Updated: Jun 12, 2022
Identifying and resolving limiting factors in the planted tank is a crucial step on the road to a successful aquarium. We’ll start by determining which factors are limiting the growth of your plants and increasing them to balance with other factors. First however, we must define what a limiting factor is and which factors are most commonly limited. Then we can ascertain how to resolve these limits and what can be done to prevent them in the future.
What is a Limiting Factor?
A limiting factor is any resource that is lacking in quantity to the point where it impedes the growth of an individual, population, or community within an ecosystem.
In biology, a population is defined as a group of individuals of the same species living together. A community is a group of populations living together in roughly the same area. An ecosystem is all of the biotic (living) and abiotic (non-living) components of a common area.
For example, a riverine (river) ecosystem includes the biotic populations of bacteria, algae, plants, molds, insects, crustaceans, mollusks, fishes, reptiles, amphibians, birds, etc. It also includes the abiotic components of rainfall, sunlight, water flow, nutrient and pollution levels, etc.
A limiting factor can be any one or more of these many biotic and abiotic factors. For example, if a building is constructed next to the river and it blocks 80% of the sunlight over a certain portion of the river, then sunlight becomes a limiting factor, which reduces the individual and population growth of certain plants and algae. The other factors which lead to plant and algae growth, nutrients and water, have not been affected and therefore are not limiting growth.
Limiting factors don’t always have to be a lack of resources. An example would be if pollution increased downriver from a factory and began to poison and thus reduce the ecosystem’s communities.
Another example would be a significant increase in nutrient levels from a farm upriver. This point source effluent (single source pollution), can cause massive increases in one species, like cyanobacteria, which will choke out and reduce the population of other species.
In this last case, the limiting factors are technically defined as the resources which are used up by the population explosion from the nutrient increase. In some cases, such as an increase in cyanobacteria, there is a toxic secretion component as well.
Limiting Factors in the Aquarium.
In regard to plant growth within an aquarium, we’ll focus on three major components; light, CO2, and nutrients.
For the purposes of this article light is defined as any photons reaching the tank. CO2 is defined as the addition of carbon dioxide gas into the water column. Nutrients are defined as organic and inorganic elements that contribute to plant growth, excluding carbon. Organic nutrients are elements that come from organic sources (nitrogen, phosphorus, potassium). Inorganic nutrients are elements that come from inorganic sources (calcium, magnesium, iron).
The Importance of Identifying Limiting Factors
It is important to identify limiting factors because imbalanced resources can cause nuisance algae growth. This results in unnecessary consumption of energy, fertilizers, and CO2 gas, as well as an elevated risk of crashing your aquarium. Crashing refers to a sudden and drastic change in the aquarium that results in elevated toxicity or a severe imbalance that results in a negative effect. For example, ammonia increasing rapidly can cause fish death and CO2 increasing rapidly can quickly lower, or crash pH.
Nuisance algae will occur if your light is too high, your nutrients are too high, or CO2 gas is too low. Excessive light and nutrient levels are caused by high lighting and overuse of fertilizers. The aquarium can crash by a rapid drop in pH if CO2 is used too heavily. It all comes down to proper balance.
The Basic Relationship
The advantage of identifying the limiting factor(s) within your aquarium is that it will allow you to precisely balance these three components. Nothing is wasted, and each one is used in proportion to the others.
If your plants have access to plenty of light and nutrients, but not enough CO2, their growth will suffer. If you then increase your CO2 gas, your plant growth will increase.
However, if you increase CO2 too much, then light or nutrients will become your limiting factor. If you increase your light, and your plant growth increases, then light was your limiting factor. However, if you increase light and your plant growth does not increase, then light was not the limiting factor in your tank, nutrients were.
Photosynthesis is not limited by nitrogen or carbon in any concentration found in natural environments (Chapin 2007). It is, however, eventually limited by light intensity (Chapin 2007). At a certain light intensity, photosynthesis rates level off. That level of photosynthesis is highest within middle-aged leaves, as opposed to the newest or oldest leaves (Pallardy 2008).
Therefore, photosynthesis can not increase infinitely. And because light intensity is the ceiling on photosynthesis, we can assume that an aquarium with nutrient and CO2 levels which match that maximum light intensity will have no limiting factors for plant growth.
This scenario would require the sun to be in your living room. And plant growth would be so fast that you would be spending thousands of dollars on CO2 and fertilizers.
This is why you want to thoroughly understand the limiting factor equation. You want to bring all three factors to a balanced ratio which will promote healthy plant growth at a rate we are happy with, and which does not cause nuisance algae growth.
