Randy is correct -- the black zones which we associate with sulfide reduction zones are not the visible result of H2S per se (rather the reduction of various other compunds such as metals - I discussed this in greater detail in the FAMA articles), but they are a reliable marker for the presence of anaerobic respiration in the sandbed and in the end that is all that really matters to effectively managing a sandbed, IMO.
It is sort of the last explanation that Randy offers which can account for the presence of H2S not causing serious problems to infauna in an aquarium. I point out in a number of places (often in bold) that a large and continuous production of H2S, methane or other anaerobic decomposition products will likely be detrimental to your tank inhabitants, but zones of anoxia deep in the sediments and the accompanying sulfate reduction or methanogenesis can perform a variety of useful functions, including binding toxic heavy metals, regenerating some nutrients, possibly even supporting populations of chemoautotrophic bacteria that can directly take up compounds toxic to aerobes and convert them into a useful nutrient source for other bacteria and infauna in the aerobic layers above them. If the system is continually overloaded, the aquarium is bound to collapse and the eruption of “sulfide volcano” or some other consequence of sulfide poisoning in the aquarium is
one potential outcome – of course, given continued nutrient overloading in a system, there is
no other method that could prevent a similar outcome...
Having said that, I explained the activity of the meiofauna (animals that live between sand grains in a sandbed) in the FAMA articles as well. The punchline is essentially that these animals are quite tolerant of high sulfide concentrations and, in fact, frequently burrow well into the sulfide reduction zones, which causes slow relase of sulfide as suggested by Randy. These reduction products are typically oxidized once they penetrate the aerobic layers of the sandbed if their movement is slow, and are also often consumed by animals in those layers. In fact, the reason that these animals burrow into the sulfide zones is for feeding -- they eat the reduced compunds present in the anoxic regions of the sandbed. In fact, the meiofauna provide an important and major pathway for nutrients to re-enter the aerobic zone of the sandbed above the sulfide reduction zones (as I mentioned above). This explains why people often observe animals or animal tracks/burrows in anoxic regions of their sandbeds. For example, anaerobic sulfur bacteria may account for nearly 1/3 of the diet of ciliates, and grazing rates by infauna averaged something on the order of 3% of the total bacteria and 1% of the total diatoms in the sediments per hour in highly productive regions of natural estuaries.
The diet of meiofauna examined consisted primarily of diatoms, flagellates, sulfur bacteria and “other” bacteria (including cyanobacteria and chemoautotrophic bacteria), which together accounted for over 90% of the diet, the relative contribution of each depending on the species. In all cases however, when researchers examined the diet of specific taxa of sand infauna, anaerobic bacteria were important enough to be named specifically as one of the main diet items rather than being lumped into the "other" category.
Why should anyone care about this -- well bacteria process nutrients best while actively growing, and these data argue that meiofaunal grazing probably represents an important stimulatory effect on the benthic microbial community, and is also essential to the long-term health and stability of sandbeds in closed systems.
In general my feeling is that the release of a large amount of H2S into the aquarium is a symptom of a larger problem with mechanisms of nutrient export. I have never seen or spoken personally with anyone who experienced a problem with H2S in their aquarium that I would trust to look after my tank while I went on vacation. I believe that as with many aquarium myths, as Randy and I have both said already, the dangers of H2S in the aquarium seem to be quite overblown. I can honestly say that I've had the same problems that everyone else likely experiences sometime (e.g., a powerhead fall down and blast a huge hole in my sandbed) and suffered no great tank disasters as a result. Perhaps I was just really lucky...
Rob
[This message has been edited by Biogeek (edited 14 June 2000).]
