chamber, but is not forced through the media, encouraging the diffusion process and preventing the media from becoming clogged (Jaubert, 1991). The confined water behind the partition in these reactor tanks has an average oxygen concentration of 1 mg/L. If it should rise above 1.5 mg/L, denitrification is incomplete, producing excess nitrite. If it should fall below 0.5 mg/L, the production of toxic hydrogen sulfide occurs. The thickness of the sand layer or fibrous material used determines the oxygen level, because the reactors are also illuminated and aerated, to maintain high oxygen content in the circulating water above the sand or fibrous material (Jaubert, 1991)-
Burrowing animals in the sand break down large organic debris into DOC, and dissolved nitrogenous compounds. The nitrogenous waste is mineralized by aerobic bacteria, producing nitrite and nitrate that diffuses downward into the anaerobic layers. There the heterotrophic denitrifying bacteria convert nitrite and nitrate to nitrogen gas, while using organic matter from the water as a food source (Jaubert, 1989). Acid secretions by bacteria and other organisms in the substrate are neutralized by the calcareous sand, causing it to dissolve, and the resulting input of calcium ions in the water seems to be adequate to maintain high, stable calcium levels despite excellent growth of the hard corals in the system. Some sand must be replenished periodically, of course (Jaubert, 1991). The use of DOC for food by heterotrophic anaerobic bacteria, and some "unknown process" removes yellowing substances from the water. (Jaubert, 1989, 1991).
It is interesting that Jaubert (1989) refers to the substrate in the tank as "living sand". This is the same term used by Riseley (1971), in a book describing the methodology of Lee Chin Eng. Riseley stressed the importance of lighting, live rock, and "live sand" taken directly from the reef. Presumably this sand contains organisms like the ones Jaubert uses in his system. Walter Acley, whose system we shall describe shortly, also is a proponent of live sand. Some of the microorganisms and worms that live in the sand should also be present in live rocks, which are often collected in sandy areas. These organisms will colonize the bottom sand layer. We should point out as well that thick live rocks with large hollow spaces inside will work to denitrify the water by the same diffusion principle as Jaubert's system, and thick sand layers on the bottom
without a partitioned off confined water compartment will also denitrify the water. We have not experimented to determine whether the confined water space offers special advantages.
Water chemistry parameters for a 2 m- system set up by Jaubert were recorded for a 4 year period with no water changes. This test aquarium contained numerous growing hard corals, soft corals, anemones, and several large fish. Lighting was from natural sunlight for 4 to 5 hours per day, and from 5 Osram HQI(R) TS 250 W/D lamps ( Jaubert and Gattuso, 1989). Nitrate levels, after an initial value of 0.350 mg/L, dropped steadily to a level of 0.013 mg/L after four years. Ammonium and nitrite levels remained close to 0.001 mg/L. The pH of the wrater varied daily, with a typical morning low of 7.8 and an afternoon high of 8.25. Calcium ion concentration peaked at 480 to 520 mg/L, but never fell below 460 mg/L ( Jaubert, 1989, 1991). Jaubert (1992) discusses variation in the pH and calcium ion concentration in his systems, and proposes that low pH at night further allows some calcium carbonate from the coral sand substrate to dissolve.
Dr. Jaubert's experimental system photograped in 1989. T. A. Frakes.
The success of this system is evident, and the growth of the hard corals has been well documented (Jaubert and Gattuso, 1989). A 40 m3 aquarium has been set up with coral fragments cemented to the walls, and growth is so good that the corals are being farmed (Jaubert et al., 1992).
aquarium literature for quite some time (Brandenburg, 1968, Wilkens, 1975, deGraaf, 1968). For an excellent review of the
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