The Effects of Water Temperature in Aquaponics

By: Joe Pate

The Effects of Water Temperature in Aquaponics

Today we’re going to expand on the idea of water temperature in your aquaponics system, and describe how temperature affects water quality and its relation to fish and plant growth.

Temperature 

  • Temperature is a measure of how fast the molecules in a substance are moving; the faster they move, the higher the temperature. Fish and invertebrates are poikilothermic or “cold-blooded” animals; this means that they are unable to regulate their body temperatures like humans or other mammals. Instead, cold-blooded animals must rely on their external environment to regulate their body temperature.
  • Typically, fish are classified into three categories: cold-water, cool-water, and warm-water. Cold-water species prefer temperatures <60, cool-water 60-68, and warm-water >68. This is a general classification.

Temperature and Oxygen Saturation 

  • Temperature and oxygen saturation levels have an inverse relationship; as the temperature rises, the dissolved oxygen concentration decreases, as the temperature decreases the dissolved oxygen concentration increases. 
  • As water temperatures increase the vapor pressure of gases inside the liquid increase; this is a result of molecules inside the liquid starting to move faster; the faster they move, the easier it is for gases to escape, and become equalized with their respective atmospheric pressures. Thus, fewer concentrations of gases are held in the water before being released into the atmosphere.
  • As water temperatures decrease,  the molecules move slower, the partial vapor pressure of oxygen decreases, and fewer gases are released into the atmosphere. This results in a higher dissolved oxygen potential because more oxygen has to be trapped in the water before its gases off at a rate of equilibrium.  

Temperature and Metabolic Rate 

  • Metabolic rate is a measure of total energy consumed by an animal over a given time. For poikilotherms, temperature is often the controlling factor in metabolic rate. As temperature increases, so does the organism’s metabolic rate. As the organism consumes more energy, there is a relative increase in oxygen demand. To meet these increased demands, more feed and more oxygen must be supplied to the fish. A general rule is that for every 10 degrees Celsius increase in body temperature; the metabolic rate increases by 2-3 times. (When temperature is outside of the optimal range, increased temperature can inhibit growth)

Temperature and Ammonia Toxicity

  • The Total Ammonia-Nitrogen (TAN) of the system is comprised of unionized ammonia (NH3) and ionized ammonia or ammonium (NH4+).  NH3 is more toxic to fish than NH4+. The form in which the ammonia exists is based on three factors; temperature, pH, and salinity. In a future post we will discuss this relationship in full detail, but today we will focus on temperatures effect on the toxicity of ammonia. As the temperature rises, the pKa (which is a measure of a compounds ability to gain or lose protons) decreases. Therefore as temperature increases, ammonium’s ability to hold on to its extra hydrogen atom decreases, making more of the TAN become unionized and thus more toxic. This relationship is represented in the graph below.

    Assumes pH of 7.0. Adapted from Emmerson, Russo, et al. 1975.

How does this affect me? 

  • This relationship between metabolic rate and temperature can be exploited to increase the growth of poikilotherms. As more feed is given to meet the increased energy requirements, faster growth is achieved. Unfortunately, the exploitation of this relationship is limited by increased oxygen demand and decreased oxygen saturation, resulting in reduced growth after a certain point. DON’T compromise your fish to increase growth. Know your species tolerance ranges and stick with them. To help with this, we have created the following chart:

What about plants?

  • Temperature can significantly affect the growth and development of plants; the ideal water temperature is dependent on the species of plant and its stage of life. Suboptimal temperatures can result in decreased growth nutrient uptake, bolting, or delayed fruit/flower onset. For this reason, it is essential to look up and know your ideal temperature ranges for your plants.

What is optimal?

The optimal temperature rates for these species are based on aquaculture standards, not aquaponic standards. In aquaponic systems, we often have to compromise between the needs of the plants, bacteria, and fish to create a well-functioning ecosystem.

For instance, Tilapia prefer a temperature between 82-88 F; however, many leafy greens like lettuce prefer cooler temperatures (65-72 F). For this reason, we often try to maintain our temperatures between 72-76 F.

Koi, however, prefer a range of 65-75 F, which is also more ideal for plants, therefore in this system, we would try to maintain our temperatures between 66-70 F.

There are several types of heaters that are either put in-line or directly into the culture water. In the future, we will discuss how to size your water heater as well as differences between electric, natural gas, and propane heaters.

Till next time, happy farming!