting organic matter continues to accumulate in these sediments, nutrient levels continue to rise, and algae becomes more of an issue. If the process is permitted to continue, algae begins to choke out corals like xenia and zoo's, and even these corals begin to suffer. Cases of bacterial, or fungul, infections in fish also become more common as nutrient levels rise. Eventually, someone declairs that the system is suffering from "Old Tank Syndrome", and needs to be broken down and started all over again.
We could avoid this whole ugly scenario, all the loss of life, frustration, and expense, if the hobbyist simply understood the importance of managing nutrients, both dissolved and in particulate form.
One operative word is missing from your sentence. "traps and holds nutrients" does not actually say any is released. Trapping and holding is, I think you'd agree, fine, if no release takes place.
Yes I agree. If you can come up with a method that traps and holds nutrients within our systems, indefinantly, without releasing them into the water, you'll be a very rich man. Unfortunatly, it doesn't work that way, and I didn't take the time to go into that in the statement you quoted.
Just to clarify my statement. Yes. Nutrients are released from solid organic material. That's the bad part. If they remaind bound in the solid particles, they could cause our systems no harm. However, as they decompose, the nutrients they contain, like nitrogen and phosphorus, are converted into liquid form, and released into the surrounding environment. The more solid particulate organic matter there is within the system, the more nutrients its releasing into the water, and the higher the overall nutrient content of the system will be. If the accumulation of rotting organic matter is kept to a minimum, the amount of nutrients being released into the water, will also be kept to a minimum, and the overall nutrient level of the system will remain low. Provided that both systems have the same basic routine maintenance performed, like water changes.
Is there a net release of nitrogen from such a sand bed? Maybe, maybe not.
I've read research papers done in the field, articles written for the hobby, and posts online about nitrate reduction in sediments. I've seen nothing convincing that would suggest that denitrification has the potential, in aged aquarium sediments, like DSB's or unchanged Miracle Mud, to reduce nitrate more efficiently than periodic manual removal of the solid organic matter before it breaks down.
There's a side to this that's largely overlooked in the hobby. All of us have cyanobacteria in our systems. Even the pristine looking, SPS dominated, tanks of the month, have them. The difference is in the number we have. Their numbers typically rise as nutrient levels rise. If you look at the photos of the sand bed in this link,
http://www.google.com/url?sa=t&rct=...2MCkDA&usg=AFQjCNHo2MEMMTPt5v_LekPwrIA1Q27eOw ,
you'll see that cyanobacteria are prolific. There's no doubt that its the decomposition in the sediments that's fueling this cyanobacterial growth. Cyanobacteria have the ability to fix nitrogen gas, and produce organic nitrogen, leading to more nitrate in the water of the system. Rotting organic matter also release nitrate that can then enter the water of the system.
So, what we have is denitrification reducing nitrate levels deep in the bowels of the sediment, cyanobacteria raising nitrate levels in the open water, and decomposition raising nitrate levels in the open water. How efficient can this system be at reducing the nitrogen content in the open water of the system? To me it sounds more like a method to produce nitrate, than a method to reduce it. Which is pretty much what we see in systems that employ these methods.
Finally, whether trapping detritus is a "problem" depends on what else might happen to that detritus. If it is skimmed or otherwise removed, then I agree it will probably cause more released nutrients (at least phosphate). If it just settles out somewhere else, then the difference may be moot.
Again, I agree. It doesn't really matter where the organic matter is within the system. It's going to rot and release nutrients. The rate of decomposition my vary, depending on where it settles, but the end result will be the same. The only way to stop this is to remove the organic matter before it has a chance to break down.