Every day in a completely cycled tank with a stable fish load, some of the bacteria are naturally dying and others are reproducing. The rate for each of these things is based on the ammonia levels in the tank. When this is fairly stable, then so are the death and reproduction rates.
Now what happens if the ammonia creation rate changes because new fish are added or some fish are removed. The rates of death and reproduction will also change accoedingly. However,compared to other species of bacteria the nitrifying bacteria are slow reproducers . Plus they do not form spores. So, is there is a larger drop in the amount of ammonia being reproduced the rate of death may increase due to not enough nitrogen compounds being there to feed them.
But if the ammonia spikes up, the bacteria cannot reproduce fast enough to handle it quickly which is when we would see spikes in ammonia and nitrite.
Next, because the nitrifiers do not produce spores, they use other strategies to be able to have surived on the planets for an extremely long time. One of the strategies is when any of the major things they need to survive and thrive become absent- think ammonia/nitrite, oxygen and inorganic carbon, the bacteria can sense this and they will go dormant. It is important to understand the difference between a decrease in one or more of these these things as opposed to the total absence of them or such a reduction in the oxygen or inorganic carbon that they will respond by going formant until what is missing returns.
While dormany activity is really minimal, However, the bacteria are not immortal and death will go on, albeit at a slowed rate. If they are dormant long enough then so many will die that to get a colony back the the same level it was when it went dormant takes the same or more time than it did for them to establish. But as long as a single viable survives, it will reproduce.
Bow for the bigger picture. Bacteria are organic forms of life. When this dies the process is it becomes ammonia. So, if there is a big enough loss of the nirtifying bacteria, they will add to the ammonia being produced in the tank they are in. This is ammonia above and beyond what the fish and other decaying organisms are normally creating. Plus the bacteria dying are then no longer oxidizing ammonia and nitrite.
What does not happen if ammonia creation is reduce but not eliminated is that some of the bacteria go dormant. it is an all or none proposition. Cut ammonia creation in half and you will lose about 1/2 of the the nitrifying bacteria. None go dormant and no ammonia or nitrite spikes happen. These only occur when there is more ammonia being produced than the existing bacteria can oxidize so the test reading are 0 ppm.
Joke Geets, Nico Boon, Willy Verstraete, Strategies of aerobic ammonia-oxidizing bacteria for coping with nutrient and oxygen fluctuations,
FEMS Microbiology Ecology, Volume 58, Issue 1, October 2006, Pages 1–13,
https://doi.org/10.1111/j.1574-6941.2006.00170.x
Abstract
In most natural environments as well as in engineered environments, such as wastewater treatment plants, ammonia-oxidizing bacteria (AOB) experience fluctuating substrate concentrations. Several physiological traits, such as low maintenance energy demand and decay rate, cell-to-cell communication, cell mobility, stable enzymes and RNAs, could allow AOB to maintain themselves under unfavourable circumstances. This review examines whether AOB possess such traits and how these traits might offer advantages over competing organisms such as heterotrophic bacteria during periods of starvation. In addition, within the AOB groups, differences exist in adaptation to and competitiveness under conditions of high or low ammonia or oxygen concentrations. Because these findings are of importance with regard to the ecology and activity of AOB in natural and engineered environments, concluding remarks are directed towards future research objectives that may clarify unanswered questions, thereby contributing to the general knowledge of the ecology and activity of ammonia oxidizers.
You can read the full paper here:
https://academic.oup.com/femsec/article/58/1/1/468326?view=extract
Salem, S., Moussa, M.S. and Van Loosdrecht, M.C.M., 2006. Determination of the decay rate of nitrifying bacteria.
Biotechnology and bioengineering,
94(2), pp.252-262.
Abstract
The growth and decay of nitrifying organisms determines the amount of nitrifying bacteria in activated sludge systems. The growth rate of the nitrifying organisms is reasonable, well defined, and studied, while the decay rate is still rather uncertain. Experiments in previous studies were over periods up to 14 days and obtained results were not confirmed. Contradicting decay rates of nitrifiers in different bacterial communities is reported. No differentiation between ammonia and nitrite oxidizers was made. Therefore, in this studyper day the decay rate of the nitrifying organisms was studied. The starvation condition (aerobic, anoxic, or anaerobic), temperature, type of bacterial community, and the presence of higher organisms are the main aspects that were investigated. A simple and reliable method (adapted from previous studies) for determining the decay rate of nitrifying organisms under different starvation conditions and different temperatures was developed. The test procedure has been used for determining the decay rate of ammonium and nitrite oxidizing bacteria in an enriched nitrifying culture and in activated sludge. The test was successfully applied at starvation periods up to 30 days. The decay rate of the enriched culture of nitrifiers was very low compared to values for nitrifiers in activated sludge. The decay rate of the nitrifiers in activated sludge was found to be to 0.2, 0.1, and 0.06 per day for aerobic, anoxic, and anaerobic conditions, respectively. The decay rate of ammonia oxidizers and nitrite oxidizers was the same at the corresponding conditions. © 2006 Wiley Periodicals, Inc.
(Institutional access is needed to view the whole paper)
What the above means is that if the bacteria are well fed when they go dormant they last longer and recover faster than if they are somewhat starved when they go dormant.
As to what can cause a big die off of the nitrifying bacteria, it is not the partial removal of ammonia from the system, and it is lack of it which triggers dormancy. So, how can we know what may have caused an ammonia spike in one of our tanks? It takes a bit of detective works to track down what is going on. If we experience an ammonia spike in an established tank which has been fully cycled for a while, there has to be a reason. It might be removing some off the media in one's filtration and/or the surface layer of substrate becauce one changes their gravel or sand.
And then there are issues with live plants. They can use ammonium faster than the bacteria can use ammonia. So the more plants one has, the less bacteria there will be. But it will never be none. So, removing a chunk of ones plants can result in an ammonia spike.
