For this month’s column, reef tanks and corals are put aside and the effects of plants in freshwater are discussed. As you walk into the lobby of our building, you see a 150-gallon freshwater aquarium full of South American fishes and lot of plants. Many aquarists then seem to do a double take and can’t believe that the substrate in the tank is fine silica sand. After all, they claim, it goes anaerobic, turning black and when disturbed, releases all sorts of toxic substances which kill the fish.

The facts are that this aquarium has been set up for 4.5 years, has no signs of black zones and rarely do fish die much less a complete wipe-out occur. Why? The secret is in the use of the plants. And this month a recently published study is reviewed, which will explain what is probably occurring in this system.The study is entitled: Nitrification and denitrification in the rhizosphere of the aquatic macrophyte Lobelia dortmanna L. by Nils Risgaard-Peterson and Kim Jensen. Published in the May issue of Limnology and Oceanography Volume 42, issue 3 pages 529-537.

In this experiment, 6 identical systems were constructed, which were 110 mm high and 45 mm inside diameter. The tube was divided into 2 chambers, the upper 70 mm tall, the lower 45 mm tall by an acrylic plate which was perforated. On top on the perforation, 35 mm of sandy sediment was laid (the sediment came from a local lake and was pre-filtered through 0.5 mm mesh screen). The top and bottom of each chamber was sealed with a glass plate. In the bottom chamber, under the plate was a port to introduce water with a constant concentration of ammonia (950 µM). The upper compartment also had a port through which artificial freshwater was supplied at a constant rate. Artificial freshwater is deionized water in which certain chemicals are added back to make a constant pH, hardness and alkalinity. The benefit of using artificial freshwater is that the researcher knows exactly what is in the water since it started as pure water and only high grade chemicals of known composition were added. To three of these chambers, a single plant (Lobelia dortmanna) was added and to the other three, no plant was added –they were the controls.

The goals of the experiment were to see if the roots of the plants released oxygen into the sediments and, if so, how this oxygen affected the nitrification and denitrification process in the sediments. To determine this, the researchers used microelectrodes to measure oxygen, ammonia and nitrate in the sediments. These special electrodes, along with a computer controlled micromanipulator, allow chemical measurements to be made in the sediments at every 0.5 mm (yes–every half millimeter) through the sediment. Further, the ammonia used to dose the bottom chamber was a special type of ammonia in which the nitrogen consisted of what scientist call labeled nitrogen. Normally nitrogen has two isotopes, call 14N and 15N, which are stable (another isotope, 13N, is radioactive). Most nitrogen is 14N (>99%) with very little being 15N. But through manufacturing processes the percentage of 15N can be greatly increased. Any nitrogen compound made with this 15N nitrogen can be detected using mass spectrometers and as the 15N is used in different reactions the pathways of 15N can be followed. Thus, the 15N is called labeled. In the case of this study, the ammonia used was made up of labeled nitrogen so that when the ammonia was converted to nitrite and nitrate those compound now contained the 15N so the amount of ammonia and the time it took to get from one compound to the next could be determined.

After setting up the chamber and adding the plants, the systems were allowed to run for 11 days before measurements were begun. They had a light cycle of 24 hours on and 24 hours off in order for stable chemical profiles to become established. Then water samples were taken and ammonia and nitrite plus nitrate measured. Chemical profiles through the sediments were taken and fluxes (the movement over time) of the chemical compounds determined.

The results between the two groups (plants, non-plants) were significant in some aspects. Oxygen profiles showed that the oxygen concentration in the sediments with no plants quickly went to zero-4 mm from the top of the sediments the oxygen was gone. In the upper 4 mm, ammonia was consumed; thus nitrification was occurring here. As soon as the oxygen went to zero, ammonia was no longer consumed and actually increased in the deeper sediments.

They found that in the sediments without plants, there was a zone of denitrification in the sediments which released dinitrogen to the upper chamber. They suspect that the ammonia supplied in the lower chamber is subject to nitrification in the sediments and then quickly to denitrification, which produces the dinitrogen. Thus nitrification and denitrification are said to be coupled.

The oxygen profile of the sediments with plants was much different. Here there was oxygen in the upper 4 mm, then went to zero from 4 to 9mm. But then oxygen increased from 9 mm to 23 mm sediment depth. Further, they found that ammonia was consumed in both these aerobic areas and there were peaks of nitrate production in the middle of the two aerobic zones. Therefore, in the sediments of the chambers with plants, they found that there were two zones of denitrification which were characterized by low (nearly zero) oxygen concentrations. Above the first anaerobic zone and between the two anaerobic zones were zones which had much higher oxygen values. In the second aerobic zone at 9 to 23mm was the plant root system. They found that the plant roots had a significant effect on the sediment chemistry by pumping oxygen into the sediments. They also found that nitrification and denitrification were stimulated by the presence of the plant in the sediments.

The results of this experiment can be applied to the home aquarium with some caveats. First, not all plants are the same. Some plants transport more oxygen to the area around their roots than others. Thus, some will stimulate the nitrification-denitrification process more or less depending upon their ability to put oxygen in the sediments. So a blanket statement that all plants release oxygen in the sediments would be incorrect–more research would have to be done to categorize various typical aquatic plants.

However, the results show that plants can have a significant effect on the sediments which can be beneficial to the nitrification and denitrification process in aquaria. Going back to that lobby aquarium, we do the water chemistry on that tank twice a week. It was set up 24 May 1993, the current nitrate concentration (20 Oct 1997) is 1.67 mg/L. It has never been higher than 25 mg/L and that was soon after we set it up. In the past year, the highest nitrate value was 6.6 mg/L. The tank contains about 400 fish (mostly cardinals, pencilfish, and several species of tetras), plus hundreds of plants. It does not have a denitrification system and we change 30 gallons of water every 2 weeks. We feed (probably overfeed) twice a day, seven days a week. We treat the tank like a hobbyist would, we don’t do anything special and we don’t have to spend much time cleaning it. The secret to this low maintenance tank is the plants which keep the sediment oxygenated and extend the nitrification zone. The plants add oxygen to the sediments, which is why there are no black zones characteristic of extreme anaerobic zones.

The research also shows that if you use fine sand in your tank with no plants, that there will be no oxygen in the substrate after just a few millimeters. Thus, disturbing these sediments could release toxic substances such as hydrogen sulfide, which can kill your fish, which is why the initial reaction of visitors in that the lobby tank is a disaster waiting to happen. However, the use of plants prevents this and provides other benefits, as well.

©1997, Timothy A. Hovanec, Ph.D.
Originally published in Aquarium Frontiers, Nov. 1997