To put it mathematically:
x = ideal light intensity/temperature
y = ideal nutrient level
z = ideal CO2 level.
x + y + z = (balanced plant growth)
There is no actual numerical value for each of these, as each aquarium is different. I am defining x, y, and z as your ideal level, which you’ll determine through experimentation.
Your aquarium’s current equation may look like this;
½ x + ¾ y + 1 ½ z = (unbalanced plant growth)
In this example, your light is the most limiting factor, followed by your nutrients. The CO2 is too high. To fix the equation, you’ll double the light intensity, increase nutrients by about 25% and decrease CO2 by about 50%.
So how do we determine which factors are limiting in our equation?
Identifying the Limiting Factor.
If you have a picturesque aquarium with no algae growth, bright and vibrant plant growth and no tissue damage or otherwise worrisome signs of disease, you may not have any limiting factors. For the rest of us, there are few ways to visually determine if your tank is lacking a certain resource.
Lack of light intensity usually results in slow plant growth and etiolation. Etiolation refers to plants growing thin and spindly toward the light source, trying to get as close to the light source as possible. If your plants are thin and have few leaves, but are growing up quickly, then your light source is too dim.
If the light source is way too dim, plants will not be able to photosynthesize at all. You’ll start to see yellowing leaves, necrosis (death) of the tissue, and eventually, the plants will die and decay.
There are dozens of symptoms of nutrient deficiencies in plant tissues. Some symptoms are unique to a certain nutrient as they have specific roles in plant growth. Other symptoms, such as yellowing leaves, or wilting, are commonly caused by several different limiting factors. Further research into the species of plant(s) you have can help you to decipher what may be unbalanced in your tank. A separate article will go into detail on plant nutrient deficiencies. However, organic or inorganic nutrients may be your limiting factor if you are seeing any of the following: tissue necrosis, yellowing leaves, pinholes in leaves, stunted growth between nodes, yellow or brown edges of leaves, new leaves misshapen, new leaves yellow or white, and darker than normal leaves, just to name a few.
Too little carbon dioxide is most commonly identified by an increase in algae growth (this can also be caused by too much light), stunted growth, white spots on leaves, and yellowing leaves, followed by melting or dissolving of leaves. This eventually results in reduced plant volume.
Another method to determine if you have limiting factors is to test your water parameters. This article on freshwater aquarium parameters lists the ideal values for CO2 and a few major nutrients. You can test your nutrient parameters using one of several commercially available kits. CO2 can be measured with a drop checker, or by calculating Co2 levels using pH and Alkalinity readings.
Increasing Limiting Factors
Once you have determined which factors may be limiting your plant growth, it's time to increase them. The key here is to increase slowly. If you drastically change a plant's chemical environment, it may drop its leaves in order to grow new ones which are better suited to the new chemical environment. This would not be ideal because a limiting factor is already causing stress on the plant, and losing its leaves would lower its photosynthetic rate, and slow down its growth.
In addition to increasing your limiting factor slowly, it is important to only change one factor or one nutrient at a time. By only changing one variable at a time, any differences you notice in your tank after can be attributed to that variable alone. If you increase calcium and potassium simultaneously because you saw misshapen new leaves and pinholes on old leaves, but then only one symptom is resolved, you don’t know whether it was the potassium or the calcium that caused it.
Remember, when we increase a limiting factor, another factor becomes the limiter. For this reason, going slowly and only increasing one variable at a time allows us to deliberately bring all our factors to that ideal x + y + z = (balance plant growth) equation.
Decreasing Abundant Factors
As I mentioned at the beginning of this article, you can sometimes have overabundant resources. Increasing nutrients and CO2 to match light intensity is not a good idea if your light intensity is too high (excessive algae growth). Instead, decrease light intensity to balance with your nutrient and CO2 levels, then determine which of those two is now limiting plant growth.
This is one of those times where experimentation and use of the scientific method is crucial to the process. When determining limiting factors of plant growth in your aquarium, there are so many variables at play, you really don’t want to go in half-hazard and see what sticks. Slow, calculated, and deliberate changes are the most efficient way to determine if you have a limiting factor problem, and how to solve it.
Don’t forget to write everything down. Keeping track of what you have added, and the changes which resulted, will ensure your success in this sometimes tedious, but ultimately very rewarding process.
Chapin, F.S., Eviner, V.T. (2007) “Nitrogen Limitation” Treatise on Geochemistry: Elsevier Science. 8: pp 1-35.
Pallardy, S.G. (2008) “Age of Leaves and of the Plant” Physiology of Woody Plants: Academic Press. 107-167.
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